P0421 in 2026: Warm-Up Catalyst, Software, Leak or Converter — What 119 Documents and 210 Complaints Show

P0421 is often read as "the catalytic converter is bad, buy one." The documents do not say that.

P0421 is a catalyst efficiency code: the computer has judged that a catalyst on bank 1 converts less than its limit allows. Ram prints it as "P0421-00 - Catalyst 1 Efficiency Below Threshold Bank 1," Mitsubishi as "Warm Up Catalyst Efficiency Below Threshold," Mazda as "P0421:00 (Catalytic converter system)," and GM titles its engineering paper "DTC P0421 Catalyst 1 Efficiency Below Threshold." The catalytic converter cleans the exhaust; the warm-up (or first) catalyst is the one near the engine that heats up first. The computer does not look inside it. It compares readings from the air-fuel (A/F) or oxygen sensors in front of and behind the converter and decides, and that decision can be wrong, as Mazda and Hino say in their own papers. Its neighbours in the papers are P0422 (GM and Volvo list it in the same kind of document), P0431 (Mitsubishi's pair for P0421) and P0432 (no paper), and P0420 and P0430, the sister codes that our P0420 guide and P0430 guide cover.

We went through NHTSA's index of manufacturer communications for papers that name P0421, P0422, P0431 or P0432 or discuss a warm-up, front or pre-catalyst efficiency fault, and an independent second search added 49 more: 166 files (149 manufacturer papers and the 17 papers of Kia's Soul recall), 119 distinct documents once exact copies are set aside. We went through 88 in full and compared 78 revisions and copies with them line by line. 70 files name at least one of P0421, P0422, P0431 or P0432 in their own text (P0421 48, P0422 26, P0431 1, P0432 none), mostly heavy-truck and diesel pickup papers.

We also went through 210 complaints from NHTSA's file, received from January 2015 through September 2026, that name P0421, P0422, P0431 or P0432, or describe a catalyst efficiency fault in owners' words. Each was sorted twice, independently, by the same written rules; the two sortings matched on the main category in 90% of them (189 of 210), and written rules settled the rest. 191 describe a catalyst fault on the owner's car; 19 do not. The number that matters most for this page is small: only 9 of the 191 print P0421, P0422 or P0431 themselves, six print P0421 and none prints P0432 (one owner typed "P04320").

By the end you will know which of five routes your P0421 is on, what each costs, and when to stop driving.

Claims marked 🟢 rest on a document in the Sources section; claims marked ⚪ are shop-price estimates from our repair guides. Owner-reported figures are what people told NHTSA, published with ODI numbers, and every complaint count here comes from the 210 complaints sorted twice and published.

Words on this page. Catalyst or catalytic converter: the part that cleans the exhaust. Warm-up catalyst (WU-TWC, "pre-cat", "front cat"): the converter nearest the engine. Bank 1: the side of the engine that holds cylinder 1. A/F sensor, O2 sensor: exhaust sensors. Engine computer (ECM, PCM). DTC: a trouble code such as P0421. Check engine light (MIL). Bulletin (TSB): a maker's repair instruction to dealers, not a recall. Recall: a repair a maker must offer; an emissions recall concerns emissions equipment. ATC: Hino's name for the diesel catalyst. Mode 6: the scan tool's self-test results. Flash: a software update. Monitor: the computer's self-check of a part. ODI number: NHTSA's reference number for a complaint.

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).

What P0421 says: a catalyst judged below its limit, not a dead converter

What the code reports. 🟢 Mazda's bulletin describes the check it asks a technician to make: the warm-up three-way converter is tested with the scan tool's Mode 6 "catalyst monitor bank 1" value: a first reading of 0, or of 2 or higher, goes straight to updating the software; a reading in between is updated, driven and read again, and only a value that then stays below 1 means "If the value is "less than 1", WU-TWC should be deteriorated." The same paper says the code can appear with no driving problem at all: "Some vehicles may experience the check engine light on with DTC P0421:00 (Catalytic converter system) without experiencing poor engine performance or misfire (Mitsubishi PCM only)." Mitsubishi's paper for its V6 cars says the light comes on when "certain exhaust manifold mounted catalytic converters may experience catalyst layer separation." GM's engineering paper for the 2018 diesel Equinox and Terrain says the condition "may be caused by a software anomaly."

What you feel. Sometimes nothing but a light (Mazda's paper says so for its cars). GM's paper for the same diesel cars adds an "exhaust smell within the cabin." Among the owners who wrote to NHTSA the worse cases appear: 33 of 191 complaints describe a stall, a loss of power or a limp mode (complaints, not a failure rate), 6 an emissions test or inspection that was failed or at risk, and 5 are marked as a tow in NHTSA's own field.

What it is not. It is not a misfire code (P0300); a misfire can ruin a catalyst, but the papers treat it as a cause to find first. It is not a lean code (P0171); it is not an oxygen sensor circuit code (P0138), though a bad sensor reading can fool the catalyst test. And it is not automatically the same as P0420; the makers use the two numbers differently, as the next section shows.

