Key takeaways
P0420 means your car's computer has decided the catalytic converter on bank 1 is no longer cleaning the exhaust as well as it should. It is usually safe to drive on for a short while if the light is steady and the engine runs normally. Do not buy a converter yet: an oxygen sensor or an exhaust leak can set the same code.
- 01The code describes a result, low converter efficiency. It does not name the part that failed.
- 02Check for other codes first. A misfire or fuel mixture fault often comes before P0420 and has to be fixed first.
- 03A catalytic converter replacement averaged $1,511 in 2025, and an oxygen sensor $287, according to CarMD's 2026 Vehicle Health Index.
- 04The federal emissions warranty covers the catalytic converter for 8 years or 80,000 miles, including on a used car.
This guide belongs to our OBD2 guide: most common codes section, which covers the wider topic.
What Does the P0420 Code Mean?
The standard definition is Catalyst System Efficiency Below Threshold (Bank 1).
The catalytic converter sits in the exhaust and turns the most harmful gases from the engine into less harmful ones. Your car watches it with two oxygen sensors: one before the converter and one after it.
A healthy converter stores and uses oxygen, so the sensor after it reads steady while the sensor before it swings up and down. When the converter wears out, the second sensor starts to copy the first. Once the two signals look too much alike, the computer stores P0420 and switches on the check engine light.
Bank 1 is the side of the engine that holds cylinder 1. A four-cylinder engine has only one bank. On a V6 or V8, the same fault on the other side is P0430. The definition is the same on every make, because generic codes follow one industry standard, the SAE J2012 trouble code definitions. You can see where P0420 ranks among the most common check engine codes in our OBD2 guide.
A code names a symptom, not a part
P0420 says the exhaust leaving the converter is not clean enough. It does not say the converter has failed. Anything that fools the second oxygen sensor, or that stops the converter from working, sets the same code.
P0420 Symptoms
Most drivers notice nothing but the light. The other signs appear when the converter is badly worn or blocked.
- A steady check engine light. Often the only symptom.
- A failed emissions test. Where the test reads the car's computer, a fault that keeps the light on has to be repaired before the car can pass, as the California Air Resources Board's OBD II fact sheet explains.
- A rotten-egg smell from the exhaust.
- Less power, most of all uphill or at highway speed, when a converter is partly blocked.
- A rattle from under the car, when the material inside the converter has broken up.
- Worse fuel economy than you are used to.
If the light is flashing
A flashing check engine light means a fault that could be damaging the catalytic converter right now, usually a severe misfire, as the EPA's check engine light guide warns. Ease off, pull over when it is safe and do not keep driving.
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Check Engine Light Checker. Tell it how the light is behaving and how the car feels, and it tells you whether to keep driving.
What Causes P0420? Most Likely First
Work down this list in order. The cheap causes are at the bottom, and they are the ones worth ruling out before you spend real money.
A worn catalytic converter
The coating inside loses its ability to clean the exhaust with age and heat. This is the most common reason on a high-mileage car.
A converter damaged from upstream
Misfires send unburned fuel into the exhaust, where it overheats the converter. An engine that burns oil or runs rich does similar damage over time.
A failing rear oxygen sensor
The sensor after the converter does the measuring. If it reads wrongly, the computer blames the converter.
An exhaust leak
A leak ahead of the rear sensor lets outside air in. The sensor sees extra oxygen and the reading no longer reflects the converter.
An engine that is not running right
A lean or rich fuel mixture, or a misfire that has not set its own code yet, changes what both sensors see.
Software that needs an update
Carmakers sometimes revise how the computer judges the converter. A dealer can check your VIN for a service bulletin.
Causes 1 and 2 end with the same repair, but they are not the same problem. If a misfire or an oil-burning engine killed the converter, a new converter will die the same way unless the cause is fixed first.
How to Diagnose P0420
You need an OBD2 scanner that shows live data. A basic code reader gets you through the first two steps.
- Read every stored code. Plug the scanner into the port under the dashboard and write down all the codes, not only P0420. Our OBD2 code lookup explains each one.
- Fix other codes first. Misfire codes such as P0300, mixture codes such as P0171 or P0172, and any oxygen sensor code come before P0420. Repair those, clear the codes and see whether P0420 returns.
- Look at the freeze frame. The scanner stores the engine speed, load and temperature at the moment the code set. It tells you the conditions to repeat on a test drive.
- Check for exhaust leaks. With the engine cold, start it and listen along the exhaust for a ticking or puffing sound that fades as it warms up. Look for black soot around joints and flanges.
- Compare the two oxygen sensors. Warm the engine fully and hold a steady speed. On live data, the sensor before the converter should swing quickly and the one after it should stay nearly flat. If the rear sensor swings like the front one, the converter is not doing its job.
- Check the warranty before you pay. If the car is within 8 years and 80,000 miles of its first sale, ask a dealer whether the converter is covered by the federal emissions warranty.
Clearing the code does not fix it
Erasing the code also erases the car's self-test results. The converter test has to run again before the light can return, and in California a car with too many unfinished self-tests fails its inspection, according to the state's Air Resources Board.
