P0138 Code in 2026: Wet Connectors, Software Flashes, and the Sensor Toyota Says Not to Replace

P0138 is a code about a sensor that mostly watches rather than steers.

A modern gasoline engine has an oxygen or air-fuel sensor before the catalytic converter and a second oxygen sensor after it. The front one tells the computer how to meter fuel. The rear one — sensor 2 — reports what comes out of the converter, so the computer can judge whether the converter still works. P0138 — "O2 Sensor Circuit High Voltage, Bank 1 Sensor 2," which Toyota writes as "Oxygen Sensor Circuit High Voltage [Bank 1 Sensor 2]," Nissan "HO2S2 (B1)," and Chrysler's Ram diesel bulletins "1/2 02 Sensor Shorted To Voltage" — means the signal from that rear sensor stayed higher than the computer allows.

That is a statement about a voltage, not a verdict on a part. We found 84 manufacturer documents that deal with the downstream oxygen sensor — service bulletins, emission recalls, warranty extensions, campaign letters, engineering requests and tech tips from GM, Ford, Toyota, Lexus, Nissan, Infiniti, Acura, Mazda, Subaru, Jeep, Ram, Land Rover, BMW, Audi and Porsche — and downloaded and read 82 of them in full, two of them byte-identical copies; two have no published PDF and are known only from NHTSA's summaries. 38 name P0138 in their own text. The causes they document fall into five groups: water and wiring, software, the upstream sensor, the rear sensor itself, and the engine or exhaust behind the reading.

The federal complaint file is thin on this code, and that is a finding in itself. Of 780,071 complaints received by NHTSA from 2015 to September 2026, 72 matched a downstream-sensor code or phrase, and only 11 name P0138. We went through all 72 and sorted each one twice, independently, by the same written rules; the two sortings matched on 59 of 72 (82%). In 10 of the 72, the sensor itself was blamed; 8 pointed to the engine computer. And 22 of the 72 are Chevrolet or GMC vehicles.

By the end you will know which of five routes your P0138 is on, what each costs in 2026, which recall, extension or bulletin applies to your VIN, and the cheap cause that gets skipped when a new sensor is quoted first.

Claims marked 🟢 rest on a document listed in the Sources section, read in full unless marked otherwise. Claims marked ⚪ are shop-price estimates carried from our repair guides. Owner-reported figures are what people told NHTSA, published with ODI numbers, and every complaint count here comes from all 72 complaints checked individually, sorted and published.

I built PulsCar after a $380 "full diagnostic" in Phuket in 2025 led to roughly $6,000 over six months across several shops (the story is on the About page). P0138 is a code where the part named in the code is not always the part at fault — one manufacturer tells its technicians outright not to replace it — so every claim below that rests on a document names it.

What P0138 says: a rear sensor reading high

Two sensors, two jobs. The sensor after the converter — downstream, rear, post-catalyst or "catalyst monitor," depending on who is writing — watches the result. The front sensor does the main fuel control; on many engines the rear one also makes small, slow corrections — the post-catalyst fuel trim behind codes P2096 and P2097 — but its main job, as our O2 sensor guide puts it, is to watch the converter. That is why a P0138 on its own usually leaves the car driving normally, and why the code is easy to live with and easy to misread.

What "high" means. An oxygen sensor of this kind produces a higher voltage when the exhaust is rich — little oxygen left — and a lower one when it is lean. 🟢 Toyota's T-SB-0151-18 turns that into a test a technician can run: with the computer forcing the mixture 25% rich, the rear sensor must read above 0.55 volts, and with it forced 12.5% lean, below 0.4 volts. A sensor that passes is working. P0138 means the signal stayed high when it should have come down — either because the exhaust really was rich after the converter, or because something other than oxygen was pushing the voltage up.

Why the circuit matters as much as the sensor. A heated oxygen sensor has two circuits in one plug: a heater that warms it and a signal wire that reports. 🟢 GM's bulletin 10-06-04-010A, for the 2003-2010 Pontiac Vibe with all-wheel drive, describes what happens when water reaches that plug: "This creates a path for current to flow from the heater circuit to the HO2S signal circuit." The computer sees a high voltage and sets P0138 — with nothing wrong with the sensor at all. The Ram diesel bulletins name P0138 for what it is: the 1/2 sensor "Shorted To Voltage."

Bank and position. Bank 1 holds cylinder 1; sensor 2 is downstream; P0158 is P0138 on bank 2. Chrysler writes the positions as "1/2" and "2/2", and Acura calls P0138 on the 2014 MDX the "Rear Secondary Heated Oxygen Sensor Circuit High Voltage" — rear cylinder bank, downstream sensor.

First: which downstream code do you have?

P0138 belongs to a family, and the family members point in different directions. Reading the right one is the first check, and it is free.

High and low often point different ways. In several documents the low code P0137 comes from things that make the sensor see too much oxygen or lose its signal: 🟢 oil from a leaking valve cover coating the sensor (FCA's tech tip 9003790 for 3.6-liter V6 engines, and STAR case S1918000008), 🟢 an aftermarket alarm's electro-tap connectors cutting into the rear harness (Mazda), and 🟢 low battery voltage on 2016 Ford hybrids (SSM 46076). The high code P0138 comes from water and short circuits, from a mixture that really is rich, and from software. Many wiring and sensor-cracking bulletins list both codes, so the direction narrows the search rather than settling it — but a shop that treats the two as interchangeable — "O2 sensor code, O2 sensor" — has skipped the most useful piece of information on the scan.

Look at the neighbors. P0138 with P0158 means both rear sensors are reading high at once, and two sensors rarely fail on the same day; think about what they share. P0138 with P0036 — the rear sensor's heater — sent 🟢 Ford to wet connectors on the 2022 Maverick and 2021-2022 Bronco Sport. P0138 with P0607, a computer processor code, sent 🟢 GM to the Vibe's wet plug. And P0138 with the rich fuel-trim code P0172 points at the mixture, not the sensor: our P0172 guide covers that side.

Heater codes are a different job. P0036, P0037, P0038 and P0141 concern the rear sensor's heater; our P0135 guide covers that family.

The maker atlas: who wrote about P0138, and what they blamed

The routes below quote each document; this is the map.