Is P0421 a dead catalytic converter, and how is it different from P0420? In most of the papers we read, the converter comes late, not first. PulsCar went through 210 complaints from 2015 to September 2026 that name a catalyst code or describe a catalyst efficiency fault, each sorted twice by the same written rules: 191 describe the fault on the owner's car, and only 9 print P0421, P0422 or P0431. In the maker papers P0421 is a software false alarm on Mazda, a misaligned clamp on a 2018 diesel Equinox, a diesel catalyst on Hino, a layer separation in Mitsubishi's manifold converters, and a converter only when the checks above agree. P0420 is the same family on bank 1 with different labels by maker. Read your car's own paper before you buy the part, and ask which test showed the converter.

First: P0421, P0422, P0431, P0432 or P0420? One monitor, many labels

The same word means different parts at different makers, so read the exact code and the make before you buy anything.

Warm-up does not tell you where the part is. 🟢 Honda's bulletin calls the converters behind its P0420 and P0430 the "WU-TWC (Warm Up Three Way Catalyst)" and says a lost A/F sensor cap "may have fallen off and bored a hole through the affected WU-TWC"; Mitsubishi prints "P0420: Warm Up Catalyst Efficiency Below Threshold" for a false light on its 2008-2010 Outlander and 2009-2010 Lancer 2.4L cars; Mazda's P0421 is the warm-up three-way converter on bank 1; a Hino truck's P0421 is its diesel catalyst, which the engine computer tests during a parked particulate-filter regeneration. A P0421 on one car and a P0420 on another can be the same kind of part. The label is not a map.

Search traps. Owners write PO421 (a letter O) or only "the catalytic converter code"; the "-00" form is the dealers' and makers' spelling. In our file 178 of the 191 relevant complaints name no catalyst code number (three typed a garbled one such as "P04320"), 4 name only P0420 or P0430 and 9 name P0421, P0422 or P0431, so a search for P0421 alone misses almost everything owners wrote.

The maker atlas: who prints a clamp, software, a converter or a recall

The documents split by maker: a diesel Equinox and a V6 Eclipse with P0421 are on different roads. Document numbers link to the NHTSA PDF; only the claims are repeated here.

The Mitsubishi paper is the clearest P0421 manifold-converter paper in the file. 🟢 It names the cause as "catalyst layer separation" from "variations in production," limits the repair to "only replace the necessary side," and for the 2006 Eclipse V6 tells the technician to confirm that the computer was already reprogrammed under an earlier emissions recall (EMR-06-002). The extension is coverage, not a recall, and it paid only when the light was on with P0421 or P0431 stored.

Most mentions are incidental. In the Ram flash bulletins P0421 is one code among 16 (18-015-17) or about 45 (18-010-21), and in GM's 21-NA-214 one of 16; the printed fix is software, and none tells a shop to replace the converter.

The do-not-replace lines: what the papers tell a shop not to buy first

None of these lines says the converter is never the cause; each says that for the condition described, the part should wait.

GM, 2018 diesel Equinox and Terrain, P0421. 🟢 PIP5615 says "If published diagnostics have been performed, and are inconclusive, do NOT replace any parts at this time." Mazda, listed 2014-15 Mazda3, 2014-16 Mazda6 and 2014-16 CX-5, P0421. 🟢 01-010/16: "It is not necessary to order a PCM part for this repair procedure". Hino, 2015 J08 trucks, P0421. 🟢 SB-14-012: reprogram "prior to suspecting a fault with the ATC." Subaru, P0420 programs. 🟢 WVH-18R: "Catalytic converter(s) should ONLY be replaced when indicated during the Catalytic Converter Efficiency Test Procedure. They should not be replaced for any other reason." Ford, 2022-2023 F-150 hybrid. 🟢 24E04: replace the converter only where the stored oxygen-storage value is above 0.4 after a prescribed drive.

In the papers we went through, software, a clamp, a leak or a sensor reading usually comes before the converter, though in owners' complaints the converter is the most common named repair (46 of 191). The exceptions are the papers that name a defect in the converter itself: Mitsubishi's layer separation (once the code is stored), Toyota's rattling Tacoma converters and the Chrysler recalls replace it without the diagnostic detour.

The 10-minute check

1. Write down how it started and read the code (2 minutes). Did the light come on with a rough idle or a stall, an exhaust smell in the cabin, a rattle from under the car, or by itself? Is it steady or flashing? Read the code with a reader that shows maker codes and write down every code beside P0421: a misfire code (P030X), a lean code or an oxygen sensor code changes the road. Do not clear it before the shop sees it unless you are about to test a fix (our advice, not a maker's rule). Note the mileage and whether the engine uses oil between changes.

2. Look under a cold car (4 minutes). With the engine cold, and the car on level ground or ramps, never held only by a jack, look along the exhaust from the engine back for soot around flanges and clamps, a hanging pipe, a cracked manifold and a converter that moves when you push it. 🟢 GM's paper for the 2018 diesel Equinox and Terrain asks the technician to remove the turbocharger heat shield and look at the clamp between the turbo and the converter, and says "If the clamp is properly aligned (1) and there is NO smell of diesel exhaust, no further action is required." 🟢 Toyota's 2015 campaign letter for 2010-2014 Tacoma trucks describes a converter whose internals "begin to rattle". 🟢 Land Rover's paper tells the technician to "Inspect for and repair any leaks in the exhaust system" before any converter is replaced.

