P0137, P0136, P0139 and P0140 in 2026: The Sensor After the Converter, the Wiring That Fools It, and the Fixes That Replace No Sensor

P0137 is a code about a sensor that mostly watches rather than steers.

A 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 much fuel to add. The rear one — sensor 2 — reports what comes out of the converter, so the computer can judge whether the converter still works. P0137 — "O2 Sensor Circuit Low Voltage, Bank 1 Sensor 2" — means the signal from that rear sensor stayed lower than the computer allows; P0136 calls the circuit faulty, P0139 calls the response too slow and P0140 reports no activity at all. Bank 1 is the side of the engine with cylinder 1; P0156, P0157, P0159 and P0160 are the same four codes on bank 2. The same code family has a high-voltage member, P0138, which has its own guide; this page covers the other four.

Each of these is a statement about an electrical signal, not a verdict on a part, and certainly not on the converter. We searched for every manufacturer communication that names these codes or describes the rear sensor, plus a second, wider search for what our first one missed: 446 NHTSA communications, 447 PDF files, 55 of them byte-identical copies. Of the 392 distinct files, 64 print at least one of the eight codes in their own text — 47 print P0137, 25 print P0139, 11 print P0140 and 5 print P0136 — and we read 249: 184 in full or at the pages that mention the codes or the rear sensor and 65 as near-duplicates compared with their originals. The other 132 contain neither a code nor rear-sensor wording, and 11 have no readable text layer. One limit: the search started from NHTSA's index summaries, so a document whose summary names none of these terms can be missed; GM PI1006, which lists P0137 only in a keyword line, turned up only afterwards, and others like it may exist. The causes the documents give fall into five groups: wiring and connectors, software, the sensor itself, oil, leaks and the engine around it, and — rarely — the converter.

The federal complaint file is thin on these codes, and that is a finding. Of 782,908 complaints received by NHTSA from 2015 to 4 October 2026, we read 1,389 that mention an oxygen or air-fuel sensor or a sensor-2 code, each sorted twice, independently, by the same written rules. 110 were not about an oxygen-sensor fault. In the other 1,279, 165 place a sensor behind the converter or say all sensors were replaced, and only 19 name one of the eight codes — P0137 in 9, P0136 in 4, P0139 in 2, P0140 in 1, and three on bank 2.

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

Claims marked 🟢 rest on a document listed in the Sources section, read in full or at every page that mentions these codes or the rear sensor. Claims marked ⚪ are shop-price estimates carried from our repair guides. Complaint counts come from the published sorting of all 1,389 complaints, and owner-reported figures are published with ODI numbers.

I built PulsCar, a car-diagnosis app, 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). The rear-sensor codes are a place where the part named in the code is often not the part at fault, so every claim below that rests on a document names it.

What these codes say: the rear sensor is complaining about its signal

Two sensors, two jobs. The sensor after the converter — downstream, rear, post-catalyst, "catalyst monitor" or "secondary," depending on who is writing — watches the result. The front sensor does the main fuel control; the rear one's main job, as our O2 sensor guide puts it, is to watch the converter. That is why a rear-sensor code on its own often leaves the car driving normally, and why it is easy to live with and easy to misread.

What each code claims. 🟢 Subaru's 09-115-24R prints the definitions: "P0137: O2 Sensor Circuit Low Voltage Bank 1 Sensor 2" and "P0140: O2 Sensor Circuit No Activity Detected Bank 1 Sensor 2". Nissan's NTB13-108a prints P0139 as "P0139 (heated oxygen sensor 2 (B1) circuit slow response)" and its bank-2 twin as "P0159 (heated oxygen sensor 2 (B2) circuit slow response)". None of the four says the converter has failed; the converter's own code is P0420, which our P0420 guide covers.

The same sensor under six names. 🟢 Ford writes "catalyst monitor sensor (CMS)"; Honda "secondary heated oxygen sensor"; Nissan "rear O2 sensor" and "HO2S2 (B1)"; Chrysler "1/2"; Mazda "HO2S" and "rear harness"; and Jaguar Land Rover maps P0136, P0156 and P0157 to the mid-catalyst sensors of its three-sensor engines. If a bulletin is quoted to you, ask which sensor it means.

A code can lie about its own sensor. 🟢 GM's bulletin 18-NA-009 for 2016-2017 Enclave and Traverse, 2016 Acadia and 2017 Acadia Limited prints "P0137: HO2S Circuit Low Voltage Bank 1 Sensor 2" — and then, if fluid is found, replaces the "PRE-CATALYST BANK 1" oxygen sensor, sensor 1, along with a power-steering hose and the engine harness. The sensor named in the code and the part in the repair are not the same sensor.

First: which of the four do you have?

The four codes point in different directions, and reading the right one is the first check. It is free.

Low points toward a short or lost signal; slow points toward software or contamination. 🟢 Nissan's rear-sensor documents turn the low direction into a number: with the engine off and cooled for 15 minutes, "If the HO2S2 (B1) voltage is less than 0.13 V, the rear O2 sensor has a short to ground condition" — and the repair follows "the diagnosis steps outlined in the ESM for DTC P0137 (HO2S2)", "even if DTC P0137 is not present." The slow code P0139, by contrast, has the cleanest software-only fixes: Mitsubishi, Nissan, Honda and Ford each have a bulletin that reprograms a computer for it. And 🟢 Triumph tells dealers that on its catalyst-monitoring motorcycles P0140 "may be incorrectly triggered" when codes are cleared with the engine running at low coolant temperature or after a bump start.

