P0420
Moderate severityCatalytic Converter System Efficiency Below Threshold (Bank 1)
Your car is safe to drive short-term, but it will fail an emissions test and the underlying issue can get worse. Get it checked within a week or two.
What does the P0420 code mean?
The P0420 code means your car’s computer has detected that the catalytic converter on Bank 1 isn’t reducing exhaust emissions as efficiently as it should. The catalytic converter’s job is to turn harmful exhaust gases into less harmful ones. Your engine measures how well it’s working using two oxygen (O2) sensors — one before the converter and one after it.
When those two sensors start reading similar values, it tells the computer the converter is no longer “scrubbing” the exhaust properly, so it stores a P0420 and turns on the check engine light. (“Bank 1” simply means the side of the engine that contains cylinder number 1.)
Can you keep driving with a P0420 code?
In most cases, yes — at least in the short term. A P0420 rarely leaves you stranded and usually won’t cause immediate damage. But two things matter: your car will fail an emissions or smog test while this code is active, and if the real cause is something upstream (like a misfire or a bad fuel mixture), driving on it can make the problem worse. The safe move is to get it diagnosed within a week or two. If you also notice a rotten-egg smell, rattling under the car, or worse fuel economy, have it looked at sooner.
What causes a P0420 code?
- Worn-out or failing catalytic converter (most common)~50%
- Faulty oxygen (O2) sensor giving false readings~25%
- Exhaust leak near the converter or sensors~15%
- Upstream misfire or rich/lean fuel condition~10%
How to diagnose a P0420 (check these first)
Before you spend money on a new catalytic converter — the expensive fix — rule out the cheap stuff:
- Scan for other codes first. A misfire (P0301–P0308) or an O2 sensor code usually points to the real cause — fix that before touching the converter.
- Inspect for exhaust leaks around the converter and the O2 sensors; even a small leak can trigger a P0420.
- Test or replace aging O2 sensors — a ~$150 sensor is far cheaper than a $1,500 converter.
- Make sure the engine is running smoothly and you’re using the correct fuel.
P0420 repair cost
Costs vary a lot by vehicle. These are typical US ranges including parts and labor:
| Oxygen (O2) sensor replacement | $150 – $300 |
| Exhaust leak repair | $100 – $400 |
| Catalytic converter replacement | $900 – $2,500 |
Tools & parts that help
A $20 OBD-II scanner lets you check for other codes before spending on a converter. If you confirm it’s the cat, you can price O2 sensors or a converter for your exact vehicle.
See top-rated OBD-II scanners ›P0420 — Frequently asked questions
Is it safe to drive with a P0420 code?
Usually yes, in the short term — but it will fail an emissions test and the cause can get worse, so get it checked within a week or two. If your check engine light is flashing, that’s a different, urgent problem (an active misfire) — stop driving.
Will a P0420 code clear itself?
The light may turn off temporarily if the converter happens to perform within range again, but the underlying issue almost always returns until it’s actually fixed.
How much does it cost to fix a P0420?
Anywhere from about $150 for an O2 sensor to $900–$2,500 for a catalytic converter, depending on the real cause and your specific vehicle.
Can a loose gas cap cause a P0420?
No. A loose or faulty gas cap triggers EVAP codes like P0455 or P0457. P0420 is specifically about the catalytic converter and exhaust.
Related codes
General guidance based on the standard definition of the P0420 code. A real diagnosis depends on your make, model, mileage and other symptoms. When in doubt, have a professional confirm the cause before replacing expensive parts.