3. Ask about oil and fuel (2 minutes). 🟢 Land Rover's list for P0420 on the 2.0L Discovery Sport and Evoque includes oil consumption and misfire checks, and Kia's paper for the 2012-2016 Soul ties a catalyst code to a compression check. In our file 38 of the 191 relevant complaints describe an engine burning or using oil.

4. Find your VIN and paperwork (2 minutes). If the car is under 8 years and 80,000 miles, also ask whether the federal emissions warranty covers the converter (our P0420 guide explains it). The recalls above are emissions recalls and may not show at nhtsa.gov/recalls, so ask a dealer with your VIN whether a bulletin, update or program exists for your model and engine; bulletins are dealer repair instructions, free only inside a warranty or a program. Kia's Soul recall 19V120 is the one safety recall in this file (nhtsa.gov/recalls?nhtsaId=19V120).

Five P0421 situations, and which is yours

"A listed 2014-15 Mazda3, 2014-16 Mazda6 or 2014-16 CX-5, steady light, runs normally." Mazda prints a false P0421 and a reprogram. → Route 1.

"A 2018 diesel Equinox or Terrain with P0421 or an exhaust smell in the cabin." GM prints a misaligned clamp for a 2018 diesel. → Route 2.

"P0421 with a rough idle, a misfire code or an engine that burns oil." The catalyst may be the victim. → Route 5, then Route 4 if it is damaged.

"2006-2008 V6 Eclipse, Galant or Spyder with P0421 or P0431, or a failed emissions test." Mitsubishi prints a manifold converter and an extension, in listed states only, that has run out. → Route 4.

"A 2012-2016 Kia Soul with a catalyst code." Kia has a safety recall and a follow-up program about converter overheating. → Routes 4 and 5, and ask for 19V120 by number.

The P0421 price ladder, from the bottom up

Our guide figures are typical independent-shop prices from our catalytic converter replacement cost guide and our P0420 guide; owners' figures include dealer quotes and bills. Nine of the ten owner figures for a converter sit between $497 and $2,462; the tenth is $13,500 (an estimate on a 2014 Audi RS5). One more owner was quoted more than $8,000 for repairs after a ruined converter on an engine that burned oil. The expensive mistake is not the converter; it is paying for it and finding that a leak, a software false alarm or an engine that burns oil was the cause.

The five P0421 routes, cheapest first

A catalyst efficiency code is a judgement, and each route below is a place that judgement can be wrong or right, ordered by what it costs to rule out. Owner counts come from the 191 relevant complaints; "came back" means the owner says the same light, code or symptom returned or the repair did not hold. Complaints come from owners whose trouble did not end, so none of these shares is a forecast for your car.

Route 1 · The reading: software, a false alarm, a recall or a paper — $0 yourself, free in a program

What it is. The catalyst is fine or only a little worn, and the computer decided otherwise: its own logic, a sensor reading it misread, a monitor that runs too early, or a trip rule.

What the documents say. 🟢 Mazda's bulletin names "improper control logic of the PCM that may detect false error of the A/F sensor, resulting in false detection of P0421" and prescribes a Mode 6 check, a reprogram and a drive; when the first Mode 6 reading is between 0 and 2, the converter is judged deteriorated only if the value stays "less than 1". 🟢 Hino's bulletin for 2015 model-year trucks says to reprogram "prior to suspecting a fault with the ATC". 🟢 GM's engineering papers PIP5615 and PIP5615A for P0421 on the 2018 diesel Equinox and Terrain end at a calibration; its bulletin 18-NA-120 ends at a clamp (Route 2). 🟢 Ford's 24E12 recall says a PCM calibration "could, over time, reduce the effectiveness of the emissions control system and the monitoring accuracy" so the light may come on "even though a catalytic converter repair may not be necessary"; Ford's 24E04 says a monitor on 2022-2023 F-150 hybrids "may falsely set" P0420 or P0430.

What the owners report. 6 complaints end at software or a campaign and 4 of them say the problem came back; 9 end with the shop finding nothing or only clearing the light, and 1 says it returned. A 2016 Ram 2500 diesel owner wrote that the dealer found nothing wrong and that the truck was "NOW THROWING A CEL WITH THE CODE P0421." A 2020 F-150 owner replaced the converter and oxygen sensors, passed inspection, and in "less than one month" had the same codes back; Ford's service center told the shop the computer needed to be "erased and re-installed at my cost" (the owner writes that a 2020 Ford van "also has a 5.0 liter V8", which implies the truck has that engine; Ford's 24E12 recall covers only 2018-2020 F-150 with the 2.7L EcoBoost).

The cheap cause that gets skipped. The computer's own judgement. 🟢 Mazda's, GM's and Hino's 2015 papers say it can be wrong, and none of them has the shop buy a part first.

Fix it yourself. Only the paperwork: write down the code, year, engine and VIN, and ask for the bulletin or recall by number. Reprogramming is a shop job with the maker's tool, and a weak battery can ruin it (Mitsubishi's paper for its false P0420 asks for 12.4 volts or more).