How to Fix P0420 and What It Costs
The repair depends on which cause you found. These are national averages for parts and labor, so your quote will differ.
| Repair | When it is the fix | Average cost |
|---|---|---|
| Catalytic converter | The rear sensor copies the front one and no other fault is found | $1,511 |
| Oxygen sensor | The rear sensor is slow, stuck or has its own code | $287 |
| Ignition coils and spark plugs | A misfire came first and damaged or masked the converter | $480 |
| Spark plugs alone | Worn plugs are causing the misfire | $299 |
| Exhaust leak repair | A leak ahead of the rear sensor | No published average. Ask for a quote. |
| Software update | A service bulletin covers your car | Ask the dealer. It may be free under warranty. |
Average costs: CarMD 2026 Vehicle Health Index, based on check engine repairs recommended in 2025. They are modeled national averages for parts and labor.
The catalytic converter was the single most common check engine repair in CarMD's data, at 7.4% of all repairs. It is also the most expensive one on the list, which is the reason to rule out everything else first.
If the converter does need replacing, ask the shop to confirm that the new part is approved for your vehicle and for your state's emissions rules, and ask what caused the old one to fail.
The emissions warranty may pay for it
Federal law requires carmakers to cover the catalytic converter for the first 8 years or 80,000 miles, whichever comes first. The cover starts on the date the car was first sold and it applies to used cars too. The maker can refuse if the failure was caused by misuse or poor maintenance, the EPA says. California adds its own emissions warranty on top, according to its Air Resources Board.
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Repair Cost Estimator. Price the repair for your own labor rate before you approve a quote.
Can You Keep Driving With P0420?
For a short time, usually yes. How the light behaves and how the car feels decide it.
| What you notice | What it means | Keep driving? |
|---|---|---|
| Steady light, car feels normal | The converter is working below standard, or a sensor is misreading | Yes, for now. Have it diagnosed soon. |
| Steady light with less power or a rattle | The converter may be blocked or breaking up | Only as far as you need to. Have it checked this week. |
| Flashing light | A fault is damaging the converter now | No. Pull over when it is safe. |
Leaving it for months has a price. A blocked converter raises the pressure in the exhaust and costs you power and fuel. The car will also fail any emissions test that reads its computer. We cover the time limits in more detail in how long you can drive with a P0420 code.
Codes Related to P0420
These often appear with P0420 or are mistaken for it. The descriptions are the standard definitions.
| Code | Standard description | How it relates |
|---|---|---|
| P0430 | Catalyst System Efficiency Below Threshold (Bank 2) | The same fault on the other side of a V engine |
| P0421 | Warm Up Catalyst Efficiency Below Threshold (Bank 1) | The small converter close to the engine is under standard |
| P0431 | Warm Up Catalyst Efficiency Below Threshold (Bank 2) | As P0421, on bank 2 |
| P0300 | Random/Multiple Cylinder Misfire Detected | A misfire is a common cause of converter damage. Fix it first. |
| P0171 | System Too Lean (Bank 1) | A mixture fault changes what the oxygen sensors read |
| P0172 | System Too Rich (Bank 1) | Excess fuel overheats the converter over time |
Descriptions follow the SAE J2012 standard definitions.
Any code can be checked in the OBD2 code lookup, and the full code list shows the rest of each family.
Frequently Asked Questions
What is the most common cause of a P0420 code?
On a high-mileage car it is most often a catalytic converter that has worn out with age and heat. On a newer car, look harder at the rear oxygen sensor, exhaust leaks and any misfire or fuel mixture codes stored alongside it, because each of those can set P0420 without the converter being at fault.
Can a bad oxygen sensor cause P0420?
Yes. The oxygen sensor after the catalytic converter is the part that measures how well the converter works. If that sensor is slow or reads wrongly, the computer concludes the converter is failing. Compare the two sensor readings on live data before replacing the converter, since a sensor costs far less.
How much does it cost to fix a P0420 code?
It depends on the cause. CarMD's 2026 Vehicle Health Index puts the average catalytic converter replacement at $1,511 and the average oxygen sensor replacement at $287, parts and labor included. A converter still inside the federal 8-year, 80,000-mile emissions warranty may cost you nothing at a dealer.
Will P0420 clear itself?
Only if the fault goes away. The car reruns its converter test as you drive, and the light can switch itself off if the test keeps passing. If the converter is worn, the code comes back. Clearing it with a scanner resets the car's self-tests, which can cause a failed emissions inspection until they finish again.
Will my car pass an emissions test with P0420?
Not where the test reads the car's computer. California's Smog Check, for example, requires any fault that is keeping the check engine light on to be repaired before the car can pass. Rules differ by state, so check with your own state's emissions program.
What is the difference between P0420 and P0430?
They are the same fault on opposite sides of the engine. P0420 is bank 1, the side that holds cylinder 1, and P0430 is bank 2. A four-cylinder engine has a single bank, so it can only set P0420. Both codes are diagnosed the same way.
Sources and References
- CarMD. 2026 Vehicle Health Index (calendar-year 2025 repair data)
- U.S. Environmental Protection Agency. Frequent Questions related to Transportation, Air Pollution, and Climate Change: federal emission control warranties
- California Air Resources Board. On-Board Diagnostic II (OBD II) Systems Fact Sheet
- SAE International. J2012: Diagnostic Trouble Code Definitions
- U.S. Environmental Protection Agency. What To Do With the Check Engine Light (EPA420-F-00-041)
Written by
CarTipsMentor Team
Researched from the sources listed above and checked against them before publishing. Nobody paid to be mentioned on this page. It is general information, not advice about your own vehicle.
Published . Last updated .