Look at the neighbors. P0137 with P0141 or P0037 — the signal and the heater of the same sensor — often points to a shared plug, fuse or harness (GM PIT5336 lists signal and heater codes with an open fuse), so look there first; our P0135 guide covers the heater side. P0137 with P0420 or P0430 is a different question, and our P0420 guide and P0430 guide cover the converter. With a fuel-trim code, see our P0171 guide or P0172 guide.

Bank 2 is a different sensor. On a V6 or V8 the two rear sensors sit on opposite banks. 🟢 Nissan's flow chart in NTB15-051 for 2015 Altima V6, Pathfinder and Murano models tells the technician to reprogram if the computer number matches and then replace only the rear sensor of the bank in the code: "Do NOT replace both rear O2 sensors if the DTC(s) is for one rear O2 sensor."

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

The routes below quote each document; this is the map. Only the documents that print one of the eight codes are listed with their numbers.

Ford: replace the catalyst monitor sensor, and reprogram. 🟢 15-0072 (2015 F-150 3.5-liter EcoBoost, built by 14 April 2015), 15-0078 (2015 Mustang 2.3-liter, built by 19 December 2014), 16-0128 (2015 Edge 2.0-liter), 19-2005 (2018-2019 EcoSport 2.0-liter automatic, built 1 September 2017 to 28 November 2018) and SSM 50230 (2019-2020 Edge and Nautilus 2.0-liter, built by 14 October 2020) name P0137; SSM 46076 blames low battery voltage on 2016 hybrids; 24-2066 (pickups) and 24-2036 (chassis cabs) reprogram 2023-2024 Super Duty 6.7-liter diesels for P0139.

GM: harnesses, hoses and an axle shaft. 🟢 PIP5123B (2013 Silverado and Sierra, bent terminal), PIT5336 (2013-2015 Enclave, Traverse and Acadia, chafed harness), PI0937 (2013 Verano, sensor against the axle shaft), 18-NA-009 (power-steering fluid on the wiring) and PIP5529 (loose intake manifold bolts).

Nissan and Infiniti: software, and the rear-sensor program. 🟢 NTB13-108a (2012-2014 370Z, Frontier, Xterra), NTB15-056 (2013-2015 Titan and Armada), NTB15-053b (2015 Juke), NTB15-051 (2015 Altima V6, Pathfinder, Murano), NTB22-006 and NTB22-007A with campaign sheets P1A25 and PC857/PM997.

Mazda and Mitsubishi. 🟢 Mazda's tech tip SA288 and the 2016 Mazda6 tip name P0137 and rear-harness damage; its 01-025/13 cures P0139 on a 2010 Mazda5 by cleaning the throttle body; Mitsubishi's TSB 16-13-002 reprograms the 2014-2016 Outlander for a false P0139.

Subaru and Toyota. 🟢 Subaru's 09-115-24R supplies a redesigned rear sensor for P0137, P0138, P0140 and P0141 across six models; Toyota's campaign G0G replaces an oxygen sensor on certain 2015-2016 Sienna vans built in 2015.

Honda, Jaguar, Triumph and GM vans. 🟢 Honda's 25-040 (2017 Accord Hybrid, P0139 software), Jaguar's JTB00392NAS3 (2013-2015 XF, XJ and 2014-2015 F-TYPE, P0136), Triumph's reference 26407 (motorcycles, P0140) and GM's PI1006 (2011-2013 Express and Savana cutaways, P0137).

Chrysler, Jeep and Ram. 🟢 Chrysler's STAR case S1918000008 (three revisions) and tech tips 9003790 and 05149180A name P0137 and oil on the oxygen sensor; the Ram Cummins diesel bulletins list it as "1/2 02 Sensor Shorted To Ground" among some three dozen oxygen-sensor codes.

The 10-minute 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 (the snapshot of speed, temperature and fuel trims your reader saves when the code set). P0137 alone, with P0141, with P0138 and P0139, with P0420 and with a fuel-trim code are five different conversations.

2. Note what the car is doing (1 minute). Steady or flashing light, fuel smell, misfire, loss of power? A rear-sensor code on its own often leaves a car driving normally; if yours drives badly, the code is probably not the whole story (75 of the 165 downstream complaints in our file also describe a drivability problem).

3. Look for oil, water and chafing (3 minutes, engine off and cool). The rear sensor screws into the exhaust pipe just after the converter, under the car; use ramps or stands on level ground, never a jack alone. Is there oil on the rear sensor or its wire, oil or power-steering fluid on the harness, a pigtail lying on the axle shaft or the exhaust, or an aftermarket alarm spliced into the rear harness? Four groups of documents above are about exactly these.

4. Run the VIN (2 minutes). At nhtsa.gov/recalls, and with a dealer for emission campaigns, warranty extensions and special service campaigns, which do not all appear as safety recalls.

5. Check your model against the atlas (2 minutes). If your car is named, write the bulletin number down for the dealer; the atlas lists what we found, not every bulletin.

The price ladder, from the bottom up

The sensor, converter and leak figures come from our O2 sensor guide and our catalytic converter guide, the diagnostic fee from our diagnostic guide and the reprogram from our P0441 guide. The owner-reported sensor figures include front sensors and sets of several sensors, which is why their top is well above the guide's range.

What does it cost to fix P0137, P0136, P0139 or P0140 in 2026, and when is a new oxygen sensor the wrong purchase? Our repair guides carry an oxygen sensor at $150 to $500 installed, a downstream sensor at $200 to $400, an engine computer reprogram outside warranty at $0 to $200 and an exhaust leak at $100 to $400. In PulsCar's reading of 1,389 federal complaints, owners put 42 oxygen-sensor figures between $30 and $2,400, with a median of $501, and eight converter figures between $500 and $3,400, six of them quotes. A new rear sensor alone is the wrong purchase in the documented cases: where Nissan's table lists the engine computer's number, where a harness, a valve cover or a battery is named, where the throttle body is carbon-fouled, and where an alarm installer has cut into the rear harness.