At the shop — say this: "Is there a bulletin, recall or software update for P0421 on my model and engine, and was the computer's software level checked before any part was ordered? Please write the catalyst test values on the invoice."

Route 2 · The leak before the converter: clamp, flange, weld, manifold — about $100 to $400

What it is. Exhaust gas escapes or air gets in ahead of the sensors, or a joint has separated.

What the documents say. 🟢 GM's 2018 diesel Equinox and Terrain paper says P0421 "may be due to an exhaust leak between the turbo assembly and the catalytic convertor due to a miss-aligned clamp", with "No parts are needed for this condition" and 0.4 hour of labor; the follow-up service update adds 0.2 hour to inspect, 0.9 hour if the converter has to come out, and says "less than 1% involved vehicles will require parts replaced."

What the owners report. 5 complaints end at an exhaust part, and none says it came back. All five are broken exhaust parts rather than clamps: a 2017 Titan owner heard "a loud explosion sound" and found the front converter "completely separated/broke off from the exhaust system" at 38,000 miles; two Touareg owners wrote that the weld at the pre-cat pipe broke; a 2014 Cadillac XTS owner found the front converter "separated from the exhaust manifold" after exhaust entered the cabin; a 2016 Equinox owner wrote that the codes, among them a catalyst efficiency one, were "All caused by crack on exhaust manifold."

The cheap cause that gets skipped. A clamp, a gasket or a hairline in a manifold. Fumes in the cabin make this a safety item as well as an emissions one.

Fix it yourself. Only the looking. ⚪ Our guides carry $100 to $400 for a weld or gasket repair and warn that on a converter job a snapped stud turns a $700 job into a $1,500 one. Exhaust fumes in the cabin mean stop and ventilate, not diagnose.

At the shop — say this: "Please check every joint from the manifold or turbo to the converter, with the engine running if the paper for my car says so, and show me where it leaks. Is the clamp the one the maker prints?"

Route 3 · The sensor and the fuel it reads — $20 to $100 for a part, $200 to $400 installed

What it is. An A/F or oxygen sensor reads wrong, or a fuel problem on one bank makes the mixture swing, so the catalyst test fails although the converter works.

What the documents say. 🟢 Mazda's bulletin blames the logic that misreads the A/F sensor and orders no computer part; it sends the car on to the shop manual only if the converter still tests low after the update. 🟢 Honda's 14-002 says a lost A/F sensor cap "may have fallen off and bored a hole through the affected WU-TWC" and has the sensor and the converter replaced together, but only if the cap is missing ("If the cap is not missing, this bulletin does not apply"). 🟢 Toyota's 2022-2024 Tundra and 2023-2024 Sequoia Hybrid paper says "This condition may indicate a fuel restriction in the bank 2 fuel rail"; it replaces the left-hand converter only "IF DTC P043000 is present."

What the owners report. None of the 191 complaints ends at a sensor alone. Sensors appear beside converters: a 2013 Lincoln MKT owner was told two converters and four sensors, $4,189 plus tax.

The cheap cause that gets skipped. A sensor that is only reporting what the engine does. ⚪ Our catalytic converter guide carries the downstream-sensor prices; our oxygen sensor replacement guide has the full range of $150 to $500 installed.

Fix it yourself. ⚪ A downstream sensor is a $20 to $100 part and a ramp-and-socket job on many cars; the boundary is a seized sensor after years of salt, and the diagnosis that says it is the sensor is the hard part. (Our advice: never replace a sensor on the code alone.)

At the shop — say this: "What did the front and rear sensor readings show, and what are the fuel trims? Please give me the numbers before you replace a sensor or a converter."

Route 4 · The converter itself: manifold, front or warm-up cat — $200 to $4,500, or free in a program

What it is. The catalyst really has lost efficiency: a layer separating, a cracked ceramic, a clogged unit, a converter overheated by something else.

What the documents say. 🟢 Mitsubishi's paper replaces the exhaust manifold with its built-in converter on 2006-2008 V6 cars because of "catalyst layer separation", one side at a time, with labor of 2.4 hours (right) or 1.2 (left), under an extension that ended on its printed terms. 🟢 Toyota's 2015 campaign for 2010-2014 Tacoma 2TR-FE trucks replaces the front exhaust pipe with the converters because "the deteriorated components could become dislodged and restrict the exhaust flow." 🟢 Chrysler's recalls U90, U67 and Z98 replace the catalyst and update the computer on specific cars.

What the owners report. 46 complaints end at a converter (24% of 191), and 12 say the problem came back (26%). Other owners in the file: a 2008 Eclipse owner with 83,000 miles wrote the car "DIDN'T PASS THE EMISSIONS TEST. THEY ASKED ME TO FIX CODE P0421"; a 2006 Eclipse owner with about 80,000 miles in Washington wrote that the extension "IS NOT HONORED, DUE TO THE 80000 MILE WARRANTY", which matches Mitsubishi's printed state list (Washington is not on it); a 2003 Lancer owner paid "496.76$" for a new converter and had P0421 again within two years, and a Mitsubishi employee said the engine was burning oil.

The cheap cause that gets skipped. What destroyed the converter. If it was oil, misfire or a leak, the new one follows the old one.

