The downstream oxygen sensor shows the catalytic converter on bank 1 is not cleaning the exhaust well enough. The driver often notices only the engine light.
Likely causes, most common first
- Worn catalytic converter
- Exhaust leak near the sensors
- Misfire or rich mixture that damaged the catalyst
- Faulty downstream oxygen sensor
Parts usually involved
- Catalytic converter
- Oxygen sensor
- Exhaust gaskets
Test before you replace
Fix any misfire or fuel trim fault first, then compare upstream and downstream sensor signals and check for exhaust leaks.
General guidance for generic OBD2 codes. Always follow the maker's procedure for the car in front of you.