The five routes, cheapest first

Route 1 · Wiring, connectors and harness — $0 and up

What it is. The sensor may be fine; the signal reaching the computer is not. A bent terminal, a harness rubbing on a hose or an axle, fluid on the wires or an installer's splice can pull the signal low or drop it, and the computer calls it P0137, P0136 or P0140.

What the documents say.

🟢 Mazda's SA288 tech tip and the 2016 Mazda6 repair tip. SA288 (NHTSA index label; the tip is titled "All Skyactiv models - check engine light on with DTC P0137") says: "If you encounter a vehicle with DTC P0137 (Circuit Low Input), check the rear harness at the red circled area in the following diagram." The 2016 Mazda6 tip gives the cause, "This may be caused by rear harness damage from an aftermarket alarm system installation," and the repair, "Repair or replace the rear harness as needed." The cost lands on the owner: SA288 says "This damage is not covered under Warranty," and the Mazda6 tip says repairs "are not covered under Mazda's warranty policy."

🟢 GM PIP5123B, 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. "Concern of DTC P2270 setting along with any of the following codes P0140, P0139, P0138, P0137, P013A, P013B" — and one instruction: "Check connector X131 for a bent terminal F as indicated in the attached PIC and repair per SI procedures." No part is replaced.

🟢 GM 18-NA-009, 2016-2017 Enclave and Traverse, 2016 Acadia and 2017 Acadia Limited, LLT engine. P0137 is one of seven codes, and the cause is on the next line: "This may be caused by a power steering leak from the power steering hose above the harness for the Bank 1 oxygen sensor, contaminating the O2 sensor wiring causing the readings skewed." If fluid is on the hose, GM replaces the bank 1 sensor, the power-steering gear inlet hose and the engine wiring harness — 7.2 hours of labor in version 2.

What the owners report. Eight of the 165 downstream complaints were sorted as wiring, a connector, a fuse or a damaged harness. 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 complaint filed on a 2020 Explorer with a bank 2 sensor 2 heater code reads "Found massive corrosion at connector" and that the maker had no pigtail kit, so "entire harness would be needed to repair." A 2020 Ascent owner's dealer wrote "Found pressure sensor leaking oil through all harnesses" and estimated about $8,933 for three harnesses; the scan listed P0137 with P0700, P2096 and other codes. A 2006 300C owner had two sensors replaced by the dealer for P0038 for $501.98, and a week later the car "CONTINUED TO CUT OFF WHILE DRIVING"; the dealer's next list was an engine harness and computer. Rodents appear twice, in a 2019 Forte and, in a dealer finding the owner disputes, a 2021 Insight.

The cheap cause that gets skipped. The harness. With the engine off and cool, follow the wire from the rear sensor to its plug and look at every place it touches something: a hose, the axle, a heat shield, the exhaust, an aftermarket alarm's splices. A rub mark or a cut costs nothing to find and decides the repair.

Fix it yourself — the honest split. Looking, photographing and re-routing a loose pigtail away from a hot or moving part are owner jobs on most cars. Cutting out an alarm installer's splices, repairing a harness and replacing the engine harness are shop work; on a 2016 Mazda6 or any Skyactiv car with an aftermarket alarm, Mazda says the warranty will not pay.

At the shop — say this: "Before replacing the sensor, can you check the rear harness and the connector for bank 1 sensor 2 for chafing, fluid and splices, and tell me whether there is a bulletin for my VIN about the harness? I would like the old sensor back."

Route 2 · Software, and a computer that reports a fault that is not there — $0 to $200 for a reprogram

What it is. The computer's own threshold sets the code, or its monitor misreads the sensor. The fix is a reprogram, sometimes with a sensor, and in the cleanest cases nothing but a reprogram. This is where most P0139 bulletins live.

What the documents say.

🟢 Mitsubishi TSB-16-13-002, 2014-2015 and early-2016 Outlander 2.4-liter. "Affected vehicles may exhibit false MIL illumination with DTC P0139 (Heated Oxygen Sensor [rear] Circuit Slow Response)." The cause: "The current malfunction threshold value used during rear O2 sensor response monitoring may cause false MIL illumination during fuel cut operations." The repair reprograms the ECM; the warranty line prints "8/80 Emissions Coverage."

🟢 Nissan NTB13-108a, 2012-2014 370Z, 2013-2014 Frontier V6 and Xterra, and NTB15-056, 2013-2015 Titan and Armada. P0139 and P0159 with the light on: NTB13-108a says "If this bulletin applies, reprogram the ECM," and the match is by the computer's part number in a table. NTB15-056 adds the shop rule: "Do not replace O2 sensors for the DTCs above if the vehicle's current ECM part number is in TABLE A on page 5." NTB13-108a adds that "Diagnosis and repairs beyond ECM reprogramming are not covered by this bulletin."

🟢 Nissan P1A25 and NTB22-006, 2013-2016 California-specification Sentra SULEV. Not a code fix but a check the software cannot make for itself: the campaign reprograms the ECM and measures the rear sensor's voltage, because "The current level software installed on the affected vehicles may not detect a rear oxygen (O2) sensor short to ground (SCG) condition." P0137 and P0139 are in its code table.