Fix it yourself. Not the converter. ⚪ Our guides call a bolt-in unit on a rust-free car reasonable to do at home and anything cut, welded or cast into a manifold not one; the old unit has scrap value of $50 to $500.

At the shop — say this: "Which test shows the converter, not the sensor or a leak? Is my car on a maker program or an extension, and is the part a maker part or an aftermarket one that my state accepts?"

Route 5 · The engine behind the code: misfire, oil, fuel, overheating — $80 to $150 to find, more to fix

What it is. Something in the engine overloads or poisons the catalyst: a misfire, burned oil, too much fuel, or heat from a controller that lets the converter overheat.

What the documents say. 🟢 Kia's SC176Y paper for the 2012-2016 Soul ties a catalyst code to an engine check: "This Subsequent Repair Action can only be initiated/opened with a DTC P0420", with an active P030X sending the technician to a compression test ("Minimum 149 PSI") before the converter is replaced, and no charge to the owner. 🟢 Kia's safety recall 19V120 says "High exhaust gas temperatures may damage the catalytic converter, possibly resulting in abnormal engine combustion and damage to one or more of the engine's pistons" and lists an engine stall and, if leaking oil reaches the exhaust, "the risk of a fire" as consequences.

What the owners report. 18 complaints end at an engine-side cause and 4 say it came back; 105 end with no cause stated, and 18 of those say it came back. 38 of the 191 are Kia Souls (10 end at the converter, 19 with no cause stated, 5 at the engine, 3 at nothing found, 1 at a software update). A 2020 Soul owner wrote the dealer "found code for catalyst efficiency PO42000", found the upper oxygen sensor failed, the converter undamaged and a scored cylinder with a failed piston; a 2016 Soul owner wrote that the dealer replaced the converter "AT A TOTAL COST OF ALMOST $1300.00" and that "THE INTERMITTENT BUCKING AND GRINDING PERSISTS."

The cheap cause that gets skipped. The oil level: a converter replaced on an engine that burns oil faces the same cause.

Fix it yourself. Check the oil level every other fill-up and write down what you add; it costs nothing. ⚪ A compression test is $80 to $150 in our guides and is the simplest way to know whether you fixed a converter or bought one for an engine that is now down a cylinder.

At the shop — say this: "Is the engine burning oil, and was a compression test or a cylinder test done? What is the maker's paper for my engine, and does a recall or a follow-up program apply?"

The coverage file: which recalls and papers run in September 2026

Most of the papers above are dealer repair instructions, not coverage. Some are recalls, some are warranty extensions, some were limited-time offers that have ended. The status column is our computation from the printed terms, as of September 2026, and it is not a promise from any maker.

Emissions recalls are not in the safety-recall lookup. We found no NHTSA safety campaign for the Chrysler, Ford, Nissan or GM catalyst-efficiency items above in NHTSA's recall file as of October 1, 2026; the papers call them emissions recalls. nhtsa.gov/recalls may therefore show nothing; the dealer, with your VIN, is the lookup. Kia's 19V120 is the exception: it is a safety recall, and its renotification is dated September 15, 2026.

What 210 complaints about catalyst codes show

How we went through them. We went through every complaint in NHTSA's file received from January 2015 through September 2026 that names P0421, P0422, P0431 or P0432 in any spelling we searched, or describes a warm-up, pre- or front catalyst, or a catalyst efficiency fault, in owners' words (leaving aside 90 that name only P0420 or P0430 with the generic efficiency words, which belong to our P0420 guide): 76 from our first search, and 134 more from an independent second search (catalyst codes and efficiency wording written without the numbers we had searched). Every complaint was sorted twice, independently, by the same written rules; the two sortings matched on the category in 90% (189 of 210). Every disagreement was settled by a written rule, and a final consistency check under the same rules changed 4 complaints on which both sortings had agreed. We publish the sorting rules and how we did it. 191 describe a catalyst fault on the owner's car, 19 do not.

The same search found 1,389 more complaints about a converter blamed or replaced with a warning light but no catalyst code number and no efficiency wording. We did not sort them: they cannot be told apart from any other converter fault, and they belong with our catalytic converter cost guide.

Is it the converter? Of the 191, 46 end at a converter (24%). Of those 46, 12 say the light, the code or the symptom came back or the repair did not hold (26%). The returns by group: software or a campaign 4 of 6, the engine side 4 of 18, no cause stated 18 of 105, checked and cleared 1 of 9, exhaust parts 0 of 5. Owners whose trouble ended are mostly not in the file, so these shares describe complaints, not cars. No complaint ends at a sensor alone.

Which code. Only 9 of the 191 print P0421 (6), P0431 (2) or P0422 (1); 4 more print only P0420 or P0430. The other 178 write "the catalytic converter code", describe a catalyst efficiency fault or a recall letter.

Which cars. Kia is 42 of the 191 (Soul 38), Chevrolet 22, Jeep 21, Nissan 16, Hyundai 16 and Ford 12. Those are complaints, not failure rates. Of the 38 Souls, 23 mention a recall, a campaign or a bulletin. Among the 191, 18 complaints on Chrysler-group cars (12 of them Jeeps) name a catalyst-efficiency emissions recall: 17 have no repair stated and one had the recall done and the problem came back. Ten complaints in the file describe an owner told that the part for a recall or repair was not available.

