What P0420 Means
The engine control module is comparing oxygen sensor readings before and after the catalytic converter, and the difference isn't big enough — meaning the converter isn't cleaning up exhaust gases as effectively as it should.
Common Causes
- Aging or failing catalytic converter
- Engine misfire allowing unburned fuel to damage the converter
- Oxygen sensor failure giving a false reading
- Exhaust leak upstream of the converter
- Running rich or lean for an extended period, which contaminates the converter
Symptoms You Might Notice
- Usually no noticeable drivability symptoms
- Check engine light on
- May fail an emissions/roadworthiness test
- Slight sulfur or rotten-egg smell in severe cases
What to Check First
- 1
Rule out a misfire or fuel trim problem first — fix any P03xx or P017x codes before replacing the converter
- 2
Inspect the exhaust for leaks between the engine and the converter
- 3
Check both upstream and downstream oxygen sensor readings with a scan tool
- 4
If sensors and fuel trims are healthy, the converter itself is likely worn and due for replacement
Frequently Asked Questions
Can I drive with a P0420 code?
Yes, in most cases it won't affect how the car drives. But it typically means the car will fail an emissions or roadworthiness test, and ignoring the underlying cause (like a misfire) can lead to bigger repairs.
Will a cheap aftermarket catalytic converter fix this?
It can, but a low-quality converter can trigger the same code again quickly. Address any misfire or fuel trim issue first, since a healthy engine won't damage a new converter the way a misfiring one would.