What the owners report. Ten of the 165 downstream complaints were sorted as pointing to the engine computer. A 2020 Cherokee owner had "ODB CODE P0139" on day 29, the sensor replaced, the light back on the drive home, and the engine computer replaced the next day — and "AFTER ALMOST TWO WEEKS THE ENGINE LIGHT CAME BACK ON WITH THE SAME CODE." A 2013 Prius owner with P0136 had the downstream sensor swapped and the wiring checked, and "BOTH MECHANICS HAVE DETERMINED THAT THE ISSUE IS WITH THE ECM." A 2004 PT Cruiser owner with P0037 had the oxygen sensors replaced and wrote "IT DIDN'T FIX IT. THEY SAY THE COMPUTER IS BAD"; the owner's computer was sent for repair and, the owner writes, was found fine. A 2007 Accord owner with P0420, P0139 and P2647 was told to reprogram the computer and could not find a campaign for the VIN.

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 Nissans in NTB13-108a and NTB15-056 the whole repair is the update when the number is in the table.

Fix it yourself — what to write down. The computer's part number or calibration ID if your reader shows it, and whether the car drives normally. A code that sets on a car that drives normally, in a model a bulletin names, is a software conversation, not a parts one.

At the shop — say this: "Is there a software update or bulletin for this code 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 · Oil, carbon, battery and loose parts elsewhere in the car — from a battery test to a valve-cover reseal

What it is. The sensor and its wire are honest; something else in the car is feeding them the wrong thing. Oil from a valve cover runs onto the sensor or into its wiring, carbon in a throttle body changes what reaches the exhaust on deceleration, a weak 12-volt battery drops a signal, intake bolts loosen. The code names the sensor; the cause is next door.

What the documents say.

🟢 Chrysler STAR case S1918000008, three revisions to August 2021. For "P0133, P0137, P0171, P0300, P2096" the case record says: "The root cause could be due to oil leaking from Bank 1 valve cover seal on to the 1/2 (Bank 1 downstream) O2 sensor." The instruction comes before ordinary diagnosis: "If oil is found, replace the 1/2 O2 sensor and inspect Bank 1 valve cover for signs of leakage." The record states that it "does not authorize warranty repairs." 🟢 Chrysler's tech tip 9003790 says it again for 3.6-liter V6 engines: "Oil leaking from ... the valve cover onto the O2 sensor causing faults codes P0140, P2096, P0137, P0157" — and the part it lists is the valve cover and gasket. A 2015 JK tip, 05149180A, tells technicians that before replacing the right downstream sensor for P0137 on a 3.6-liter, contact a named Chrysler engineer (and go on with the repair if there is no answer in 15 minutes): "Engineering is investigating whether the sensors are getting contaminated with oil."

🟢 Mazda 01-025/13, 2010 Mazda5. P0139 "may be caused by carbon forming inside the Electric Throttle Body (ETB)." The bulletin cleans the throttle body, relearns the idle and calls for no sensor: "Idle speed: 650-750 rpm" is the diagnostic yardstick, and a drop of more than 100 rpm points to the carbon.

🟢 Ford SSM 46076, 2016 Fusion, MKZ and C-MAX hybrids with the 2.0-liter engine. "Diagnostic Trouble Code P0137(O2 Sensor Circuit Low Voltage (Bank 1 Sensor 2) in Powertrain Control Module(PCM) memory and U3003 (Battery Voltage)" — caused by low battery voltage. The repair is the battery tests of the workshop manual, then clear the codes; if P0137 and U3003 do not repeat, return the car. Ford adds, "Engineering is currently investigating this concern."

What the owners report. Twenty-five of the 165 downstream complaints were sorted as an engine cause. A 2007 Wrangler owner wrote that the engine "BURNS EXCESSIVE AMOUNT OF OIL" and that "THE OIL ALSO DESTROYS THE FOUR OXYGEN SENSORS WHICH HAVE TO REPLACED EVERY 75K MILES." A 2016 Legacy owner's specialist found oil first in the engine computer and then in the sensors: "Oil seems to be traveling through the wiring harness from the engine into the electrical components." On a 2023 Telluride, Kia's inspection found, in the owner's words, "OIL LEAKING FROM LOWER 02 SENSOR" and a cylinder with no piston or rod left. A 2016 Terrain owner changed the upstream and downstream sensors and wrote that a converter would be required next "because the excessive oil consumption has clogged it almost completely."

The cheap cause that gets skipped. The dipstick, the oil-change receipts and a flashlight. Oil on top of the rear sensor, a wet valve-cover seam or a quart added every thousand miles all point away from the sensor. A battery load test and a throttle-body look are minutes of work.

At the shop — say this: "Is there oil on the rear oxygen sensor or its wire, and is the valve cover seam wet? Is the engine using oil between changes? Can you test the battery and check the intake bolts before replacing the sensor?"

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

What it is. Sometimes it is the sensor. The documents describe sensors that crack inside, seals that let exhaust in, wires that wear through at the sensor body and harnesses that were routed to fail. In several of them the maker has a new design, and the part number may not tell you which you are holding.

What the documents say.

🟢 Subaru 09-115-24, revised as 09-115-24R in April 2025, 2017-2024 Impreza and Crosstrek, 2019-2023 Forester, 2020-2024 Legacy and Outback 2.5-liter, 2022-2023 BRZ. For P0137, P0138, 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." 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." The labor line is "MFI OBDII REAR OXYGEN SENSOR R&R," 1.0 hour, and coverage is the basic new-car warranty, the applicable emission warranty or an active Subaru Added Security Gold plan.

🟢 Ford 16-0128, 19-2005, 15-0072, 15-0078 and SSM 50230. Ford's catalyst monitor sensor bulletins replace the sensor and then reprogram: "Yes - replace the catalyst monitor sensor (CMS)" for the 2015 Edge with P0054, P0137 or P0141, 0.9 hour front-wheel drive and 1.8 hours all-wheel drive. 19-2005 for the 2018-2019 EcoSport blames "a faulty catalyst monitor sensor as well as powertrain control module (PCM) software." SSM 50230 for the 2019-2020 Edge and Nautilus 2.0-liter says "This may be due to an exhaust gas oxygen sensor seal" and installs "an updated oxygen sensor, part number K2GZ-9G444-D." The 2015 F-150 bulletin goes by bank: the right sensor, the left, or both.

