P0420Powertrain
Powertrain · SAE
Catalyst System Efficiency Below Threshold (Bank 1)
Meaning
The downstream (post-catalyst) oxygen sensor reports that the catalytic converter is no longer reducing emissions as it should. There are often no drivability symptoms.
Common causes
- Aged or damaged catalytic converter
- Faulty downstream oxygen/lambda sensor
- Exhaust leak near the sensors
- An upstream misfire or wrong air/fuel mixture that damaged the catalyst
- Oil or coolant contamination of the catalyst
Symptoms
- Check-engine light
- Frequently no driving symptoms
- Possible emissions/inspection test failure
- Sometimes a sulphur smell
Severity
Moderate: an emissions problem rather than a risk of immediate mechanical damage.
What to do
- Compare upstream vs downstream oxygen-sensor signals on the scan tool
- Check for exhaust leaks
- Rule out misfire or a wrong mixture before replacing the catalytic converter