P0420 Code: The Converter Is Usually the Victim

Short answer

A converter worn out by a misfire or lean fuel

A P0420 code is the car reporting that the catalytic converter has stopped storing oxygen. Replacing catalytic converters is the most common check engine repair in America, averaging $1,511, though what one actually costs runs from $650 to $2,500 per converter. It's also usually the last step in a chain that started somewhere cheaper.

OilAndMiles.com - The P0420 code is the single most searched OBD-II code in the United States, a DTC in the words a shop and a scan tool use, and almost every page answering it opens with the price of a catalytic converter. That's the right part to worry about and the wrong place to start.

What follows is what the code measures, why the converter is usually the thing that got killed rather than the thing that failed, and what 39 million repairs say it costs.

Everything here applies equally to P0430, which is the identical fault on the other bank of a V engine. The two codes differ by which side of the engine reported it, and by nothing else.

A P0420 code is a measurement, not a diagnosis

The engine computer doesn't inspect the converter. It runs a test, and the test works by comparison. If you aren't sure the converter is even the problem yet, the symptoms of a bad catalytic converter split into two different failures and only one of them sets this code.

There are two oxygen sensors in the exhaust: one upstream, before the converter, and one downstream, after it. The computer deliberately commands rapid rich and lean swings in the fuel mixture, then confirms on the upstream sensor that those swings are happening. Having established that, it watches the downstream sensor.

A healthy converter stores oxygen. It absorbs those swings, so the downstream signal comes out relatively flat. When the downstream signal starts mirroring the upstream one, the converter has stopped absorbing anything, and that's the moment P0420 is set.

One commanded rich and lean swing, read on the upstream sensor and then sent into two catalytic converters drawn in section. In the one that still stores oxygen the swing dies away inside it and the downstream sensor reports a flat trace, so no code sets. In the one that stores nothing the swing reaches the outlet at full size and the downstream trace matches the upstream one, which sets P0420 — The test is a comparison, not an inspection. A converter that still stores oxygen flattens the swing; one that doesn't, can't. Diagram: Oil & Miles

What the data says

The two bank codes are the same measurement and they aren't reported at the same rate. Across 2010 to 2025 model years, complaints name P0420 276 times and P0430 87 times, roughly three to one toward bank 1.

One make inverts it. Honda is the only one where the bank 2 code outnumbers bank 1, 46 against 27, and the Pilot alone carries 40 of those 46. Every other make with double figures runs the other way, Kia 98 to none and Chevrolet 40 to 2.

A code that is supposed to be side-agnostic isn't arriving side-agnostically, and on one V6 it arrives mostly on the side you can see from the front of the engine bay.

NHTSA consumer complaints, FLAT_CMPL bulk file dated August 25, 2026, counted September 1, 2026. Model years 2010 to 2025, one row per complaint after de-duplication. Counts are mentions of the code inside the complaint text, not a failure rate.

What owners report

On Honda Pilot Forums the same sequence is described over and over by different people, and it's the sequence this page is about.

December 2023, a 2010 Pilot at 147,000 miles: “I got a p0430 and the car was burning oil so I thought it is probably the cat. So I got cat Bank 2 replaced and the light came back after 50 miles.”

February 2024, a 2011 Pilot: “ive already replace both catalytic converters and 02 sensors and for some reason keep getting the same codes.”

June 2024, a 2010 Pilot at 175,000 miles, is the whole argument in one post. Both codes in January 2023, all three original-equipment converters and new sensors fitted at once, and “About 18 months later, light on again with codes p0301, p0302, p0303.” Three misfire codes, on the cylinders, after the converters had been bought.

Honda Pilot Forums, read at source September 16, 2026. Taken for the sequence each owner describes and what it cost them, never for the diagnoses offered in the replies.

Worth knowing

This is why "just replace the cheap oxygen sensor first" is worse advice than it sounds. A working sensor is what makes the test possible.

If the sensor had failed, the computer would set an oxygen sensor code instead, because it couldn't run the catalyst test at all. Getting a P0420 means the sensors were healthy enough to measure with.

Why the converter is usually the victim

A catalytic converter has no moving parts. It's a ceramic honeycomb coated in precious metal, sitting in the exhaust stream, and left alone it lasts a very long time.

What kills a converter is something arriving from upstream. The fault that usually sent it there is a misfire or a lean running condition. It also helps to know what is actually inside one: three precious metals on a ceramic honeycomb, none of which wears out on its own.