What happened. 33 describe a stall, a loss of power or a limp mode. 6 describe a failed or threatened emissions test or inspection. 82 mention a recall, a campaign, a bulletin or an emissions warranty (43%). In NHTSA's own fields, 1 of the 191 is marked as a crash, none as a fire, an injury or a death, and 5 as a tow.

When and how old. 16 relevant complaints in 2015, 2 to 15 a year from 2016 to 2022, then 28 in 2023, 31 in 2024, 27 in 2025 and 21 in 2026 through September. The median model year is 2015; among the 84 that give a plausible mileage, the median is 100,000 miles.

Is a stall with a catalyst code dangerous, and does a new converter stop it? PulsCar sorted, twice and independently, the 210 complaints in NHTSA's file from 2015 to September 2026 that name a family code or describe a catalyst efficiency fault: 191 describe the fault on the owner's car, 33 of them a stall, a loss of power or a limp mode, 1 a crash in NHTSA's own field and 5 a tow. Where a complaint ends at a converter, 12 of 46 owners say the problem returned. None of the maker papers about P0421 says to stop driving, but Kia's recall lists an engine stall as a consequence of an overheated converter. Our rule: a stall, a loss of power, a flashing check engine light or fumes in the cabin is a tow, not a drive.

The nine complaints that print a family code

The nine complaints that print P0421, P0422 or P0431 are a small, mixed group, and the nearest thing the file has to owners describing your code. Four are Mitsubishis and one is a Mazda; the rest are a Ram, an Audi, a Volkswagen and a Volvo. Four end with no cause named, 2 at a converter, 2 at an engine fault and 1 with the dealer finding nothing.

The Mitsubishis line up with the printed paper. A 2006 Eclipse owner (ODI 10940222) and a 2008 Eclipse owner (ODI 10966701) wrote about P0421, the code and the model years of the manifold-converter extension (the 2008 owner is in Maryland and does not name the engine); the 2003 Lancer (ODI 11258370) and the 2001 Galant (ODI 10679585) are outside its 2006-2008 range. A 2010 Mazda3 owner (ODI 11290720) wrote that "3 AUTO REPAIR SHOPS LOOK AT MY CAR, AND THEY ALL SAY THAT THE CATALYTIC CONVERTER IS NOT OPERATING PROPERLY" and that Mazda would not cover it because the car was not in a state that uses California standards; Mazda's bulletin covers listed 2014-2016 cars, not a 2010. A 2022 Tiguan owner (ODI 11682731) had P0431 with three other codes and a failed crankcase pressure regulator after a loss of acceleration on the highway; a 2019 XC40 owner (ODI 11607460) wrote "Diagnostic code failure P0422 Evap system failure", so owner and code table disagree on the system (counted because the printed code is P0422).

Two things stand out: in three of the nine the code sits beside other codes, and 3 of the 9 describe a stall, a loss of power, a car that would not rev or a no-start, so the code that sounds mild comes with symptoms that are not.

What owners say they paid

Of the 191 relevant complaints, 14 state a dollar figure tied to the repair, 16 figures in all: 9 quotes, 3 bills and 4 amounts paid. The figures are what owners wrote, not audited invoices, and we publish them with ODI numbers; every exclusion has a reason in our notes.

How much does it cost to fix P0421 in 2026, and when is the converter the wrong purchase? PulsCar took every dollar figure in the 191 relevant complaints that meets one written inclusion rule: 16 figures in 14 complaints. A converter replaced or quoted runs $497 to $13,500 with a median of $1,633 across 10 figures, a diagnostic fee $100 to $235. Our repair guides carry $100 to $400 for an exhaust leak repair, $200 to $400 for a downstream sensor installed and $200 to $4,500 for a converter, and a recall, warranty or program can make the fix free. The cheap rungs rule out the software, the leak and the engine first; 12 of the 46 owners whose complaint ends at a converter say the problem came back, so a part bought on the code alone skips the cheap causes.

Five of the converter figures are on Kia Souls ($1,300 billed, $1,486 and $1,780 paid, $1,300 and $2,000 quoted). Kia's recall and follow-up program replace the converter and, if needed, the engine free of charge on covered VINs, but refunds are limited: one of these owners was refused because Kia reimburses only repairs made before the recall and the owner's repair came after the recall service, and another writes that the VIN is not in it. A Soul owner who paid should ask, with the VIN, about both papers. The $13,500 estimate (ODI 11524023, a 2014 Audi RS5 with 60,000 miles) is for a car whose dealer said there was "no reason why the converters should be bad based on the vehicle history."

Why P0421 comes back after a repair

The converter was the victim. Burned oil, a misfire or a leak gives the same code with a new converter in the car; a 2003 Lancer owner paid $496.76 for a converter and had P0421 again within two years, and a Mitsubishi employee pointed to oil burning. The paper was skipped. A false alarm comes back when the computer's software is not updated; one owner had a converter and sensors fitted, the same codes back within a month, and was told the computer needed reprogramming. The fix was a clear. A cleared code on a leak that is only partly sealed can return as soon as the self-checks run again. In the file, 39 of the 191 say the same light, code or symptom returned or the repair did not hold.

