Volkswagen Engine Codes - P0420

Catalyst System Efficiency Below Threshold - Bank 1

What it means: OBD-II code P0420 indicates a problem with the catalyst system on your Volkswagen vehicle.

The engine control module, or ECM, gathers information from the frequency ratio of the front and rear oxygen sensors. A three way catalyst converter indicates when a low switching frequency of the heated oxygen sensor is present. 

VW TSI Engine

As the oxygen storage decreases, the oxygen sensor switching frequency will increase. When the frequency ratio of the 1 and 2 oxygen sensors reaches a specific limit, the three way catalyst malfunction is detected and the trouble code is prompted. Possible causes of the P0420 DTC include:

  • Leaking exhaust tube
  • Leaks in the intake
  • Faulty fuel injectors
  • Leaking fuel injectors
  • Worn spark plugs
  • Improper engine timing

Finding the exact cause of the problem should start with a visual inspection and confirmation that all parts are working as they should before looking to the catalytic converter. However, it is almost always the catalytic converter that needs to be replaced to remedy this issue. Alternatively, other diagnostic trouble codes may accompany the code P0420 that can help you pin down the root of the issue.

While every effort was made to provide accurate and expert information about this problem, please always consult a professional mechanic for conclusive troubleshooting and repairs. This guide is merely meant to explain the possible triggers for DTC P0420.