GM: connectors, chafe, water, software and recalls. 🟢 PIP5123 (three versions) for early-2013 Silverado and Sierra trucks, PIT5336 for 2013-2015 Enclave, Traverse and Acadia 3.6-liter models, 10-06-04-010A for the Pontiac Vibe, 19-NA-077 for 2018-2019 Equinox and Terrain, emission recalls 14096, N192268180 and N192268181, and special coverage N192287100.

Ford: water in the plug. 🟢 TSB 20-2380 and 21-2090 for the 2020 F-Super Duty, SSM 50973 and 50993 for the 2022 Maverick, SSM 51048 for the 2021-2022 Bronco Sport, and TSB 16-0144 for 2014-2015 Explorer and Taurus V6 models, which we know only from NHTSA's summary (cause not stated there). Ford's other six documents here concern P0137.

Toyota and Lexus: look upstream first. 🟢 T-SB-0001-10 for listed 2005-2010 V6 models and T-SB-0151-18 for the 2.5-liter RAV4 and Camry blame the upstream air-fuel sensor; a Lexus campaign on the 2010 RX 450h replaced the No. 2 sensor and its bracket; Toyota's Sienna campaign G0G lists P0138 and replaces a sensor free.

Nissan and Infiniti: software, and a 20-year extension. 🟢 NTB18-062 (three versions) and ITB18-031 for 2018 V8 trucks and the QX80; NTB15-051, ITB15-022 and NTB15-053a for 2015 models; NTB22-006 and NTB22-007 with their campaign bulletins for 2013-2016 Sentra and Altima models; and campaigns R1620 and R1621.

Subaru, Acura and Mazda. 🟢 Subaru's 09-115-24R supplies a redesigned rear sensor for P0138 and its siblings across six models; Acura's BER14010C collected parts from 2014 MDX vehicles with P0138 and gives no fix; Mazda's two tips concern P0137.

Jeep and Ram. 🟢 Eight Jeep bulletins, from 18-071-22 and 18-042-22 to their 2026 successors 18-006-26 and 18-007-26, fix P0138 on 2021-2022 Gladiator 3.6-liter trucks with software. Ram's 25-004-12, 25-003-13 and 25-002-14 for 2010-2012 6.7-liter diesels fix connectors and terminals.

Land Rover, BMW, Audi and Porsche. 🟢 Land Rover's SSM72448 lists P0138 among dozens of oxygen-sensor codes and admits "The root cause has yet to be identified and is currently under investigation." BMW (SI B01 03 19) and Audi (AWA-20-05) extend rear-sensor coverage to 10 years or 120,000 miles; Porsche's WLB7, WKC7 and WKE8 replace contaminated sensors or fix software. Only Land Rover's document, of these four makers, uses the code P0138.

The 10-minute P0138 check

1. Read every code, not just one (2 minutes, free at many parts stores). Write down each code, pending and history included, and the freeze frame. P0138 alone, with P0158, with P0036, with P0172 or with P0607 are five different conversations.

2. Note what the car is doing (1 minute). Steady or flashing light, fuel smell, misfire, loss of power? P0138 on its own rarely changes how a car drives; if yours drives badly, something else is going on.

3. Think about water (2 minutes). Has the car been through deep water, a flooded road, a car wash with an underbody spray, or a heavy rain before the light came on? Is there water under the carpet near the center of the dash? (On the Vibe, GM warns the carpet may stay dry.) Water is behind six of the documents that name P0138, and on at least one car the plug sits inside the cabin.

4. Run the VIN (3 minutes). At nhtsa.gov/recalls, and with a dealer for emission recalls, extensions and campaigns, which do not all appear as safety recalls.

5. Check your model against the atlas above (2 minutes). If your car is named in a bulletin, write the number down for the dealer.

Six P0138 situations, and which is yours

"P0138 on its own, car drives normally, light steady." The common case. Start with the connector and the VIN before the sensor. → Routes 1, 2 and 4.

"P0138 and P0158 together." Both rear sensors reading high at once. Look at what they share — a harness, a connector, a fuse, the software, or a rich mixture. → Routes 1, 2 and 3.

"P0138 with P0036 or P0141." Signal and heater on the same sensor, at once. Ford's Maverick and Bronco Sport notes (with P0036) and its 2020 Super Duty bulletin (with P0141 among the codes) point at water in a connector. → Route 1.

"P0138 on a Toyota with an air-fuel sensor code, or with no other code." Toyota tells its technicians to read the freeze frame first and, when the air-fuel and rear-sensor values match its window, to replace the air-fuel sensor and not the rear sensor. → Route 3.

"P0138 with P0172, a fuel smell or black smoke." The mixture may really be rich. → Routes 3 and 5.

"The light flashes, the engine knocks or loses power." The oxygen sensor code is a passenger. → Route 5.

The P0138 price ladder, from the bottom up

The first four lines cost little or nothing and decide whether any of the rest is a candidate. The sensor and converter figures come from our O2 sensor guide, the reprogram from our P0441 guide and the exhaust leak from our catalytic converter guide, so the numbers match across pages. The Toyota line is a Sienna owner's "more than $1000" and a Venza bill of $2,168.77 that also paid for camshaft gears.

What does it cost to fix P0138 in 2026, and when is a new oxygen sensor the wrong purchase? Our repair guides carry a downstream sensor at $150 to $300 installed, an upstream air-fuel sensor at $200 to $500, an engine computer reprogram outside warranty at $0 to $200 and an exhaust leak at $100 to $400. In PulsCar's reading of the 72 federal complaints about the downstream sensor, owners name $30 to $500 for oxygen-sensor work across four figures, and $1,000 to $2,169 for two Toyota bills that included the rear and air-fuel sensors, the higher with camshaft gears. A new rear sensor alone is the wrong purchase in three documented cases: on Toyotas whose freeze frame matches the bulletin; on cars with a named software fix, such as the 2018 Armada; and when water has reached the connector, where Ford replaces the harness with the sensor.

The five P0138 routes, cheapest first

Route 1 · Water, wiring and connectors — $0 and up

What it is. The sensor may be fine; the signal reaching the computer is not. Water in a plug, a bent terminal, a chafed harness or a corroded inline connector can push the signal wire toward a voltage it should never carry, and the computer calls it P0138.