🟢 Toyota special service campaign G0G, 2015-2016 Sienna, about 24,100 vehicles. The sensor "could experience wire degradation inside the Oxygen Sensor wiring harness caused by vehicle vibration during normal operation," and the campaign's question-and-answer sheet lists the codes: "if this condition occurs, DTC P0037, P0136, P0137, P0138, P0139 may be set." A warning follows that matters to everyone else: "Please note these DTCs may be set for reasons not related to this condition." Replacement was free: 0.7 hour, part 89465-08140. The letter prints no end date, and it does not say whether the sensor is the front or the rear one.

🟢 GM PI1006, 2011-2013 Express and Savana cutaways with 4.8- or 6.0-liter engines built before 26 November 2012. Its keyword line lists P0137, P0140, P0157 and P0160 among thirty-odd codes, and its cause is mechanical: "Moisture may be splashing on the hot O2 sensors and causing them to crack." The repair: "Replace both O2 sensors and install the new ECM service calibration."

What the owners report. Forty-nine of the 165 downstream complaints were sorted as the sensor itself. A 2017 Altima owner'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 (sorted under the throttle body, not the sensor) was quoted $250 by the dealer for the rear sensor and had the owner's husband, an "on the side mechanic," fit it for $30; the light stayed on, and the dealer then said the front sensor had been the bad one. A 2012 RAV4 owner paid $750 for both sensors and the three dashboard lights came back on the drive home. Repeated replacement runs through the set — owners count sensors replaced two and three times, and one writes of four sensors "WHICH HAVE TO REPLACED EVERY 75K MILES" — which is the pattern Route 3 explains.

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.

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. If the sensor is seized or buried, stop: a snapped sensor is a bigger repair. One 2012 Sentra owner bought two sensors for P0420, took them to a shop, and the dealer said "THE PORT WAS INDEED THREADED" and that the only fix was a new exhaust manifold; the owner calls it "A $70 SENSOR" causing the need for "A $1000 MANIFOLD" — a cautionary tale about a damaged sensor port, here on the upstream sensor.

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 oil? Which bank is it, and is there a new-design part or a campaign for my VIN?"

Route 5 · The converter, the leak and the engine behind the reading — $100 to $15,864

What it is. The rear sensor sits after the converter and sees what the engine and converter send it. A leak in the exhaust, a converter that has failed and an engine that is failing all reach it. Here the oxygen-sensor code is the smallest item on the bill — and the documents warn against jumping to the converter.

What the documents say.

🟢 GM PIP5110 family, 2011-2015 Silverado HD, Sierra HD, Express and Savana. For P0420 with no other code, GM tells the technician to "check for a possible leak in the exhaust system behind the bank 1 sensor 2 O2 sensor," and states a rule the converter's price makes worth remembering: "This repair must be attempted before considering the replacement of a catalytic convertor."

🟢 GM recalls N192268180 and N192268181, 2012-2013 Equinox and Terrain 2.4-liter, Bin 4 emission versions. Two recalls on the same models: the first, about 327,328 vehicles, says "Dealers are to replace the post-catalyst oxygen sensor"; the second, about 60,626, says "Dealers are to replace both catalytic converters and both oxygen sensors." Both state a fact owners should know: "The malfunction indicator lamp (check engine light) will not illuminate." Reimbursement for earlier repairs ran to 31 October 2021.

🟢 GM PI0968, 2013 Cruze and Sonic 1.8-liter. On a new vehicle the converter must break in, and until then the rear sensor's readings may be shifted: "the heated post-O2 (sensor 2) may respond with an output that is shifted from its normal operation until the converter break in is completed." On vehicles with very low mileage, "if no trouble is found, no repair attempts should be performed and no parts should be replaced" — clear the code and test drive. The code in that bulletin is P0133, not one of ours, but its explanation concerns the rear (sensor 2) sensor. 🟢 Audi's 2044668/1 says much the same of the 4.0-liter S6, S7, A8 and S8 (2013-2016) and RS 7 (2014-2016) under 5,000 miles, where break-in fumes slow the rear sensor, for delayed-response codes P013E00 and P014A00: "delete the DTC(s). Do not replace the oxygen sensors" or the converter.

What the owners report. In the 165 downstream complaints, the converter was the stated cause or repair in 16, an engine failure or engine repair in 25, and an exhaust leak or part in 5. At the top of the ladder is a 2020 Explorer: a shop estimate of $15,863.61 with $11,534.51 for an engine that had failed internally, and a line for a "BAD HEATER CIRCUIT OXYGEN SENSOR BANK 1 SENSOR 2." A 2013 Terrain owner paid $1,100 of $3,023.17 in repairs and was then quoted $7,000 for an engine. A 2013 Lincoln MKT owner, after a recall repair that Lincoln said had replaced two converters and four sensors, called it "POORLY EXECUTED" and got another dealer's estimate: "$ 1,400 EACH CATALYTIC ($ 2,800), 4 SENSORS $ 786, LABOR $ 600 = $ 4,189." A 2017 Edge owner paid $1,072.67 for P0036, with the downstream sensor replaced and the software updated under TSB 18-2212A, and $950.95 for spark plugs and a coil; an independent garage then found misfire codes and an empty coolant reservoir.

