P0420 Code: Bank 1 Meaning, Causes and Repair Cost

The P0420 code is an OBD-II code, a DTC in the words a shop and a scan tool use. The first number you'll hear quoted for it may be the price of a catalytic converter, the right part to worry about and the wrong place to start.

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.

What does a P0420 code mean?

Your car's 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 your exhaust: one upstream, before the converter, and one downstream, after it. The computer deliberately commands rapid rich and lean swings in the fuel mixture. It confirms on the upstream sensor that those swings are happening. Then it watches the downstream sensor.

A healthy converter stores oxygen, and that's what you're paying for. 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. That's the moment P0420 is set. The test only compares a reading with a limit, so a converter that still works can fail it.

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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.

A dead sensor usually sets an oxygen sensor code of its own, because the computer can't run the catalyst test without it. A worn one can still read wrong without a code, so the shop should check its signal before anyone prices a converter.

The downstream oxygen sensor

Can you drive with a P0420 code?

Yes, for a while, and that's exactly what makes it expensive for you. The car keeps running while whatever set the code keeps working on the converter.

A steady P0420 on its own doesn't threaten the engine. Your car will run, and you may notice nothing beyond the light.

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 coil job becomes a converter job.

Our guide to what a burning smell means covers the version of this you can smell before you can read it. That's when unburned fuel is already cooking the converter.

What are the symptoms of a P0420 code?

Rough idle, stalling, poor acceleration and a smell of sulfur often get listed as P0420 symptoms, but almost none of them is caused by P0420. They're caused by whatever caused the P0420.

A converter that's stopped storing oxygen may give you nothing beyond the light, and a car that idles roughly has a misfire, which is the thing killing it. The exception is a converter that's broken up inside and blocked, which does strangle the engine, and you feel it as a car that won't pull.

What causes a P0420 code?

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 yours is something arriving from upstream. The fault that usually sent it there is a misfire or a lean running condition. Three things do the killing.

Each of the five causes has a clue, a test that names it and a part not to buy before that test. P0420 alone confirms none of them, because the code names a measurement, not a part.

The first three kill a converter. The last two, an exhaust leak and the engine's software, set the code without a failed one.

Oil burning can kill a converter too. GM found that piston ring wear on 2012–2013 Equinox and Terrain 2.4L engines burned enough oil to damage it, even with no check engine light, and that coverage has now ended.

Clogged fuel injectors are another cause. Honda ties P0420 and P0430 to them on some 2016–2019 Pilot, Ridgeline and Odyssey, covered 10 years from purchase or 150,000 miles, so have a dealer run the VIN.

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1. A misfire

Unburned fuel reaches the converter and burns inside it instead of in the cylinder, and that's what melts a honeycomb. A misfire code stored next to the P0420 names this cause, and a flashing check engine light means it's happening right now.

Fix the coil or plug first, because a new converter behind a misfire melts the same way. The misfire code tells the shop which cylinder to start on.

2. A wrong fuel mixture

Running rich or lean for a long time overheats the catalyst, and a lean code such as P0171 sitting beside the P0420 points here. Fix the mixture before the converter.

A vacuum leak takes this route to the converter. Air the computer never measured leaves the mixture lean, and it shows as a lean code such as P0171, covered in our vacuum leak guide. No carmaker bulletin cited here ties P0420 to a gas cap or bad gas.

Walker, a converter maker, does list fuel quality among the faults that overheat a converter. Nissan's loose-cap bulletin ties a loose cap to the evaporative leak codes P0442, P0455 and P0456, not to P0420.

3. 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 coolant level that keeps dropping with no leak in sight points the same way. Ask for a cooling system pressure test before a converter goes in.

4. An exhaust leak

This one sets the code without killing anything. Fresh air drawn in through a leak ahead of the sensor reads as lean, and the computer corrects for a condition that's not real.

Fix the leak and the converter was never the problem. Walker says even a small leak lets in enough air for the rear sensor to set a converter efficiency code.

5. The engine computer's software

Sometimes nothing has failed at all. Ford has told its dealers that on some Ranger and Super Duty trucks a P0420 may be due to the engine computer's software, and the fix is a reprogram.

So before anyone prices a converter, ask a dealer whether a software update exists for your VIN. On the Super Duty, Ford adds that the condition doesn't affect how the truck drives.