What the documents say.

🟢 GM 10-06-04-010A, 2003-2010 Pontiac Vibe with all-wheel drive. The owner may see the check engine light or the stability-control light, sometimes with driveability complaints in hot weather with the air conditioning on, and a technician "may find DTC P0607 and/or P0138." The cause: "The evaporator drain may be restricted, causing water to drip from the evaporator case onto the post HO2S connector." The connector sits near the center of the instrument panel, under the carpet, and the bulletin warns: "There may not be carpet wet from this leak, only the connector." The repair is not a sensor. "Clean the connector with compressed air and apply a small amount of silicone grease to the connector," then clear the drain until water flows — 0.5 hours. The bulletin names only the Vibe; it does not mention the Toyota Matrix or Corolla.

🟢 Ford TSB 20-2380 and 21-2090, 2020 F-Super Duty 6.2- and 7.3-liter, built on or before 5 June 2020. P0132, P0135, P0138, P0141, P0152, P0155, P0158 and P0161 from "water intrusion into heated oxygen sensor (HO2S) connectors C141, C142, C1571 and/or C1572 on the transmission harness where it connects to the engine exhaust sensors." If the pins of the sensor the code names show moisture or corrosion, the affected sensors are replaced along with the 7C078 transmission harness, 1.0 to 1.5 hours; if they are dry, the bulletin does not apply. And one warning that runs against shop habit: "CAUTION: Do not use dielectric grease in the sensor or harness connectors as the sensors breathe through the wiring strands for proper sensor venting." The 2021 version, 21-2090, limits it to trucks with the 10R140 transmission and updates the parts list; we read it from the page images.

🟢 Ford SSM 50973 and 50993, 2022 Maverick 2.0-liter built on or before 4 April 2022. A charging-system warning, and possibly "a malfunction indicator lamp (MIL) with DTCs P0036:00 and/or P0138:00 stored in the PCM," from "water intrusion in the connector C1050." The fix is a pigtail kit; the first note says to "apply dielectric grease," the second "apply electrical grease." 🟢 SSM 51048 says the same for 2021-2022 Bronco Sport 2.0-liter models: P0036 and/or P0138 from "corrosion or water intrusion in connector C201 and/or C1570," repaired with a pigtail kit and Motorcraft electrical grease. So Ford forbids grease in the Super Duty's sensor connectors and prescribes it in these inline connectors, without reconciling the two — worth asking about by name.

🟢 GM PIP5123, 5123A and 5123B, 2013 Silverado and Sierra with 4.8-, 5.3- and 6.0-liter engines built at Fort Wayne or Silao in January or February 2013. P2270 together with any of P0140, P0139, P0138, P0137, P013A or P013B, and one instruction: "Check connector X131 for a bent terminal F as indicated in the attached PIC and repair per SI procedures." Later versions add the Silao plant and January builds, and point to GM's standard wiring-repair warranty bulletin.

🟢 GM PIT5336, 2013-2015 Buick Enclave, Chevrolet Traverse and GMC Acadia 3.6-liter. About thirty codes including P0138 and P0158; the technician may also find "that the Emission 1 Fuse is open." The inspection point: "the engine harness to the Bank 1 O2 sensor for chafing on the A/C hose coupling/connection at the back side of the engine." The repair insulates the damaged circuits and ties the harness away from the coupling.

🟢 Ram 25-004-12, 25-003-13 and 25-002-14, 2010-2012 Ram 2500 and 3500 with the Cummins 6.7-liter diesel. These list P0138 as "1/2 02 Sensor Shorted To Voltage" among 36 oxygen-sensor codes and replace connectors, terminals and finally an overlay harness — some 2011 harnesses had tin terminals where gold ones belonged. 25-002-14 puts it first: "Do NOT replace any parts prior to performing this bulletin and the Verifications test below." The diesel sensor system differs from a gasoline car's, but the principle carries.

What the owners report. Six of the 72 complaints were sorted as wiring, connector, harness, fuse or contamination. A 2007 Tahoe owner found the bank 1 sensor 2 connection had come loose from its frame mount: "IT CAUSED THE WIRING TO GET SLACK AND WRAP AROUND THE DRIVE SHAFT." A 2016 Legacy owner's shop found oil in the engine computer and then in the oxygen sensors: "Oil seems to be traveling through the wiring harness from the engine into the electrical components." A 2020 Ascent's dealer wrote "Found pressure sensor leaking oil through all harnesses" and quoted about $8,933 for three harnesses; that complaint names P0137, not P0138. Rodents appear twice, in a 2019 Forte and a 2021 Insight, whose owner disputes the finding. Outside that category, a 2022 Maverick owner writes, "I have replaced the downstream oxygen sensor twice"; the complaint gives neither the code nor the build date, so whether Ford's connector notes applied cannot be told from it.

The cheap cause that gets skipped. The plug. Unplugging the connector the code points to and looking at the pins costs nothing and takes minutes on most cars. Green or white corrosion, water droplets or a bent pin changes the whole repair.

Fix it yourself — the honest split. Finding the connector, unplugging it with the engine off, looking for moisture and corrosion, and checking the harness for rub marks where it passes the exhaust, the transmission or the air-conditioning lines are owner jobs. So is clearing a blocked air-conditioning drain on a car where the plug sits inside the cabin. Replacing a harness or crimping new terminals is shop work.

At the shop — say this: "Before replacing the sensor, can you unplug the connector for bank 1 sensor 2 and check the pins for water or corrosion, and check the harness for chafing? Is there a bulletin for my VIN about water in the oxygen sensor connectors, and does it say to use grease or not?"

Route 2 · Software, and a computer that reports the wrong sensor — $0 to $200

What it is. The computer's own logic sets the code, sets it on the wrong bank, or misses a fault it should have caught. The fix is a reprogram, sometimes with a sensor, and in the cleanest cases nothing but a reprogram.

What the documents say.

🟢 Nissan NTB18-062, 062a and 062b, 2018 Armada, Titan and Titan XD with the VK56VD V8, and Infiniti ITB18-031, 2018 QX80. P0138 and/or P0158 with the check engine light on, and a sentence that defines who the bulletin is for: "There are no drivability issues." If the engine computer's part number is on the list, "If this bulletin applies, reprogram the ECM." If it is not, "If there is not a match, reprogramming does not apply. Refer to ASIST for further diagnosis." No sensor is replaced. The 2019 revision added the Titan and Titan XD.

