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