P0421 MITSUBISHI Tech NotesAs 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 Lancer2003 Mitsubishi Lancer2004 Mitsubishi Lancer2005 Mitsubishi Lancer.
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The blinking patterns of both LED1 (green) and LED2 (red) indicate the types of abnormality when it occurs. Types of abnormality can be indicated in details by connecting an optional part ‘A-Control Service Tool (PAC-SK52ST)’ to connector CNM on outdoor controller board. Mitsubishi Trouble Code Info 96 & later are OBD2 systems and require a scanner. Accessing Diagnostic Trouble Codes. Turn ignition switch to OFF position.
P0421 MITSUBISHI MeaningThe Engine Control Module 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.
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