CarMD publishes an annual index built from repairs recommended across the United States. In the 2026 edition, covering 39,116,755 repairs in calendar year 2025, catalytic converter replacement ranks first, and the report says this about it:

"A catalytic converter usually won't fail on newer vehicles unless a related root cause like a faulty spark plug or O2 sensor is ignored."

That's the company whose data puts converter replacement at number one saying the converter is normally a consequence. Three things do the killing, and they're the three the drawing numbers:

Line diagram of the exhaust path running left to right: an upstream oxygen sensor, the catalytic converter, a downstream sensor and the tail pipe. Three numbered causes sit above it and feed into the pipe ahead of the converter: 1 a spark plug not firing, 2 a mass air flow sensor, 3 a head gasket letting coolant into the chamber. A band under the pipe steps thicker at each one and stops at the converter inlet. A flanged joint before the upstream sensor is drawn open, with fresh air arrows entering it — Everything that kills a converter reaches it from in front. The band gets heavier at each cause and stops where the converter starts. Diagram: Oil & Miles

A misfire. Unburnt fuel reaches the converter and burns inside it instead of in the cylinder, and that's what melts a honeycomb. Ignition coils and spark plugs are ranks three and five on the same CarMD list.
Running rich or lean for a long time. A mixture that is persistently wrong overheats the catalyst. Mass air flow sensor replacement is rank four.
Coolant reaching the combustion chamber. A failing head gasket puts coolant where fuel should be, and it poisons the catalyst on the way past. If white smoke came before the code did, our guide to white smoke from the exhaust is the place to start rather than here.

A fourth thing sets the code without killing anything, which is why the drawing leaves it unnumbered: an exhaust leak ahead of the sensor. Fresh air drawn in through a leak reaches the sensor and reads as lean, and the computer corrects for a condition that's not real. Fix the leak and the converter was never the problem.

If you have a misfire code or a lean code sitting alongside the P0420, that's not a coincidence. That's the sequence, showing you both ends of itself at once.

Which cars report a P0420 code most

From the national complaint record

0.30%

of Kia complaints name P0420. That's four times the next one, so this isn't a leaderboard with a close race at the top. It's one outlier and a pack. Within that make it is almost one vehicle: 87 of the 98 are the Soul.

Share of each make's NHTSA complaints that name P0420, model years 2010 to 2025, 645,169 complaints read; the Kia (0.30%) is left out of the bars because at that distance every other bar would be a hairline.
Hyundai	0.08%	33 of 40,008
Chevrolet	0.07%	40 of 58,248
Honda	0.06%	27 of 46,500
Ford	0.03%	34 of 130,824

Source: NHTSA consumer complaints, the public database, published 25 August 2026 and read 22 September 2026. We counted every complaint filed on model years 2010 to 2025, 645,169 of them. Makes naming the code fewer than 10 times are left out, because below that a ranking is noise. Complaints measure who reported a fault, not how often a part fails.

What the repairs cost

These are US averages across 39 million recommended repairs in 2025, parts and labor together, from the CarMD Vehicle Health Index.

Read it as a sequence rather than a menu. Ranks three, four, five and ten are the things that kill converters, and they run $256 to $480 fitted. Rank one is what it costs after they were left alone, and it is $1,511.

The figure on each row is what the job bills, parts and labor together. The part on its own sits underneath it, and the gap is labor, the shop's margin, and on a converter the hours of getting the old one off a car driven on salted roads.

What it costs

What a P0420 code costs to fix, every repair fitted

Six separate repairs, so there's no total. The cheapest is what gets tried first, and the dearest is what the others prevent.

Catalytic converterRank 1, and 7.43% of 2025 repairs. Normally the last step rather than the first. The EPA-grade part alone is $107 to $500 across the three cars, and the CARB part California, Colorado and New York require runs $218 to $924	$1,511Parts and labor, US average
Oxygen sensorRank 2, 6.68%. The part alone is $53 to $189 upstream, Bosch at both ends of that, and $32 to $91 downstream, SKP up to Bosch. The upstream one is the sensor this code depends on	$287Parts and labor, US average
Ignition coils and spark plugsRank 3, 6.00%. This is the misfire that melts a honeycomb, and it's the commonest thing to fix first. One coil is $13 to $70, SKP up to Spectra Premium, and one plug $3.22 to $16, Champion Platinum Power up to Denso Iridium Long Life	$480Parts and labor, US average
Mass air flow sensorRank 4, 5.77%. A mixture persistently rich or lean overheats the catalyst, and this sensor is the usual cause. The part alone is $31 to $272, SKP up to Pierburg. Remanufactured ones carry a core charge of $15 to $62 on top	$346Parts and labor, US average
Ignition coilsRank 5, 5.02%. At $13 to $70 a coil, a four-cylinder set is $52 to $280 and the V8 needs eight, so the parts can be a fifth of this bill or more than all of it	$256Parts and labor, US average
Spark plugsRank 10, 1.84%. At $3.22 to $16 a plug, a set of four is $13 to $66 depending on the metal, and the V8 needs eight	$299Parts and labor, US average