The cheap cause that gets skipped. The leak. A leak at a flange or clamp costs $100 to $400 to repair in our guides; a converter costs $200 to $4,500. For P0420 on those GM trucks (4.8- and 6.0-liter), GM writes the order into its bulletin: repair a leak behind the rear sensor before considering a converter.

At the shop — say this: "Before a converter, can you check the exhaust for a leak around the rear sensor, including behind it, and show me the live data from both sensors? Is the engine using oil or coolant, and is there a recall for my VIN on the converter or the sensors?"

The coverage file: which programs exist, and which still run in October 2026

Most documents here 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 is our own calculation from the terms printed in each document, as of October 2026 — not a statement by the manufacturer — so check the exact VIN.

The Nissan extension, read closely. 🟢 NTB22-007A 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." The campaign sheet 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 document says when the 20 years start; even counted from the earliest production date they run to March 2032, so the time limit has not run out for any eligible car and the 200,000-mile limit decides. The check itself: "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." Codes P0137 and P0139 are in the bulletin's table, and a clause brings other table codes in when they are found only after that repair.

The rest. GM's N192268180 and N192268181 recalls and Toyota's K0A fix a problem the dashboard may not show: "The malfunction indicator lamp (check engine light) will not illuminate" (GM) and "this software may not detect a malfunctioning sensor" (Toyota), so a VIN lookup is the surest way to find them. The Audi and BMW extensions run from the original in-service date, so by time they are open only for cars that entered service within ten years; Lexus's campaign ended on its printed date, more than eight years ago.

What 1,389 complaints show

We searched every complaint in NHTSA's file for a sensor-2 or related oxygen-sensor code (P0136 to P0141, P0156 to P0161, P0036 to P0038, P0056 to P0058, P013A to P013F, P014A, P014B, P2270 to P2273), for a bare number after a code word, and for oxygen-sensor wording — "O2 sensor," "oxygen sensor," "lambda," "A/F sensor," "HO2S" and a few common variants. Those searches found 1,291 complaints. A second, wider search, run without sight of ours, went through the candidates it returned and added 98 we had missed: misspellings ("O2 censor"), a bare "O2," an exhaust sensor named without "O2," and upstream sensor codes our first list left out. All 1,389 were sorted twice, independently, by the same written rules, which were fixed and dated before the sorting began (plus a second dated addendum for the added batch). The two sortings matched on the category for 91 percent of the 1,091 complaints of the main run, and every disagreement was settled by a written rule. 110 turned out not to concern an oxygen-sensor fault.

The file cannot tell the rear sensor from the front for most complaints: 990 of the 1,279 relevant ones name neither position. The study therefore reads the 165 complaints that place a sensor behind the converter or say that both or all the oxygen sensors were replaced — about a third rest on that "both or all" wording; the rest use the words "rear," "downstream," "sensor 2," "B1S2," or a sensor-2 code (92 of the 165 also place a sensor upstream; 73 do not) — and says so wherever the full 1,279 are used.

The 19 that name a target code. A 2007 Camry, a 2006 Equinox, a 2018 Malibu, a 2013 Chrysler 200, a 2014 Sorento (two complaints that match on car, parts and codes, so we count one vehicle), a 2022 Malibu, a 2018 Volt and a 2020 Ascent name P0137; a 2013 Prius, a 2001 Montero, a 2017 Edge and a 2013 Sorento name P0136; a 2020 Cherokee and a 2007 Accord name P0139; a 2014 Hyundai filed as a Santa Fe (the owner's text says Sonata) names P0140; and a 2007 XL7 and a 2018 MDX (one owner, two complaints) name bank 2 codes. So the 19 complaints come from 17 vehicles. Ten were sorted as undiagnosed, four (three vehicles) as an engine failure, three as the computer, one wiring and one fuel system; none was sorted as the converter, and none as the sensor. Several are long lists: the 2022 Malibu's dealer retrieved "P00C6, P0137, P0171, P0299, P089, and P2097" and the car's engine seized.

Drivability. Of the 165, 75 describe stalling, hesitation, lost power or hard starting, and 76 report the problem coming back after a repair. Owners write to NHTSA when something frightening happened, so the oxygen-sensor code is often a passenger in the story.

By maker. Chevrolet has 32 of the 165, Ford 20, Toyota 15 and Kia 13, across 30 makers: too few to rank cars, and nothing here is a failure rate.

What do 1,389 federal complaints show about the rear oxygen sensor and its codes, and how often is the converter the answer? In PulsCar's reading of every complaint our searches and the second search found that mentions an oxygen sensor or a sensor-2 code, 165 place a sensor behind the converter or say all were replaced and only 19 name P0137, P0136, P0139, P0140 or their bank 2 twins. Two independent sortings by the same written rules matched on the category for 91 percent of the main run. Among the 165, the sensor itself was the stated answer in 49, no diagnosis was given in 45, the engine was behind it in 25, and the converter in 16, about one in ten; none of the 19 code-named complaints was sorted as converter. Wiring was the answer in 8 and the engine computer in 10. A sensor code is far more often a report about something else than a verdict on the converter.

What owners say they paid or were quoted

Two hundred and one dollar figures in 161 relevant complaints survived one written inclusion rule, and the full list with ODI numbers is published; 207 other mentions were excluded, each with a reason (a vehicle price, a car payment, an unrelated repair, a running total). Of the 201, 86 are quotes, 49 are stated as paid, 15 as billed and 49 give a price without saying which, and 2 are offers. The ones that bear on these codes, by what they paid for:

The sensor alone. Forty-two figures run from $30 to $2,400, median $501, across all sensor positions. $30 for a rear sensor a 2011 Traverse owner's husband fitted, against the dealer's $250 quote. $400 for a 2017 Altima's rear sensor. $750 for both sensors on a 2012 RAV4. $232.97 for a 2014 Sonic's two oxygen sensors. $1,450 for a sensor replacement on a car whose owner says it triggered no code.