🟢 Jeep 18-071-22 and 18-042-22, and their 2026 successors 18-007-26 and 18-006-26, 2022 and 2021 Gladiator with the 3.6-liter V6. Each lists P0138 ("O2 Sensor 1/2 Circuit High") among the codes a PCM reprogram fixes, with P0158 and heater codes; later revisions add a camshaft and crankshaft relearn and misfire codes, and the 2026 versions name the cause as PCM software. They are repaired under the truck's own warranty.

🟢 Nissan NTB15-051 and Infiniti ITB15-022, 2015 Altima V6, Pathfinder, Murano and QX60, and NTB15-053a, 2015 Juke. Rear-sensor codes including P0137, P0138 and P0139 are fixed by a reprogram and the rear sensor — on the V6 models only the one the code names, in NTB15-051's words: "Do NOT replace both rear O2 sensors if the DTC(s) is for one rear O2 sensor."

🟢 Porsche WKE8, 2018-2019 911 GT3. The clearest reason not to trust a bank number blindly: "if there is a fault in the oxygen sensor on one cylinder bank, the fault code for the oxygen sensor on the other cylinder bank will be stored in the fault memory." And the consequence: "As a result, replacing the relevant oxygen sensor will not correct the stored fault and this may result in repeat repairs." The fix is software.

🟢 GM 19-NA-077, 2018-2019 Equinox and Terrain. A software update for stop/start behavior and hesitation from a stop, on cars that may also show P0132 or P0138 — with GM's own warning that it is not the fix for the codes: "This software enhancement will not correct the P0132 or P0138." The instruction: "Use SI to diagnose and repair these DTCs first and then update to the current level of software."

🟢 Nissan campaigns R1620 and R1621 fixed the opposite problem on 2015-2016 models: software under which "the rear oxygen (O2) sensor signal could fail, resulting in the rear oxygen (O2) sensor OBD monitor being disabled without the customer being alerted."

What the owners report. A 2018 Armada owner's complaint is two lines of scan data — "CODE FOUND :P0138 (02)" — at 27,000 miles, the year Nissan's reprogram bulletin covers. A 2017 Malibu owner's first dealer visit ended with the software updated "AND IT WAS NOTED GM WOULD MONITOR THE CONCERN"; the complaint lists P0138 among many other codes. A 2020 Cherokee owner had the sensor replaced for P0139 and then the engine computer, and the code returned both times. In total, 8 of the 72 complaints were sorted as pointing to the engine computer — a software update, a replacement done or recommended, or a P0607 module code.

The cheap cause that gets skipped. The calibration. A dealer can compare the software on your car with the latest in minutes, and on the 2018 Nissan V8 trucks the whole repair is the update.

Fix it yourself — what to write down. The engine computer part number or calibration ID if your reader shows it; whether the car drives normally; whether P0158 set with P0138. A code that sets with "no drivability issues" on a truck named in a bulletin is a software conversation, not a parts one.

At the shop — say this: "Is there a software update or bulletin for P0138 on my VIN, and is my engine computer on the latest calibration? The car drives normally — can we reprogram before replacing a sensor?"

Route 3 · The upstream sensor behind a downstream code — $200 to $500

What it is. The rear sensor reports honestly on a mixture the front sensor is getting wrong. If the upstream air-fuel sensor reads lean when the engine is really running rich, the computer adds fuel, the exhaust after the converter comes out rich, and the rear sensor's voltage stays high. Replace the rear sensor and the code comes back.

What the documents say.

🟢 Toyota T-SB-0001-10 Rev1, certain 2005-2010 Avalon, Camry, Highlander, RAV4, Sienna and Venza models. The applicability is set by VIN codes, and most of the repair-manual references inside are for the 2GR-FE V6. The bulletin covers "a MIL "ON" Diagnostic Trouble Code (DTC) P0138 and/or P0158 (Oxygen Sensor Circuit High Voltage Bank 1 or 2)," and says: "This condition may be caused by incorrect Air Fuel Ratio Sensor response." The test is in the freeze frame: if the upstream sensor reads between 3.35 and 3.70 volts while the rear sensor reads between 0.70 and 0.90 volts, the instruction is "Replace the appropriate bank Air Fuel Ratio Sensor ONLY (NOT O2 Sensor) to correct the P0138 or P0158 DTC condition." The freeze-frame test applies when P0138 or P0158 is the only code; with the air-fuel codes P2195 or P2197 alongside, the bulletin goes straight to checking and replacing the air-fuel sensor. If the freeze frame does not match, the technician inspects the rear sensor's circuit before anything else. Only in a separate branch, for the computer code P0606, does the rear sensor get replaced — when it reads above 1.10 volts or 300 ohms warm and the harness is good.

🟢 Toyota T-SB-0151-18, 2009-2012 RAV4 and 2010-2011 Camry with the 2AR-FE 2.5-liter, not PZEV models. "MIL ON DTC P0138 - A/F Sensor Stuck Lean" is its title. The cause: "This condition may be caused by incorrect air fuel ratio sensor response." Before anything is replaced, the technician checks for exhaust leaks — if there is one, the bulletin does not apply — and runs the active test described earlier to prove the rear sensor responds; if the car already has the latest calibration, the bulletin does not apply either. Then the engine computer is reprogrammed and the upstream air-fuel sensor replaced, 1.0 hours on the RAV4 and 1.1 on the Camry. "The ECM (PCM) should NOT be replaced as part of the Repair Procedure." The bulletin was covered by Toyota's federal emission warranty of 96 months or 80,000 miles from the in-service date, which has run out for practically every car it names. "This Service Bulletin is NOT applicable to Partial Zero Emission Vehicles (PZEV)" — check the emission label under the hood.

What the owners report. A 2010 Venza owner's invoice for P0138: the technician "found internal electrical failures in rear O2 sensor and AF sensor" and replaced both, with camshaft gears, for $2,168.77. A 2011 Sienna owner lists "A/F SENSOR BANK 1 SENSOR" and "O2 SENSOR BANK 1 SENSOR 2" together, at more than $1,000. A 2011 RAV4 owner: "I HAD THE SENSOR REPLACED TWICE IN LESS THAN 2 WEEKS TO CONFIRM IT WASN'T A FAULTY SENSOR," and the lights came back within 24 hours; the complaint does not give the code or the engine, so whether T-SB-0151-18 applied cannot be told from it. Only the Venza's code is on the record as P0138.