Figures are CarMD Vehicle Health Index 2026 US averages across 39,116,755 repairs recommended in 2025, parts and labor together, read August 25, 2026. Part prices underneath are every brand RockAuto lists for a 2010 Honda Accord 2.4L, a 2012 Ford F-150 5.0L V8 and a 2013 Nissan Altima 2.5L sedan, read September 16, 2026. Left out after reading them: wholesaler closeouts carrying a 30 day warranty, coil sets and service kits sold as one line, and the Altima coupe's third oxygen sensor, which the sedan doesn't have. Converter prices are the same three-car census set out in full on our replacement-cost page.

The average cost of addressing any check engine light in 2025 was $554, a record, on a fleet whose average age reached 12.8 years.

At the other end of the same top ten sits the evaporative purge valve at $172, and our guide to the P0456 code covers that family. One more thing before you compare quotes: estimator sites commonly put a converter replacement above $2,300.

The CarMD figure is lower because it's a different kind of number: a count of what 39 million recommended repairs actually came to, rather than a modeled estimate. Neither is wrong, and if your quote sits between them, that's ordinary.

Worth knowing

A catalytic converter isn't an ordinary part. Federal rule 40 CFR 85.2103 names it a major emission control component, and those carry a warranty of 8 years or 80,000 miles, whichever comes first. Other emission faults on a light-duty vehicle carry 2 years or 24,000 miles.

Most cars that reach a P0420 are past both figures, because the US fleet now averages 12.8 years. But the two numbers cost nothing to check, and being wrong about them costs $1,511.

People search this code with their car attached, and the five makes typed most often are Chevrolet, Honda, Ford, Nissan and Toyota. We counted how often owners of each actually filed a federal complaint naming P0420, and the two lists barely overlap.

Read the percentage rather than the count. Ford has four times as many complaints on file as Kia about everything, so a raw count would rank Ford higher for any fault at all.

Chevrolet, or Chevy, and no single model

40 complaints, 0.069% of every Chevrolet complaint filed. No single model carries it: the Equinox leads on 10 of the 40, 25%, the Traverse follows on 9, and the rest are spread across the Cruze, the Trax and a dozen others. That's unusual on this list, and it's worth knowing before a shop tells you the Equinox is known for it, because on this evidence it's not.

Honda, and the Pilot

27 complaints, 0.058%. The Pilot is the biggest single share at 10 of the 27, about 37%, and the Odyssey and Fit follow. Honda's figure is close to Chevrolet's once the different complaint volumes are accounted for, which is the point of using a percentage.

Ford, and the Focus

34 complaints, 0.026%. That is the lowest rate of the five makes that appear at all, and it happens on the largest complaint base in the file, 130,824 filings, with the Focus carrying 17 of the 34, exactly half. A raw count would have put Ford near the top of this list; the rate puts it near the bottom.

Toyota and Lexus, where the number is zero

Not a single complaint. Across 36,545 Toyota complaints and 3,708 Lexus complaints on 2010 to 2025 model years, none names P0420. Nissan is barely different: three, out of 29,502.

Now put that beside the search box, where p0420 toyota and its variants are asked more than four thousand times a month. Those owners exist. They aren't writing to the federal government about it.

That gap is the most useful thing on this page, and it's not about Toyota. A complaint is a grievance, not a diagnosis. People file when they feel wronged: when a repair was refused, when a car failed young, when a dealer said no.

A check engine light on a high-mileage car that still drives normally doesn't produce that sense of being wronged, so it doesn't produce a filing. Read this record as who was angry enough to write, never as who has the fault.

Kia, the one nobody searches and the record puts first

98 complaints, 0.301%. That is nearly four times the next make on the list and the highest rate we measured for any code in this family. And within Kia it is almost one vehicle: 87 of the 98 are the Soul, concentrated on the 2015 and 2016 model years.

Kia doesn't appear in the brand terms people search for this code. The record puts it first by a distance. Both of those can be true at once, and holding them together is the whole skill of reading a complaint database.

Can you drive with a P0420 code

Usually yes, for a while, and that's exactly what makes it expensive.