A converter. Eight figures, $500 to $3,400, median $2,500, six of them quotes — against $200 to $4,500 in our converter guide.

Wiring and computers. $449 to $8,933 for wiring, the top figure the Ascent's three harnesses; six computer figures from $450 to $1,600, among them $450 plus labor for a 2002 Grand Cherokee's computer and $500 plus labor for its full set of oxygen sensors, the second set after new converters.

The code as a passenger. Twenty-three engine figures run from $800 to $12,000, median $6,000, twenty of them quotes. One 2020 Explorer estimate reached $15,863.61 in total, $11,534.51 of it the engine, with an oxygen-sensor line among the items. None of these was mainly an oxygen-sensor repair.

Why these codes come 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.

Because the wrong sensor was replaced. P0137 is bank 1; P0157 is bank 2. On a V6 or V8 the two rear sensors look alike and sit on opposite sides, and Nissan's flow chart tells the shop to replace only the one the code names. Jaguar's three-sensor layouts add a third sensor that is neither.

Owners report the pattern. A 2020 Cherokee owner had a sensor replaced for P0139, then the computer, and the code returned both times. A 2013 Prius owner's two mechanics swapped the downstream sensor, checked the wiring and concluded "THE ISSUE IS WITH THE ECM." A 2014 Sorento owner's long list of replaced parts — a high-pressure fuel pump, injectors, spark plugs, a starter, a MAP sensor — ends with a mechanic's harness recommendation and a pile of codes that includes P0137.

Why do P0137, P0136, P0139 and P0140 come back after a new oxygen sensor, and what should be checked before a second one? In PulsCar's reading of 249 manufacturer documents, the part named in the code is often not the part at fault, and a repeat visit often means the cause was never touched. Nissan tells Titan and Armada technicians not to replace the O2 sensors for P0139 when the computer's number is in its table; Mitsubishi (Outlander) calls P0139 a false light cured by software and Honda (Accord Hybrid) says the computer misreads the sensor; Mazda says rear-harness damage from an aftermarket alarm installation may cause P0137; GM blames a bent terminal, a power-steering leak or an axle shaft; Chrysler blames oil from the valve cover. Before a second sensor, ask for the freeze frame, the connector and harness checked, oil looked for on the sensor, and the software level compared with the latest.

Can you drive with these codes?

Often, for now, if the car drives normally. The rear sensor's job on most engines is to watch the converter, not to steer the fuel, so a car with one of these codes and a steady light often keeps driving as before — though owners in our file also report stalls and lost power. 🟢 Nissan's campaign sheet for its short-to-ground extension answers "Is my vehicle safe to drive?" with "Yes." — that is Nissan on those cars, not a rule for every code — and Ford says that on 2023-2024 F-Super Duty 6.7-liter diesels built by 2 January 2024 a light with P0139 "may be due to the software in the PCM and does not affect vehicle operation."

What changes the answer. A flashing check engine light, a misfire, a loss of power, a stall, a smell of fuel or hot oil, black smoke, or oil or coolant disappearing: the engine is doing something the rear sensor is only reporting, and driving on can damage the converter or the engine. Of the 17 vehicles behind the 19 code-naming complaints, three ended in an engine failure.

Should you buy a used car with one of these codes in its history?

It depends on what fixed it. A receipt for a named software update, a harness repair, a valve-cover reseal 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 any rear-sensor code alongside oil consumption, a coolant smell or a recent engine replacement.

Four checks before you buy. Look for "harness," "connector," "reprogram," "valve cover" or a bulletin number in the service history. Run the VIN. Scan the car: a code cleared before a sale leaves the readiness monitors incomplete. And look at the rear sensor for oil and the underside for a cut or rubbed harness.

Clearing the code before an inspection

Clearing the code does not clear the cause, and inspectors look for it. Erasing it resets the oxygen-sensor and catalyst monitors to "not ready"; in many state programs a car with incomplete monitors is turned away or failed (the limit depends on your state and model year), and if the cause is still there the code returns while they complete.

What works instead. Fix the cause first, then drive normally for several days before the test. 🟢 Nissan's rear-sensor repair ends the same way: set the drive pattern and run it until the oxygen-sensor and catalyst monitors read complete, and Honda's update ends with clearing all codes after the software goes in.

What a shop actually does with one of these codes, checkpoint by checkpoint

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

Checkpoint 2: live data. Front and rear sensors side by side on a warm engine; the rear signal should be steadier than the front. 🟢 On Nissan's NTB22-006 and NTB22-007A cars only, the dealer test is: engine off, cooled 15 minutes, rear-sensor voltage under 0.13 volts means a short to ground. That threshold is Nissan's; do not apply it to other makes.

Checkpoint 3: the harness, the connector and the oil. Unplug and inspect; look along the wire for rub marks, fluid and splices, and at the sensor body for oil.

Checkpoint 4: the bulletins, the software and the leak. Bulletins and campaigns by VIN, the software level, and a leak check at the flanges and clamps.

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 traps that cost owners money

Trap 1: a new sensor into a bad harness. On a Mazda with an alarm, a GM crossover with a power-steering leak, a Silverado with a bent terminal, a new sensor reports through the same damaged wire or connector. Lesson: look at the wire before you buy the part.

Trap 2: a converter quoted for a sensor code. A converter costs $200 to $4,500 in our guides; GM writes into its P0420 bulletin that the leak comes first, and in this file no complaint that named one of these codes was sorted as a converter. Lesson: ask for live data from both sensors before a converter is quoted.

