What does the P0420 OBD-II code mean?

If your car’s computer (PCM) triggers a P0420 trouble code, it means it has detected a problem with the catalytic converter. The operation of the catalytic converter is to break down harmful pollutants created during the combustion cycle, thereby reducing the emissions expelled from the exhaust pipe. Two oxygen sensors monitor the catalytic converter, with one positioned at the front (upstream), and one at the rear (downstream). Readings for the upstream oxygen sensor fluctuate, with the downstream O2 sensor readings remaining steady. If these two sensors give similar readings, it indicates the catalytic converter is not working efficiently, with the output of harmful pollutants increasing, and a P0420 error code being triggered.


How serious is the P0420 code?

A P0420 diagnostic code is not considered a serious problem, and it may be that there won’t be any driveability problems other than the ‘check engine’ light illuminated. However, if left unrepaired, it can cause issues including lack of power or poor acceleration, together with causing severe damage to other components, including the catalytic converter which can be very costly. Therefore, it is advisable to address a P0420 trouble code as soon as possible to keep the repair costs to a minimum.

What are the common symptoms of a P0420 trouble code?

There are often no obvious symptoms that there is a problem with the catalytic converter. The four common indicators are:

  • Check Engine light illuminated
  • Lack of power/poor acceleration
  • Decreased fuel economy
  • Rotten egg (sulfur) smell which occurs due to the incorrect amount of oxygen in the catalytic converter leading to excess sulfur in the fuel tank.


What are the causes of a P0420 diagnostic code?

There are numerous causes for a P0420 trouble code triggering. The most common are:

  • Worn or faulty catalytic converter (common)
  • Oil contamination in the catalytic converter
  • Defective front and/or rear oxygen sensor
  • Defective oxygen sensor wiring/connections
  • Faulty air-fuel sensor
  • Faulty engine coolant temperature sensor
  • Engine misfire
  • Damaged exhaust pipe or a leak in the exhaust system
  • Using incorrect fuel type (e.g. using leaded fuel instead of unleaded)

How to diagnose a P0420 OBD-II code?

The simple way to diagnose an OBD-II fault code is to use an OBD-II scanner or book a diagnostic check with a trusted mechanic or garage. 

  • Scan your system to verify that the P0420 is the only trouble code. If other OBD-II codes are present, these must be addressed first.
  • Clear the fault codes, followed by a test drive. If the code persists:
  • Inspect the exhaust system for damage and leaks. 
  • Read the live data of the downstream (rear) oxygen sensor to verify it’s working correctly. 
  • Inspect the rear oxygen sensor, wiring, and connectors for damage and/or excessive wear.
  • Check that the catalytic converter is working properly.


How to fix a P0420 OBD-II trouble code?

It is recommended to test drive the vehicle after each check/work is performed to see if the P0420 fault code clears. If the code returns, move on to the next repair. The most common repairs are:

  • Confirm the error code with an OBD-II scanner, and reset the fault code. If there are other OBD-II codes present, these must be resolved first. Follow this with a test drive of the car to see if the trouble code clears. If it returns, other common repairs are:
  • Replacement of the catalytic converter (common) – check for available updates of the PCM if the catalytic converter is faulty. Updates to the PCM will be required if the catalytic converter is replaced.
  • Repair or replace any exhaust leaks or damage. Pay close attention to the exhaust pipes, exhaust manifold, gaskets and pre-catalytic converter. 
  • Replace engine coolant temperature sensor.
  • Replace front or rear oxygen sensor.
  • Repair or replace any damaged wiring and/or connectors to the oxygen sensor(s).
  • Repair or replace any leaking fuel injector(s).
  • Diagnose and repair any misfiring problems (other OBD-II codes should be present).


How to avoid a P0420 code?