P0421 Mitsubishi - Warm Up Catalyst Efficiency Below Threshold- Cylinders 1-4
- 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
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
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.