Possible causes

– Warm Up Three Way Catalytic converter deterioration or malfunction
– Exhaust system leaks
– Loose front or rear HO2Ss
– Faulty front heater O2 sensor
– O2 sensors harness is open or shorted
– O2 sensors poor electrical connection

Tech notes

As the code description implies the P0421 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 P0421 code is combined with other codes, try fixing the other codes first.

There is a factory service recall for the following Mitsubishi Models:

2002 Mitsubishi Lancer
2003 Mitsubishi Lancer
2004 Mitsubishi Lancer
2005 Mitsubishi Lancer
2002?2005 Mitsubishi Lancer Factory Service Recall OBDII Code P0421

When is the code detected?

The P0421 code is set when PCM has determined that the warm up catalyst efficiency for bank 1 is below threshold for the current engine operating condition. (Bank 1 identifies the location of cylinder #1, while bank 2 identifies the cylinders on the opposite bank)

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)

P0421 MITSUBISHI 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.

MITSUBISHI Technical Service Bulletins for P0421 code