Catalyst System Efficiency Below Threshold Bank 1
1997-2000 BMW 528i E39 2.8L I6 🇦🇺
Description
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.
Related Codes
Other Powertrain Codes
- P0001Fuel Volume Regulator Control Circuit/Open
- P0002Fuel Volume Regulator Control Circuit Range/Performance
- P0003Fuel Volume Regulator Control Circuit Low
- P0004Fuel Volume Regulator Control Circuit High
- P0010Intake Camshaft Position Actuator Circuit/Open Bank 1
- P0011Intake Camshaft Position Timing Over-Advanced Bank 1
- P0012Intake Camshaft Position Timing Over-Retarded Bank 1
- P0013Exhaust Camshaft Position Actuator Circuit/Open Bank 1
- P0014Exhaust Camshaft Position Timing Over-Advanced Bank 1
- P0015Exhaust Camshaft Position Timing Over-Retarded Bank 1
P0420 on Other Vehicles
Common Questions
What does code P0420 mean on a 1997-2000 BMW 528i E39?
P0420 stands for "Catalyst System Efficiency Below Threshold Bank 1". 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.
What are the symptoms of P0420 on a BMW 528i E39?
Common symptoms of P0420 include: Check Engine Light, Failed emissions test, Possible sulfur smell, Reduced fuel economy.
What causes P0420 on a 1997-2000 BMW 528i E39?
Common causes of P0420 include: 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.