Trap 3: a program you do not know about. Nissan's 20-year rear-sensor extension, Toyota's Sienna campaign, GM's emission recalls and Toyota's K0A are easy to miss unless someone runs the VIN, and GM's two recalls and Toyota's K0A concern problems the dashboard may never show; in this file a 2017 Altima owner paid almost $400 for a rear sensor; a 2017 is outside the extension, which covers eligible 2013-2016 VINs. Lesson: run the VIN, and ask by bulletin number.

Three scenarios, 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, P0137, quote for a rear sensor. The owner asks the dealer to run the VIN for the rear-sensor warranty extension and the voltage check. The car is eligible; the voltage reads below 0.13 volts and the short to ground is repaired under the extension. Lesson: on a covered car, the program decides who pays.

Scenario 2: 2010 Mazda5, 96,000 miles, P0139 and a quote of $300 for a rear sensor. The owner asks for the bulletin. The idle drops by more than 100 rpm in the test; the shop cleans the throttle body and relearns the idle, and the code stays away. Lesson: the sensor was the messenger.

Scenario 3: 2016 Mazda6, 61,000 miles, P0137 after an alarm was installed, quote for a sensor. The owner asks the shop to look at the rear harness. The protector tube is damaged by the alarm's electro-tap connectors; the repair is a harness, and Mazda says the warranty will not cover it. Lesson: the installer's work is part of the diagnosis.

The plan: 10 minutes, today, this week

10 minutes: read every code and the freeze frame; note whether the car drives normally; look at the wire and the sensor for oil, fluid and rub marks.

Today: run the VIN at nhtsa.gov/recalls and ask a dealer about emission recalls, extensions and campaigns by name — NTB22-007A on a 2013-2016 Altima 2.5 or 50-state Sentra, G0G on a 2015-2016 Sienna, N192268180 on a Bin 4 2012-2013 Equinox or Terrain 2.4, 09-115-24R on a Subaru, and the bulletin for your maker in the atlas above.

This week: have the harness and the connector checked, the software compared and the freeze frame read before any sensor or converter 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. Ford TSB 15-0072 and 15-0078 (image PDFs, transcribed), 16-0128, 16-0163, 19-2005, 24-2036 and 24-2066 (image PDFs, transcribed); SSM 46076 and 50230.
2. GM PIP5123B, PIT5336, PI0937, PI0968, PIP5110, PIP5110B and PIP5110C, PIP5529, 18-NA-009 version 2; PI1006, emission recalls 14096, 14096B, 14096C, N192268180 and N192268181.
3. Nissan and Infiniti NTB13-108a, NTB15-056, NTB15-051, NTB15-053b, NTB22-006, NTB22-007 and NTB22-007A; campaign sheets P1A25 and PC857/PM997 (image PDFs, read from page images); recall R1620.
4. Toyota and Lexus special service campaign G0G, K0A, T-SB-0081-24; Lexus campaign FLB.
5. Mazda and Mitsubishi SA288 (NHTSA index label) (image PDF, transcribed) and the 2016 Mazda6 tip 1011M; 01-025/13; Mitsubishi TSB-16-13-002.
6. Subaru and Honda 09-115-24 and 09-115-24R; Honda 25-040.
7. Chrysler, Jeep and Ram STAR case S1918000008 Rev. C, Rev. B and Rev. A; GPOP tech tips 9003790 and 05149180A; Ram 25-002-14.
8. Jaguar, Triumph, BMW, Audi JTB00392NAS3; Triumph reference 26407 (NHTSA index SMTCOM02); BMW SIB 01 03 19; Audi 2044668/1, AWA-20-05 and 96H5.
9. PulsCar analysis of NHTSA public data, October 2026: all 1,389 complaints about oxygen sensors, each sorted twice, independently, with disagreements settled by written rules; 201 owner-reported amounts; the document list with read status. Data and how we did it and the sorting rules; the documents list.

For the neighboring codes and parts: P0138, P0133, P0135 and the heater codes, P0420, P0430, P0171, P0172, the O2 sensor cost guide, the catalytic converter cost guide, the diagnostic cost guide and the code index.

Methodology. The complaint set is every complaint in NHTSA's file (782,908 received 1 January 2015 to 4 October 2026) that names a sensor-2 or related oxygen-sensor code, a bare number after a code word, or oxygen- or air-fuel-sensor wording, plus 98 added by a second, wider search; the exact search patterns are in the sorting rules. Those rules, with a relevance test, eight categories and four flags, were written and dated before any sorting began. Every complaint was sorted twice, independently, by them; every disagreement on any field (152 in the main run of 1,091 complaints) was settled under written rules, and a final check of the agreed sortings changed 22 more, each with a stated reason. Agreement on the category: 91 percent. The manufacturer set is every NHTSA-indexed communication whose summary names one of the eight codes, a neighbouring sensor-2 code or rear-sensor wording, plus those the second search added; 447 PDF files were downloaded, 55 byte-identical copies set aside, near-duplicates compared with their originals and every quotation checked by script against the document text. Coverage status is our computation from the printed terms, as of October 2026. Nothing here is a failure rate.

Prices verified July to October 2026 in the repair guides they are carried from, and reviewed quarterly. Technical documents verified October 2026: the 249 documents we read (184 in full or at the pages that name the codes or the rear sensor, 65 near-duplicates compared with their originals) in our published list — among them Nissan NTB22-007A and NTB15-051, Toyota G0G, GM 18-NA-009 and PI0937, Ford 19-2005 and SSM 46076, Mazda SA288 and 01-025/13, and Subaru 09-115-24R.

Had one of these codes fixed by a harness, a software update, a campaign or the Nissan extension — or refused one? Email [email protected] with the vehicle, mileage and outcome.