Beyond Toyota. A rear sensor that reads rich may be reporting a rich engine: P0138 with P0172 belongs in our P0172 guide first, because fuel in the oil or a leaking injector can keep the rear sensor high.

The cheap cause that gets skipped. The freeze frame. It records the upstream and downstream voltages at the moment the code set, and on the Toyotas above it decides which sensor to buy. It is on the scan tool already.

At the shop — say this: "Can you print the freeze frame for P0138 and tell me what the air-fuel sensor and the rear oxygen sensor read? Is my car covered by T-SB-0001-10 or T-SB-0151-18, and does the freeze frame match?"

Route 4 · The rear sensor itself — $150 to $500

What it is. Sometimes it is the sensor. The documents describe sensors that crack inside, sensors whose reference air is contaminated, sensors that fracture from condensation, and harnesses that fail at the sensor.

What the documents say.

🟢 Subaru 09-115-24, revised as 09-115-24R in April 2025. For "P0138: O2 Sensor Circuit High Voltage Bank 1 Sensor 2" and its siblings P0137, P0140 and P0141, Subaru supplies a redesigned rear sensor: "The new sensors have an optimized coating application to help prevent cracking within the internal surfaces reducing any possibility of short/open circuits." Two details matter at the parts counter: "The part numbers have not been changed. The new parts can be identified by the Denso part number engraved on the sensor." Rear sensor labor is 1.0 hour.

🟢 Porsche WLB7, 2017-2018 Panamera 2.9- and 3.0-liter, and 2018 Panamera 4 E-Hybrid. The downstream sensors had a silicone protective tube on the lead: "Gas coming from this protective tube can contaminate the measuring electrode on the reference air side of the oxygen sensor over the service life of the vehicles." The campaign replaces them with sensors that have an acrylic tube.

🟢 GM emission recall 14096, four versions, 2011-2013 Chevrolet Express and GMC Savana cutaways with gasoline or LPG engines. "The Oxygen (O2) Sensor may fracture if condensation from the catalytic converter enters the O2 sensor and causes a sudden temperature change." Dealers inspect both rear sensors, replace any of the first design, and reprogram the engine computer, "at no charge to customers, regardless of mileage, age of vehicle, or ownership, from this time forward." The first version listed both bank 2 sensors; four days later GM revised it "to clarify the involved sensors" — one rear sensor on each bank.

🟢 A Lexus campaign for certain 2010 RX 450h hybrids replaced the No. 2 sensor and a harness bracket after vibration damage; it expired on 31 May 2018.

🟢 BMW and Audi extended rear-sensor coverage on 2014 N63T V8 and 2016-2018 4.0-liter TFSI models; the coverage file below has their status. 🟢 Land Rover's SSM72448 describes 2016 model-year faults where "the fault is determined to be with the sensor" and asks dealers to keep the removed sensor for collection.

What the owners report. Ten of the 72 complaints were sorted as the sensor itself, among them a 2007 Town & Country owner — "I HAVE HAD TO REPLACE MY O2 SENSORS WITH FACTORY OEM REPLACEMENTS 3 TIMES NOW" — and a 2015 BMW 535d diesel whose owner replaced both post-catalyst sensors and had them fail again. Other owners describe the same part in other categories. A 2017 Altima's rear sensor failed at 29,900 miles and cost almost $400: "This is a part that should easily last for 150,000 miles." A 2011 Traverse owner was quoted $250 by the dealer for the rear sensor and had it fitted at home for $30 — and the light stayed on, because the dealer then said the front sensor had been the bad one.

The cheap cause that gets skipped. The part number and the design. On a Subaru, a counter can hand over an old-design sensor with the same number; on a Nissan, only the sensor for the bank in the code; on a Porsche or a GM cutaway, a campaign may already cover it.

Fix it yourself — realistic on an accessible sensor. Our O2 sensor guide puts the part at $30 to $80 aftermarket or $80 to $300 OEM, and the labor saving at $50 to $175. If the sensor is seized or buried, stop: a snapped sensor is a bigger repair.

At the shop — say this: "Before I pay for a rear sensor, can you show me the live data that proves it is the sensor and not the wiring or the upstream sensor? Which bank is it, and is there a new-design part or a campaign for my VIN?"

Route 5 · The engine and exhaust behind the reading — $100 to $11,535

What it is. The rear sensor sits after the converter and sees what the engine and converter send it. Oil burning in the cylinders, coolant in the combustion chamber, a converter that has failed and a rich-running engine all reach it first. Here the oxygen-sensor code is the smallest item on the bill.

What the documents say.

🟢 GM special coverage N192287100 and its revision, 2012-2013 Equinox and Terrain 2.4-liter. These vehicles "may exhibit catalytic converter and/or oxygen sensor performance degradation, without illumination of the malfunction indicator lamp (check engine light), as a result of excessive engine oil consumption." If an oil consumption test confirms it — or the engine was already repaired for it — GM replaces the converters and the oxygen sensors. The coverage runs "for a period of 10 years or 150,000 miles" from the in-service date. 🟢 GM's companion emission recalls N192268180 and N192268181 replace the rear sensor, or both converters and both sensors, on 2012-2013 Equinox and Terrain 2.4-liter models, and say plainly that "The malfunction indicator lamp (check engine light) will not illuminate."

The direction of an exhaust leak. A leak between the front and rear sensors lets air reach the rear sensor, which then reads lean — the P0137 direction. A leak ahead of the front sensor can do the opposite: the front sensor reads lean, the computer adds fuel, and the rear sensor reads rich. Our catalytic converter guide prices the repair at $100 to $400. 🟢 Toyota's T-SB-0151-18 simply says the bulletin does not apply if there is any exhaust leak.

