P0420: Your catalytic converter is not cleaning exhaust like it should
Catalyst System Efficiency Below Threshold (Bank 1)
This usually points to the catalytic converter or the oxygen sensors that watch it. Get it checked within a week or two. If the light flashes or the engine shakes, that is a misfire: stop driving and get help.
Stop driving and get help if
- The check engine light is flashing
- The temperature gauge runs high, or you see steam (overheating)
- The car loses power, hesitates badly or stalls
- You smell burning or see smoke
- A brake warning light comes on, or the brakes feel different
Pull over as soon as you can, turn the engine off, and call a tow or roadside help.
What it means
Your catalytic converter scrubs pollutants out of the exhaust. Two oxygen sensors watch it work, one before it and one after. When both sensors start reporting similar readings, the computer concludes the converter is not doing much anymore and turns on the check engine light. The converter usually does not fail on its own. Something else, like a misfire or a bad sensor, often kills it or fakes the symptom. That is why this is one of the most misdiagnosed codes there is.
Common causes
- Worn catalytic converterThe most common cause on cars past 100,000 miles.
- Failing downstream oxygen sensorA lazy sensor can report a healthy cat as dead. Much cheaper to fix, so rule this out first.
- Exhaust leak near the sensorsOutside air skews the readings.
- A misfire or fuel problem poisoning the converterIf other codes are present, fix those first or the new converter dies the same death.
What the fix could involve
From the cheaper fix to the bigger one. Prices depend on your car, your area and the shop, so ask for a written quote that names the part and the labor before you approve it.
- Exhaust leak repair
- Downstream O2 sensor
- Catalytic converter
Symptoms
- Check engine light on, steady (not flashing)
- Often no change in how the car drives
- Possible drop in fuel economy
- Rotten-egg smell from the exhaust in some cases
- Failed emissions test
What to do next
Do not buy a converter because of this code alone. First, check for other codes, especially misfires (P0300 series) and fuel trim codes (P0171, P0174). Fix those first. If the car is past 100,000 miles, the codes are clean otherwise, and the converter is original, the converter probably is worn out. A shop can confirm with a temperature or backpressure test before you spend real money.
Lewis reads P0420 alongside any companion codes and shows live upstream oxygen sensor, fuel trim, and catalyst temperature data, so you can see whether the engine is running clean before you blame the converter. Get Lewis on the App Store or Google Play.
Related codes
FAQ
- Will my car pass emissions with P0420?
- No. This code is an automatic emissions failure almost everywhere that tests.
- Can bad gas or a loose gas cap cause it?
- No. That is P0455/P0457 territory. P0420 is about the converter and its sensors.
- Someone told me to try a fuel additive. Worth it?
- Usually not. If the converter is clogged from oil or coolant burning, additives will not save it. If it is a sensor, additives are irrelevant.
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