P0420
Moderate severityCatalytic Converter System Efficiency Below Threshold (Bank 1)
If your check engine light is steady and the car drives normally, a P0420 rarely leaves you stranded. But your car will fail an emissions or smog test while this code is active, and if the real cause is something upstream — a misfire or a bad fuel mixture — driving on it makes the problem worse. Plan to get it diagnosed within a week or two.
Stop driving if: the check engine light is flashing, you hear rattling from under the car, you smell rotten eggs, or you notice a clear loss of power.
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.
A flashing check engine light is always different. It means an active misfire is dumping raw fuel into your exhaust, which can destroy a catalytic converter within minutes. If your light is flashing, pull over safely and stop driving, whatever the code says.
What causes a P0420 code?
Listed in the order most shops would check them — cheapest and most likely first, not as statistical odds. The real cause varies by year, make, model and engine.
- Faulty oxygen (O2) sensor giving false readingsCheck 1st
- Exhaust leak near the converter or the sensorsCheck 2nd
- Upstream misfire or a rich / lean fuel conditionCheck 3rd
- Worn-out or failing catalytic converterCheck last
On high-mileage engines that burn oil, a replacement converter can fail again within months if the oil consumption isn’t addressed first. If your car uses noticeable oil between changes, mention it to your shop before approving a new converter.
How to diagnose a P0420 (check these first)
Before you spend money on a new catalytic converter — the expensive fix — rule out the cheap causes in this order:
- 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 both O2 sensors. Even a small leak lets extra air in and can trigger a P0420.
- Check live O2 sensor data with a scanner. The downstream sensor should stay relatively flat; if it mirrors the upstream sensor, the converter is likely genuinely worn.
- Test or replace an aging downstream O2 sensor — a ~$150 sensor is far cheaper than a $1,500 converter and is a common false trigger.
- Confirm the engine runs smoothly on the correct fuel, with no rough idle or hesitation, before condemning the converter.
P0420 repair cost
Costs vary a lot by vehicle and region. These are typical US ranges including parts and labor — treat them as a ballpark for the conversation with your shop, not a quote.
| Oxygen (O2) sensor replacement | $150 – $300 |
| Exhaust leak repair | $100 – $400 |
| Catalytic converter replacement | $900 – $2,500 |
Tools & parts that help
A basic OBD-II scanner lets you check for other stored codes before spending anything on a converter — and that single step is what separates a $150 repair from a $1,500 one. If you confirm the converter really is the problem, you can then price O2 sensors or a converter for your exact vehicle.
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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 SAE definition of the P0420 code. A real diagnosis depends on your make, model, year, engine and other symptoms. This is not a substitute for a professional inspection — when in doubt, have a technician confirm the cause before replacing expensive parts.