P0420 the most common fault code?

P0420 is possibly the most common DTC retrieved from modern vehicles.
This is because it is one of the codes that is concerned with the exhaust system, any fault upstream of the exhaust can result in a P0420 code.

This why it can be cleared and not return or cleared only to return but not always immediately.
The catalyst can be damaged by many things, but misfire is the most common cause of catalyst damage. P0420 indicates the catalyst is operating below some threshold set within the ECU software. It can be a temporary fault or an indication of catalyst failure.

It is monitored by the O2 sensors, one before the catalyst and one after the catalyst. The two signals are compared and if they are similar the ECU flags the code.

Above is a scan tool image of a catalyst operating efficiently.



There are number of ways to test the catalyst efficiency

• use an infra red thermometer to check for an increase in temperature across the catalyst
• Scan tool data to monitor the switch ratio before and after the catalyst
• oscilloscope to check O2 signal output
• Gas analyser to check readings
• EOBD mode 6

Always test the catalyst under the most favourable conditions, as this is how the ECU tests the catalyst. Hot engine, fast idle/cruise and no other faults present.

The ECU will only monitor the the catalyst efficiency if there are no codes stored. Therefore it is essential to check the catalyst before returning the vehicle to the customer, if a fault has been repaired that may have damaged the catalyst, such as coil pack failure.

Remember it is the O2 sensor that is used to monitor the catalyst so these must be operating correctly.