P0420high severityPowertrain
Catalyst System Efficiency Below Threshold Bank 1
on 2010 Audi A8 4.2L V8 · AWD
What this code means
The Bank 1 catalytic converter is not performing efficiently. The downstream O2 sensor is mirroring the upstream O2 sensor, indicating the catalyst is not storing and releasing oxygen properly.
Symptoms
- Check Engine Light
- Failed emissions test
- Possible sulfur smell
- Reduced fuel economy
Common causes
- Worn catalytic converter (most common)
- Exhaust leak before downstream O2 sensor
- Faulty downstream O2 sensor
- Engine misfire damaging catalyst
- Oil burning contaminating catalyst
- Using leaded fuel or certain fuel additives
Diagnostic steps
- Compare upstream and downstream O2 sensor waveforms (downstream should be flat/steady)
- Check for exhaust leaks
- Test downstream O2 sensor response
- Check for engine misfire codes
- Inspect for oil consumption
- Perform catalyst efficiency test with scan tool
Repair notes
If catalyst is truly failed, replacement is needed. First rule out exhaust leaks and bad downstream O2 sensor. Aftermarket catalysts may not last as long as OEM.
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