Search this code and you'll find lists of symptoms: rough idle, stalling, poor acceleration, a smell of sulfur. Read those carefully, because almost none of them is caused by P0420. They're caused by whatever caused the P0420.

A converter that has stopped storing oxygen is usually silent, and a car that idles roughly has a misfire, which is the thing killing it. The exception is a converter that has broken up inside and blocked, which does strangle the engine and is felt as a car that won't pull.

A steady P0420 on its own doesn't threaten the engine. The car will run, and most people notice nothing beyond the light. What it does threaten is your emissions test, because a converter reporting low efficiency fails one in every state that runs OBD testing.

The answer changes completely if the light is flashing, or if a misfire code is present alongside it. A flashing check engine light means fuel is reaching the exhaust right now, and continuing to drive is how a $256 coil becomes a $1,511 converter. Our guide to what a burning smell means covers the version of this you can smell before you can read it.

P0430 is the same code on the other side

A V6 or V8 has two cylinder heads, two exhaust manifolds, and in most cases two converters with their own sensors. Bank 1 is the side containing cylinder number one. Bank 2 is the other one.

P0420 reports the catalyst on bank 1. P0430 reports the catalyst on bank 2: same test, same causes, same prices, different side. An inline four or six has one bank and therefore can't set P0430 at all.

Seeing both at once is worth taking seriously. Two converters rarely fail independently at the same moment, so both banks reporting usually points at something feeding them both: fuel trim, or a reading problem, rather than two dead converters.

The record doesn't treat the two sides equally, and the split is sharper than "same code, other bank" suggests. Across the complaints read for this article, P0420 is named 288 times and P0430 91 times.

The 288-to-91 ratio holds for most makes. It doesn't hold for Honda.

Honda is the only make in this data that reports P0430 more often than P0420: 46 against 27. Almost all of it is one vehicle: the Pilot alone accounts for 40 of those 46, against 10 P0420 mentions on the same model. Ford is second on P0430 with 22, fifteen of them on the F-150, but Ford still names P0420 more often overall.

Chevrolet runs the other way and just as hard: 40 P0420 against 2 P0430. Kia names P0420 98 times and P0430 not once, though nearly all of Kia's are the Soul, which has an inline four and therefore has no bank 2 to report.

Read that as a hint about where to look first on a V6, not as a failure rate. These are counts of owners who typed a code into a complaint, so they measure reporting, and 91 mentions is a small base to divide thirteen makes between. What it is good for is this: on a Pilot with a converter code, bank 2 is where the record says the story usually starts.

What to do about a P0420 code

Four questions settle whether it is really the converter, and all four are free to ask. 4 steps below:

Ask what other codes are stored. A misfire or lean code alongside P0420 names the cause. Fix that first and the catalyst code may not return.
Ask whether the exhaust was checked for leaks ahead of the sensor. This costs minutes and it changes the answer.
Ask what the fuel trim numbers are. A shop that can't tell you hasn't looked.
Ask why the converter failed. If nobody can answer that, the replacement is a guess, and the new one is exposed to whatever killed the old one.

All four are questions about the estimate rather than about the car, and how to read a repair estimate covers the lines a shop expects nobody to query.

None of that means leaving the converter alone. A catalyst that has stopped storing oxygen doesn't recover when the misfire is fixed, so on most cars that reach this code the answer is both jobs, not one. What the order changes is the bill: find the cause first and you fit one converter, fit the converter first and you may fit two.

From the documents

What is the code actually measuring?

Conversion capability, against a ratio. The rule that defines this monitor never mentions a part failing. It defines a measurement and a threshold, which is why a P0420 can be set by a converter that is physically intact:

13 CCR 1968.2 (e)(1.1), OBD II final regulation order

The OBD II system shall monitor the catalyst system for proper conversion capability.

Read at source as the published PDF, California Air Resources Board, September 16, 2026. Conversion capability, not condition.

13 CCR 1968.2 (e)(1.2.1)(B), malfunction criteria

The average FTP test Non-Methane Hydrocarbon (NMHC) conversion efficiency of the monitored portion of the catalyst system falls below 50 percent (i.e., the cumulative NMHC emissions measured at the outlet of the monitored catalyst(s) are more than 50 percent of the cumulative engine-out emissions measured at the inlet of the catalyst(s)).

Same document. The threshold is a ratio between what goes in and what comes out.

Read the second quote twice. The test compares what enters the catalyst with what leaves it, and anything that changes either side of that ratio moves the result, whether or not the catalyst is the thing that changed.