Can you drive with P0421?

It depends on how the car behaves, and this is our rule, not any maker's: none of the papers we read about P0421 says to stop driving. A steady light with normal driving, no stalls, no hesitation, no power loss, no smell: a shop within days. If it has stalled, lost power, will not pick up speed, the check engine light is flashing, or exhaust fumes reach the cabin, do not drive it to the shop: have it towed. 🟢 Kia's recall says an overheated converter can end in piston damage and "an engine stall, increasing the risk of crash," and that leaking oil on the exhaust adds "the risk of a fire"; Kia asks owners to contact a dealer immediately, and the repair is free. 🟢 Toyota's 2015 dealer letter for its 2010-2014 Tacoma campaign says a deteriorated converter's parts can "restrict the exhaust flow" and the truck "may experience a reduction in power." Our view is that fumes in the cabin are their own warning. A 2024 Blazer owner wrote of "a sudden and catastrophic loss of engine power" while merging onto a freeway, with a solid, not flashing, light.

Should you buy a used car with a P0421 in its history?

Ask what was found, not what was replaced. A history with a software update, a clamp or a manifold repair and a clean drive afterwards is better than a converter replaced and the code returning. Ask for the invoice that names the test that showed the converter, ask a dealer with the VIN about recalls and programs, and before you pay, have the car scanned, driven for a full warm-up and scanned again.

Clearing P0421 before an inspection

A cleared code comes back if the cause is still there: a leak, a worn converter or an engine that burns oil, and an emissions test needs the car's self-checks to have run again; 6 of the 191 complaints mention a failed or threatened test. One 2008 Eclipse owner failed a test at 83,000 miles and was told to fix P0421. Clearing can also cost you coverage: a program may require the code to be stored, as 🟢 Mitsubishi's now-ended extension did (the light on and P0421 or P0431 recorded). Fix first, drive several normal trips, then test. Our guide on how to reset a check engine light covers the mechanics.

What the papers tell a shop to do with P0421 and its sister codes, checkpoint by checkpoint

Checkpoint 1 — the other codes. Mitsubishi's bulletin for a false P0420 on 2008-2010 Outlander and 2009-2010 Lancer: "DO NOT reprogram the ECM until the reason(s) for other DTCs being set is corrected." Ford 24E04: other codes mean diagnosis outside the program. Checkpoint 2 — the software. Hino (bulletin for 2015 model-year trucks) and Volvo/Mack: update the engine computer first; Ford 24E12: reprogram, no parts. Checkpoint 3 — the leak. GM: the clamp between the turbo and the converter (18209: with the engine running). Land Rover: the exhaust joints. Checkpoint 4 — the test. Mazda: Mode 6 catalyst monitor, reprogram, a prescribed drive, test again. Ford 24E04: a sulfur-removal drive, then the oxygen-storage value; replace the converter only above 0.4. Subaru: a catalyst index test. Checkpoint 5 — the engine. Kia: a compression check with an active P030X. Land Rover: oil consumption and ignition. Checkpoint 6 — the part. Only then the converter, and on Mitsubishi's manifold converters, the affected side only.

Three P0421 traps

Trap 1 · Buying the converter from the code. The code says the computer judged the catalyst low; it does not say which part. GM's first P0421 paper said not to replace parts while diagnostics were inconclusive, and its revision points to a software fix; Hino's bulletin for 2015 model-year trucks says to reprogram first (its 2020 bulletin for 2016-2020 trucks replaces the ATC after an exhaust brake check); Mazda orders no part.

Trap 2 · Replacing the converter on an engine that burns oil or misfires. The new converter faces the same cause; 38 complaints in the file describe an engine burning or using oil.

Trap 3 · Assuming a program covers your car or your state. Mitsubishi's extension lists four states and, for 2008 models only, three more; a Washington owner was told it was not honored. Mazda's bulletin lists VIN ranges, so a dealer's answer by VIN is the only answer.

Three P0421 claims, decoded

"P0421 means the converter is dead." It means the computer judged a catalyst low; Mazda, Hino, GM and Ford print false alarms, a misaligned clamp and a monitor that may be wrong.

"P0421 is just another name for P0420." The makers use the numbers differently: Honda calls its P0420 converters warm-up, and Mitsubishi prints the warm-up words beside P0420 and beside P0421.

"A recall will pay for it." For specific cars, and not always the converter: the Kia Soul, Chrysler's U90, U67, Z98 and V86 cars and the hybrid F-150 (24E04) can include a new converter, while GM's 2013-2014 Cruze and Sonic 1.8L, the 2.7L F-150 and the NV200 recalls are free computer updates. For the rest, no; and ten complaints in the file describe an owner told the part was not available.

The P0421 plan: 10 minutes, today, this week

Today (10 minutes). Engine cold, look and smell around the exhaust, check the oil level, read every code, note the mileage. Today (VIN). Ask a dealer about emissions recalls, extensions and your model's P0421 or catalyst bulletins. This week. If the light stays on, pay for one diagnosis, ask for the catalyst test values in writing and for the bulletin number, and let the shop try the cheapest route first. If the car has stalled, lost power or will not pick up speed, the check engine light is flashing, or fumes reach the cabin, do not drive it to the shop: have it towed. Do not wait the week.

