CodelampCode explained
P0420: your catalytic converter tested weak
P0420
- In plain words
- The catalytic converter on bank 1 is not cleaning the exhaust as well as it should.
- Standard name
- Catalyst System Efficiency Below Threshold (Bank 1)
- System
- Catalytic converter and oxygen sensors
General information, not a mechanic's diagnosis: have a mechanic confirm before any repair.
What the computer saw
The computer compares two oxygen sensors, one before the catalytic converter and one after. When the rear one starts to look like the front one, the converter is not storing and cleaning the exhaust as it should, and P0420 is set. It means the test failed. It does not prove the converter itself is the broken part.
Likely causes, most common first
- A worn catalytic converter. The most common cause on older, high-mileage cars. Converters lose their ability to store oxygen over time.
- A faulty rear oxygen sensor. The rear sensor is the judge. If it reads wrong, a good converter can fail the test.
- An exhaust leak before the rear sensor. Outside air drawn in through a leak upsets the reading.
- An engine problem that harmed the converter. Misfires, burning oil or a rich mixture send unburned fuel and oil into the converter and wear it out early. Fix these first, or a new converter can fail the same way.
What you can note yourself
- Read every stored code, not just this one. A misfire (P0300 to P0308) or oxygen sensor code (P0130 to P0141) changes what P0420 means.
- Note whether the light came on after a recent repair, a long trip or a tank of fuel from a new station.
- Note any rattle from under the car or a smell of rotten eggs, and tell the shop.
What to tell the shop
- All the codes, including any pending ones.
- Ask whether they checked the oxygen sensor readings and looked for exhaust leaks before quoting a converter.
- Any recent work on the engine or exhaust.
This page gives no verdict on whether a car is safe to drive; a mechanic can.