What the owners report. Seven complaints were sorted as the exhaust or the engine behind the reading, and six more as a major engine failure with an oxygen-sensor code among others. A 2012 Equinox owner replaced both converters, the upstream sensor and the downstream sensor, and was still adding oil: a dealer technician told them GM had a bulletin, but the car was "OVER THE 140,000 MILES LIMIT" (GM's later special coverage N192287100 runs to 150,000 miles). A 2016 Terrain owner replaced both sensors and expects the converter to be next, "because the excessive oil consumption has clogged it almost completely." A 2012 Equinox's open recall for the rear sensor "led to the discovery of a cracked exhaust manifold." And at the top of the ladder: a 2020 Explorer estimate of $15,863.61, including $11,534.51 for an engine the shop said had failed internally — a supervisor mentioned possible coolant in two cylinders — and a line for the bank 1 sensor 2 heater circuit; and a 2019 Sportage quoted $6,700 for an engine with melted pistons.

The cheap cause that gets skipped. The dipstick and the oil-change receipts. GM's coverage treats less than 2,000 miles per quart as excessive; an engine burning oil like that is coating the converter and the sensors behind it, and no sensor fixes that.

At the shop — say this: "Before a sensor or a converter, is the engine using oil or coolant? Can you check for an exhaust leak ahead of the rear sensor, and tell me in writing whether the converter is being damaged by the engine?"

The coverage file: which programs are still running in September 2026

Most P0138 documents are ordinary bulletins: they tell a dealer how to fix the car and leave the bill to whatever warranty the car still has. A handful go further. The status column below is our own calculation from the terms printed in each document, as of September 2026 — not a statement by the manufacturer — so check the exact VIN.

The Nissan extension, read closely. 🟢 NTB22-007 says: "Nissan is extending the emission warranty on the Rear Oxygen Sensor in the case of a short-to-ground condition to 20 Years or 200,000 miles, whichever occurs first." A clause brings P0138 in: "Only if one of the DTCs in Table A are first found after a short-to-ground repair, the corresponding issue(s) will also be corrected according to the service manual at no charge for parts and labor as part of this warranty extension." The dealer announcement puts the eligible production windows at 7 March 2012 to 31 August 2016 for the Altima 2.5-liter and 4 June 2012 to 22 September 2016 for the 50-state Sentra. Neither says when the 20 years start; even from the earliest production date they run to 2032. The check itself is simple: 🟢 engine off and cooled for 15 minutes, then "If the HO2S2 (B1) voltage is less than 0.13 V, the rear O2 sensor has a short to ground condition and the inspection result is NG."

Two owners who met the program. A 2017 Altima owner paid almost $400 for a rear sensor and asked why 2017 was left out when the "same part (even the same part number)" was covered; eligibility ends with cars built by 31 August 2016. A 2013 Sentra owner at about 173,000 miles found the P1A25 campaign, and "the dealer refused to service the vehicle due to the check engine warning light and other issues that the dealer claimed would affect the recall repair." That matches the bulletin's own rule for the California cars: "DTCs stored prior to performing this ECM reprogram are not covered under this campaign." P1A25 applies only to California-specification SULEV Sentras; a 50-state Sentra falls under the extension instead, and the owner's complaint does not say which theirs was.

Status notes. The GM special coverage and the Audi and BMW extensions run 10 years from the in-service date, so each is still open only for cars that entered service after September 2016 and are under the mileage cap — practically no 2012-2013 Equinox or Terrain, mostly 2017-2018 Audis, and almost no BMWs, which were built from March 2013 to June 2014. The GM emission recalls and Nissan's P1A25 print no end date; GM's windows for reimbursing earlier repairs closed on 30 June 2015 (14096) and 31 October 2021 (N192268180 and N192268181).

What 72 downstream-sensor complaints show

We searched every complaint in NHTSA's file for a downstream-sensor code — P0136 to P0140, P013A to P013F, P0157, P0158, P2270 or P2271, with or without a two-character suffix — and for "downstream" or "rear" followed by O2, 02 or oxygen, "post-cat" or "post catalyst," "bank 1 sensor 2" or "B1S2," counting one "P136" written for P0136 and codes written with a letter O, such as "PO138": 72 complaints, two of which turned out not to concern an oxygen sensor. Every one of the 72 was sorted twice, independently, by the same written rules. The two sortings matched on the category for 59 of 72 (82%); the 13 that differed were settled by the written rules, and the published file carries both original sortings and the final one for every complaint.

The 11 that name P0138. A 2007 Silverado 1500, a 2012 Equinox, three 2017 Malibus, a 2002 Grand Cherokee, a 2014 Sorento, a 2018 Armada, a 2009 Corolla, a 2009 Corolla Matrix and a 2010 Venza. Four of the eleven were sorted as pointing to the engine computer, five as the code with no diagnosis, one as a recall and one as the sensor. The two 2009 Toyotas both set P0138 with P0607, a computer processor code; one owner cites a Toyota bulletin for P0607 that is not among the documents we read.

By maker and year. Chevrolet 20 and GMC 2, Kia 10, Toyota 7; the Malibu leads the models with 8. The busiest years — 10 each in 2019, 2020 and 2026 so far — have no model above three, nothing like the CR-V pile-up in our P0172 file.

What this file cannot tell you. Seventy-two complaints across nineteen makers is too few to rank cars. Owners write to NHTSA when something frightening happens, so many of these are about a stall, an engine or a dealer dispute, with the oxygen-sensor code along for the ride. The documents, not the complaints, carry the weight in this guide.

What do the 72 federal complaints about the downstream sensor show about P0138, and how often is the sensor itself the answer? In PulsCar's reading of every complaint that names a downstream-sensor code or describes the rear sensor, only 11 name P0138; every complaint was sorted twice, independently, by the same written rules, and the two sortings matched on 59 of 72 (82%). The sensor itself was blamed in 10 of the 72; the engine computer in 8; the exhaust or the engine behind the reading in 7; wiring, connectors or contamination in 6; and a recall or coverage program was central in 6. In 25, the code is named with no diagnosis at all, often among many codes for a bigger problem. Of those 11, four were sorted as engine computer under this guide's written rules.

What owners say they paid

Twenty dollar figures appear in the 72 complaints, and the full list with ODI numbers is published. The ones that bear on this code, grouped by what they paid for:

The sensor alone. $30 for a rear sensor a 2011 Traverse owner's family fitted at home, against the dealer's $250 quote. Almost $400 for a 2017 Altima's rear sensor. $500 plus labor for a 2002 Grand Cherokee's full set of oxygen sensors, the second set after new converters; the same shop then quoted $450 plus labor for the engine computer.

