P0420 HONDA - Catalyst System Efficiency Below Threshold Bank 1

P0420 HONDA - Catalyst System Efficiency Below Threshold Bank 1

Possible causes 


  • Faulty Three-way Catalyst Converter Bank 1 
  • Exhaust Tube 
  • Intake Air Leaks 
  • Faulty Oxygen (O2) Sensor 
  • Faulty Fuel injector(s) 
  • Leaking Fuel Injector (s) 
  • Faulty Spark plugs Improper Ignition Timing 
  • Faulty Engine Control Module (ECM) 
  • Dirty Air Filter

Tech notes 

As the code description implies the P0420 code means that the vehicle's control module has detected that the three-way catalytic converter is not working properly (is not as efficient as the factory is expecting). Replacing the oxygen (O2) sensors may sometimes fix the code, but in most cases the catalytic convert needs to be replaced to fix the problem. If the P0420 code is combined with other codes, try fixing the other codes first.

Possible Solutions 

  • 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

P0420 Honda Description

The Engine Control Module (ECM) monitors the switching frequency ratio of heated oxygen sensors 1 (front O2 sensor) and heated oxygen sensors 2 (rear O2 sensor). A three way catalyst converter (Manifold) with high oxygen storage capacity will indicate a low switching frequency of heated oxygen sensor 2. As oxygen storage capacity decreases, the heated oxygen sensor 2 switching frequency will increase. When the frequency ratio of heated oxygen sensors 1 and 2 approaches a specified limit value, the three way catalyst malfunction is diagnosed.