How do you fix a P0420 code?

Five questions settle whether it's really the converter, and all five are free to ask. To fix a P0420 code without guessing, start by asking what other codes are stored, in this order:

1. 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.
2. Ask whether the exhaust was checked for leaks ahead of the sensor. This costs minutes and it changes the answer.
3. Ask what the fuel trim numbers are. Fuel trim is how far the computer is correcting the mixture, and a shop that can't tell you hasn't looked.
4. 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.
5. Ask about a software update, a recall or the warranty. Ford reprograms some trucks for a false P0420, and Kia and GM have converter programs on some cars. Federal law may also cover a converter under 8 years or 80,000 miles.

All five are questions you ask about the estimate rather than about the car. Our guide on how to read a repair estimate covers the lines a shop expects nobody to query.

None of that means you leave the converter alone. A catalyst that has stopped storing oxygen doesn't recover when the misfire is fixed, so once the converter is below the threshold too, 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.

A converter cleaner won't skip these steps. A cleaner can only help a converter that never gets hot on short trips, and the long drive the label asks for does most of that work. It can't reach a melted, broken or contaminated converter, as our guide to whether catalytic converter cleaners work explains.

How much does it cost to fix a P0420 code?

The repairs that kill a converter cost less than the converter itself, so price those first. A shop's labor sits on top, plus the hours a rusted converter takes to come off.

On a 2021 Jeep Grand Cherokee 3.6L, a full set of six plugs runs $330–$660 at an independent shop, parts and labor. Fixing the misfire first is what saves the converter.

The converter is the expensive end: a federal-grade one fitted runs $402–$1,846, parts and labor, on the cars in our catalytic converter replacement cost guide, which lists the carmakers' own hours. Before you check a car repair quote, compare the hours a shop booked against them.

Will a P0420 code fail an emissions test?

Yes, while the light is on. A test that reads the car's computer fails it, and clearing the code first leaves the catalyst monitor reading "not ready".

The light can go out by itself. Under California's OBD II rule, which the EPA accepts nationwide, it goes out after three drives in a row in which the catalyst monitor runs and passes. The stored code can be erased after 40 warm-up cycles without it.

A converter that's still below the threshold fails the next test and sets the code again. So a light that went out is no proof of a pass.

Is P0430 the same as P0420?

Yes, it's the same test on the other bank: P0420 reports the catalyst on bank 1 and P0430 the one on bank 2. 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.

Same causes, same prices, different side. If you see both at once, look first at something feeding both banks, fuel trim or a reading problem, rather than two dead converters, and our P0430 guide covers bank 2 on its own.

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Which cars report a P0420 code most?

Kia leads, and within Kia it is almost all one car, the Soul. The next make is far behind, so read this as one outlier and a pack.

No Toyota or Lexus owner named this code in a federal complaint, which doesn't mean no Toyota has it. A complaint is a grievance, not a diagnosis, and a check engine light on an older car that still drives normally rarely produces one. Read this record as who was angry enough to write, never as who has the fault.

Who pays for a P0420 repair?

Run your VIN before you pay for a converter, because Kia replaces it on some Souls and GM covers it on some Chevy Silverado and GMC Sierra 1500 trucks. Our free VIN decoder lists the recalls for your model year, and NHTSA's VIN search shows which are still open on yours.

The federal emission warranty can also cover a converter for 8 years or 80,000 miles, whichever comes first, against 2 years or 24,000 miles for other emission faults. Most cars that reach a P0420 are past both, but the two numbers cost nothing to check.

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Common questions

Asked most often

Why do I still have P0420 after replacing the converter?

Because what killed the old one is still there, or nothing failed at all. A new converter behind a live misfire goes the same way, and an exhaust leak ahead of the sensor or a software fault sets the code with no failed part.

Is P0420 always the catalytic converter?

No, because it reports a measurement of catalyst output, and several upstream faults produce that measurement. The converter is usually the victim of a misfire, a wrong mixture or an exhaust leak upstream.

Will a P0420 code clear itself?

The light can, after three drives in a row in which the catalyst monitor runs and passes. A converter that's still below the threshold fails the next test and sets the code again.

Prices are US averages compiled from August to October 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.