Conclusion

The code names a measurement, not a part

P0420 reports what the converter stopped doing, not what broke. The computer swings the mixture, confirms it on the upstream sensor, and reads how much survives to the downstream one. A flat trace means the catalyst still stores oxygen; a matching trace means it doesn't, and that's all the code knows.

Which is why the order matters more than the number. Replacing catalytic converters averaged $1,511 in 2025 and was the most common check engine repair in America, while the coils, plugs and air flow sensors that kill converters run $256 to $480. The company whose data puts the converter first says it normally fails only when a root cause upstream was ignored.

Oil & Miles prices the parts at a counter and the jobs from a repair count, across repair costs and symptoms.

Common questions

Asked most often

What does the P0420 code mean?

Catalyst system efficiency below threshold, bank 1. The computer compared the oxygen sensor before the converter with the one after it and found the converter is no longer storing oxygen.

How do you fix a P0420 code?

By finding what stopped the converter working. Misfires, a persistently rich or lean mixture, and exhaust leaks all cause it, and all cost less to fix than the converter.

Is P0420 always the catalytic converter?

No. It reports a measurement of catalyst output, and several upstream faults produce that measurement. CarMD's own report says converters usually fail only when a root cause was ignored.

What is the difference between P0420 and P0430?

The bank. P0420 is the catalyst on bank 1, P0430 is bank 2. Everything else about them is identical, and an inline engine can't set P0430.

Can I drive with a P0420 code?

Usually yes if the light is steady, though it will fail an emissions test. If the light is flashing or a misfire code is present, stop, because that's how a coil becomes a converter.

How much does a P0420 repair cost?

Replacing the converter averaged $1,511 in 2025. The upstream repairs that prevent it, coils, plugs and mass air flow sensors, run $256 to $480.

What people ask next

Sources

Per-make counts for the section above, from the same NHTSA bulk file and verified a second time against the raw rows, because a figure of zero is worth checking twice. NHTSA consumer complaints, FLAT_CMPL, read August 27, 2026
Honda Pilot Forums, for three owner accounts of both codes returning after converters were replaced. Taken for what each owner did and what happened, not for the replies. Read September 16, 2026
CarMD Vehicle Health Index 2026, read as the published PDF, for every cost and share figure on this page. CarMD sells diagnostic tools, an interest worth naming. CarMD.com Corp, read August 25, 2026
RockAuto, every brand listed for a 2010 Accord 2.4L, a 2012 F-150 5.0L V8 and a 2013 Altima 2.5L sedan: coils, plugs, oxygen sensors, air flow sensors. Closeouts and coil sets left out. RockAuto, read September 16, 2026
40 CFR 85.2103, the federal emission performance warranty, read at source: subsection (c)(1) for the two warranty periods, (d)(1) for catalytic converters being a major emission control component. Cornell Legal Information Institute, read August 25, 2026
The catalyst monitor logic, that the computer commands rich and lean swings, verifies them on the upstream sensor and reads the downstream one, and that a working sensor is therefore a precondition for the code. ricksfreeautorepairadvice.com, read August 25, 2026
The $554 average check engine repair and the 12.8 year average vehicle age are both from the same CarMD 2026 index. CarMD.com Corp, read August 25, 2026

How this was compiled

The complaint counts come from NHTSA's published bulk download rather than its search page, so the whole record is counted rather than a sample. The file is FLAT_CMPL, and the copy read here is dated 25 August 2026.

NHTSA files one row per component a complaint names, so the count is by complaint number rather than by row: across the 2010 to 2025 model years that is 645,169 complaints, and those are what was scanned for this article.

The cost figures were taken from one source rather than averaged across several, because that source counts repairs rather than estimating them, and mixing a count with estimates would produce a number that means nothing. The mechanism section was written from how the monitor works rather than from a list of causes, which is why the order of blame here differs from most pages on this code.

What we didn't do

We didn't scan a car or clear a code for this article.
We give no figure for how often P0420 turns out to be something other than the converter. That number is quoted widely as 25% to 35% and we couldn't find a primary source for it, so it's not here.
We don't name converter prices by model. The $1,511 is a national average across every vehicle in the sample, and a specific car can sit a long way from it.
We didn't test whether a used or aftermarket converter clears the code. Emissions rules on aftermarket converters vary by state and that's a separate question.

Prices are US averages compiled in August and September 2026 and vary by region, engine and shop rate. Prices from $10 up are rounded to the nearest dollar, so $14.13 reads as $14 and $14.52 as $15. Below $10 they're exact. This article is general information, not advice about your specific vehicle, see the disclaimer.