P0420 OBD II Trouble Code Code: Catalyst System Efficiency Below Threshold (Bank 1)
How to diagnose, repair, and info on symptoms and causes of a P0420 OBD-II trouble code which means the catalytic converter is no longer performing at a normal level. Often this means the bank 1 converter will need to be replaced.
Symptoms of P0420 OBD II Trouble Code
- Leaded fuel was used where unleaded was called for (unlikely)
- A damaged or failed oxygen / O2 sensor
- Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
- The engine coolant temperature sensor is not working properly
- Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
- Failed or underperforming catalytic converter (likely)
- Retarded spark timing
- The oxygen sensors in front and behind the converter are reporting too similar of readings
- Leaking fuel injector or high fuel pressure
- Cylinder misfire
- Oil contamination
Common Causes of P0420 OBD II Trouble Code
- Check for exhaust leaks at the manifold, pipes, catalytic converter. Repair as required.
- Use a scope to diagnose the oxygen sensor operation (Tip: The oxygen sensor in front of the catalytic converter normally has a fluctuating waveform. The waveform of the sensor behind the converter should be more steady).
- Inspect the downstream heated oxygen sensor, replace if necessary
- Replace the catalytic converter