These are composite scenarios, not individual customers; the labor times and dollar figures come from the papers and owner reports above. If you drive a 2014 Mazda3 with P0421 and no driving problem, ask for 01-010/16 by number before you accept a converter quote. If you drive a 2018 diesel Equinox with P0421 and an exhaust smell in the cabin, ask for a clamp check first; GM's paper prices it at 0.4 hour with no parts. If you drive a 2006-2008 V6 Eclipse or Galant registered in California, Maine, Massachusetts or Vermont, know that the extension has ended on its printed terms.

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. General Motors: Service Bulletin 18-NA-120; Service Update 18209; Preliminary Information PIP5615 and PIP5615A; 21-NA-214 Version 5 with Version 4, Version 3 and Version 2; PIP5917; emissions recall 16186.
2. Mazda: Service Bulletin 01-010/16 and the version it replaced, 01-016/15.
3. Mitsubishi: TSB-13-15-001 (warranty extension for P0421 and P0431) and TSB-12-13-005.
4. Stellantis (FCA, Ram, Dodge, Jeep, Chrysler): 18-015-17 Rev. B; 18-010-21 Rev. B; emissions recalls U90 Revision 7, U67 Revision 11, Z98 and V86 Revision 3.
5. Hino, Volvo, Mack: Hino SB-14-012, SB-15-003 and SB-20-001; Volvo and Mack solution K14003352.
6. Ford and Nissan: emissions recalls Ford 24E04 and 24E12; Nissan R21B3.
7. Honda and Acura: 14-002; 18-022 and 18-023; 22-002; the dealer message AHPa06212018901; the July 2023 stop-sale and emissions recall message 10239702.
8. Subaru, Toyota, Land Rover, BMW: Subaru WVH-18R; Toyota Special Service Campaign F0U and T-SB-0030-23 Rev2 with the original; Land Rover SSM74404; BMW SI B12 09 14.
9. Kia: SC176Y Rev. 1; safety recall 19V120 (SC176): NHTSA acknowledgement, the dealer campaign bulletin Rev. 4 of July 13, 2023 and the owner renotification of September 15, 2026; recall page 19V120.
10. PulsCar analysis of NHTSA public data, October 2026: 210 complaints about catalyst codes, each sorted twice by the same written rules; owner-reported amounts; the document list with check status and links. Complaints, amounts, documents, the sorting rules and how we did it (CC BY 4.0).

For the neighbouring pages: the P0420 guide and the P0430 guide for the sister codes, catalytic converter replacement cost, oxygen sensor replacement cost, P0171 and P0300 for the lean and misfire codes that can sit beside a catalyst code, a car that stalls while driving, how to reset a check engine light and the check-engine code index.

PulsCar does not diagnose a catalyst efficiency code from the sound of the engine. For P0421, the test values at the converter and the documents above are the evidence.

Methodology. The complaint set is every complaint in NHTSA's file received from January 2015 to September 2026 that names P0421, P0422, P0431 or P0432 in any spelling we searched, or describes a warm-up, pre- or front catalyst or a catalyst efficiency fault in owners' words (76; 90 that name only P0420 or P0430 with the generic efficiency words were left to the P0420 page), plus 134 found by an independent second search that describe a catalyst code or efficiency fault without the numbers searched: 210 in all. Complaints about a converter blamed or replaced with a warning light but no code number and no efficiency wording (1,389) were found and not sorted. Every complaint was sorted twice, independently, by the same written rules, fixed before each batch was sorted; the two sortings matched on the category in 90% of them (189 of 210), every disagreement was settled by a written rule, and a final consistency check changed 4 complaints on which both had agreed. 191 describe a catalyst fault on the owner's car and 19 do not. Counts are counts of complaints, not cars, and nothing here is a failure rate. The document set is every NHTSA index entry whose summary names one of the four codes or discusses a warm-up, pre- or front catalyst efficiency fault (101 entries, 18 of them added as known catalyst-efficiency recalls and programs), 49 entries from the second search and the 17 papers of the Kia Soul recall: 167 entries, 166 of them with a PDF, of which 119 are distinct once 47 exact copies are set aside. We went through 88 documents in full and compared 78 revisions and copies with them line by line. Model years, engines and build dates are as printed in the documents, never from NHTSA's index. Coverage status "as of September 2026" is our computation from printed terms. Amounts follow one written inclusion rule, and every exclusion has a reason in our notes.

Prices verified July to October 2026 in the repair guides they are carried from and in the owner reports above, and reviewed quarterly. Technical documents verified October 2026: the 119 distinct documents in our published file, of which 88 were read in full — among them Chrysler's Z98 of August 2026, U67 Revision 11 of June 2026 and Kia's SC176Y of March 23, 2026.

Had P0421 cleared by software, a clamp, a manifold, a converter or an engine repair, or a converter replaced for nothing? Email [email protected] with the vehicle, mileage and outcome.