Two sensors, front and rear. More than $1,000 for a 2011 Sienna's air-fuel sensor and bank 1 sensor 2. $2,168.77 for a 2010 Venza's rear and air-fuel sensors together with camshaft gears.

After a recall. $707.74 and $978.98, paid by a 2010 Ram 2500 owner after emission recall VE5 replaced the converter and manifold and P2271 appeared; the owner believes the harness or sensors were damaged during the recall work.

The code as a passenger. A 2018 Volt quoted over $4,000 for an EGR valve and cooler behind a blown fuse; a 2020 Ascent quoted about $8,933 for three fluid-soaked harnesses; a 2019 Sportage quoted $6,700 for an engine; a 2020 Explorer estimate of $15,863.61. None of these was mainly an oxygen-sensor repair; the Explorer estimate carries one oxygen-sensor line alongside an engine replacement.

Why P0138 comes back after a new sensor

Because the sensor was fine. Every route except Route 4 leaves the cause in place when the sensor is swapped — the upstream sensor, the wet plug, the software, the engine.

Because the wrong sensor was replaced. P0138 is bank 1; P0158 is bank 2. On a V6 or V8 the two rear sensors look alike and sit on opposite sides, and a new sensor on the wrong bank changes nothing.

Because the new part was an old design. 🟢 Subaru kept the same part numbers for its redesigned rear sensors; only the engraved Denso number tells them apart.

Owners report the pattern. A 2013 Malibu owner with the rear-trim code P2097 had both oxygen sensors replaced: "ABOUT 5 MILES AFTER I LEAVE THE SHOP CHECK ENGINE LIGHT CAME BACK ON AND IT'S THE SAME CODE." A 2013 Prius owner had the downstream sensor swapped for P0136, the wiring checked, and "BOTH MECHANICS HAVE DETERMINED THAT THE ISSUE IS WITH THE ECM."

Why does P0138 come back after a new oxygen sensor, and what should be checked before a second one? In PulsCar's reading of 82 manufacturer documents, the part named in the code is not always the part at fault. Toyota's T-SB-0001-10 tells technicians to replace the upstream air-fuel sensor only, not the oxygen sensor, when the freeze frame shows the upstream sensor between 3.35 and 3.70 volts and the rear between 0.70 and 0.90. GM traces P0138 on the Pontiac Vibe to water on the connector and says its 2018-2019 Equinox software will not correct the code. Ford's 2020 Super Duty bulletin replaces a wet harness. On Nissan's 2018 V8 trucks whose engine computer is on the bulletin's list, the fix is only a reprogram. Before a second sensor, ask for the freeze frame, the connector checked for water and corrosion, the bank confirmed, and the software level compared with the latest.

Can you drive with P0138?

Usually, for now, if it drives normally. The rear sensor's job on most engines is to watch the converter, not to steer the fuel, so a car with P0138 alone and a steady light usually drives as it did yesterday. The Nissan V8 bulletins describe exactly that car: "There are no drivability issues."

What changes the answer. A flashing check engine light, a misfire, a loss of power, a smell of fuel, black smoke, or oil or coolant disappearing. Any of those means the engine is doing something the rear sensor is only reporting, and continued driving can damage the converter. P0138 with several other codes at once — especially on a car that has been through water — is an electrical problem that can grow.

What it costs you to wait. Mostly the inspection, and a light that is already on cannot warn you about the next fault.

Should you buy a used car with a P0138 history?

It depends on what fixed it. A receipt for a cleaned connector, a named software update or a campaign repair on a car the bulletin covers is a closed file. A rear sensor followed by the same code again is an open one, and so is P0138 alongside oil consumption.

Four checks before you buy. Look for "connector," "harness," "reprogram" or a bulletin number in the service history. Run the VIN for extensions, recalls and campaigns. Scan the car: a code cleared before a sale leaves the readiness monitors incomplete. And on any car with an oil-consumption history, check the oil level against the last change.

Clearing P0138 before an inspection

Clearing the code does not clear the cause, and inspectors look for it. Erasing the code resets the catalyst and oxygen-sensor monitors to "not ready," a car with incomplete monitors is typically turned away or failed, and if the cause is still there P0138 returns while they complete.

What works instead. Fix the cause first, then drive normally for several days before the test. On the Nissans covered by the extension, the dealer's repair ends with a drive pattern that runs until the oxygen-sensor and catalyst monitors read complete.

What a shop actually does with a P0138, checkpoint by checkpoint

Checkpoint 1: all codes and the freeze frame. Every code, pending and stored, and a printed freeze frame; what set with P0138 decides the route.

Checkpoint 2: live data. Upstream and downstream sensors side by side on a warm engine. A rear sensor that is stuck high while the upstream sensor switches normally points to the rear circuit; both reading rich points to the mixture. 🟢 Toyota's active test — force the mixture rich and lean and watch the rear sensor follow — proves whether the rear sensor works.

Checkpoint 3: the connector, the bulletins and the software. Unplug and inspect the pins and harness; look up bulletins, campaigns and extensions by VIN; compare the software with the latest.

Checkpoint 4: the engine. Oil consumption, coolant loss, misfire history and the fuel trims. A rear sensor reading rich on an engine with large negative fuel trims is telling the truth.

Checkpoint 5: the part. Only now a sensor — on the right bank, of the current design, with the old one kept for you to see.

Three P0138 traps that cost owners the most

Trap 1: the rear sensor on a Toyota whose freeze frame blames the front one. 🟢 Toyota's bulletin says replace the air-fuel sensor "ONLY (NOT O2 Sensor)" when the numbers match. A rear sensor bought first is money spent before the diagnosis. Lesson: freeze frame before parts.

Trap 2: a new sensor into a wet plug. On the Vibe, the Super Duty, the Maverick and the Bronco Sport, the water is in a connector on the sensor's circuit, and a new sensor still reports through the same wet connection. Lesson: unplug and look before you buy.

Trap 3: a program you do not know about. Nissan's 20-year extension, GM's emission recalls, Audi's 10-year rear-sensor coverage and Porsche's campaigns are easy to miss unless someone runs the VIN; in this file a 2017 Altima owner found the Nissan extension did not reach their car, and a 2013 Sentra owner was turned away from P1A25. Lesson: run the VIN, and ask by bulletin number.

Three P0138 quotes, decoded

These are composite scenarios, not individual customers: each combines details that come up repeatedly in owner reports and shop quotes; the dollar figures are drawn from the ranges on the price ladder above.

Scenario 1: 2014 Nissan Altima 2.5, 118,000 miles, check engine light with P0137, quote for a rear sensor. The owner asks the dealer to run the VIN for the rear-sensor warranty extension. The car is eligible; the voltage check reads below 0.13 volts and the short to ground is repaired under the extension. When P0138 turns up in the check that follows the repair, it is one of the codes the extension covers, because it was first found after that repair. Lesson: on a covered car, the order of the repairs decides who pays.

Scenario 2: 2008 Toyota Highlander V6, 142,000 miles, P0138 alone, quote for a rear sensor. The owner asks for the freeze frame. The upstream sensor reads inside Toyota's 3.35-3.70 volt window and the rear inside 0.70-0.90; the shop replaces the air-fuel sensor instead, at a price inside the $200-$500 range, and the code stays away. Lesson: the cheaper-looking part was the wrong one.

Scenario 3: 2020 F-250 6.2-liter, 64,000 miles, P0138 and P0158 after a wet winter, quote for two sensors. The owner asks about TSB 21-2090. The technician unplugs the connectors, finds corrosion, and replaces the harness and the affected sensors as the bulletin describes, 1.2 hours for two sensors — instead of two new sensors plugged into wet connectors. Lesson: two sensors failing on the same day is a wiring story.

The P0138 plan: 10 minutes, today, this week

10 minutes: read every code and the freeze frame; note whether the car drives normally; think about water.

Today: run the VIN at nhtsa.gov/recalls and ask a dealer about emission recalls, extensions and campaigns by name — NTB22-007 on a 2013-2016 Altima 2.5 or 50-state Sentra, N192268180 on a 2012-2013 Equinox or Terrain 2.4, NTB18-062b on a 2018 Armada or Titan, 18-006-26 or 18-007-26 on a 2021-2022 Gladiator 3.6, T-SB-0001-10 or T-SB-0151-18 on the Toyotas they list.

This week: have the connector and harness checked, the software compared, and the freeze frame read before any sensor is bought — and if a sensor is bought, the bank confirmed and the old one returned to you.

Sources and documents

Primary documents only. Where a claim in this guide is marked 🟢, it rests on one of the items below, and the reference is given so you can pull the document yourself. Claims marked ⚪ are shop-price estimates carried from PulsCar's repair guides — estimates, not measurements from a dataset we own.

1. GM 10-06-04-010A; PIP5123, PIP5123A and PIP5123B; PIT5336; 19-NA-077; PIP5529, PIP5529A and PIP5529B; PIP5644; emission recalls 14096, 14096A, 14096B and 14096C, 13292, N192268180 and N192268181; special coverage N192287100 and N192287100-01.
2. Ford TSB 15-0078 (image PDF, transcribed), 16-0128, 16-0144 (NHTSA summary only), 16-0163, 19-2005, 20-2380, and 21-2090 (text layer unreadable; read from page images); SSM 46076, 50230, 50973, 50993 and 51048.
3. Toyota and Lexus T-SB-0001-10 Rev1 and T-SB-0151-18; Lexus limited service campaign technical instructions for the 2010 RX 450h (three copies, one marked expired).
4. Nissan and Infiniti NTB15-051, ITB15-022, NTB15-053a, NTB18-062, NTB18-062a, NTB18-062b, ITB18-031, NTB22-006 and NTB22-007; campaign bulletins R1620, R1621, P1A25 and PC857/PM997 (the last two image PDFs, read from page images).
5. Subaru 09-115-24 and 09-115-24R, 11-169-16 and 11-169-16R, 11-160-15 (NHTSA summary only). Acura engineering request BER14010C and dealer message of January 2014. Mazda repair tips on P0137 (one image PDF).
6. Jeep and Ram 18-071-22 Rev. A, B and C; 18-042-22 Rev. C and D; 18-006-26 and Rev. A; 18-007-26 Rev. B; 25-004-12, 25-003-13 and 25-002-14; STAR case S1918000008 Rev. A, B and C; FCA tech tips on 3.6-liter oil contamination.
7. Land Rover SSM72448. BMW SI B01 03 19 and Revision 02. Audi AWA-20-05. Porsche WLB7 (three variants), WKC7 and WKE8.
8. PulsCar analysis of NHTSA public data, September 2026: all 72 complaints about the downstream sensor, each sorted twice; 20 owner-reported amounts; the document list with read status. Data and method.

For the neighboring codes and parts: P0135 and the heater codes, P0172, P0420, the O2 sensor cost guide, the catalytic converter cost guide and the code index. For the cars: Subaru Outback and Subaru Forester.

Methodology. The complaint set is every complaint in NHTSA's file that names a downstream-sensor code (P0136 to P0140, P013A to P013F, P0157, P0158, P2270, P2271, with or without a two-character suffix) or contains "downstream" or "rear" followed by O2, 02 or oxygen, "post-cat" or "post catalyst," "bank 1 sensor 2" or "B1S2" (plus one "P136" and codes written with a letter O); all 72 were read in full and sorted twice, independently, by the same written rules (the two sortings matched on 59 of 72, 82%), and the differences were settled by written rules. Every complaint quoted, and every dollar figure, was read in full. Coverage status is our computation from the printed terms, as of September 2026. Nothing here is a failure rate.

Prices verified July to September 2026 in the repair guides they are carried from, and reviewed quarterly. Technical documents verified September 2026: the 82 documents read in full in our published list — among them Toyota T-SB-0001-10 and T-SB-0151-18, Nissan NTB18-062b and NTB22-007, GM 10-06-04-010A and 19-NA-077, Ford 20-2380 and SSM 50973, and Subaru 09-115-24R.

Had a P0138 fixed by a cleaned connector, a software update, a campaign or the Nissan extension — or refused one? Email [email protected] with the vehicle, mileage and outcome.