Diagnostic procedure

WARNING: This page is about a different car, the 2005 Mazda 6. However, it is still accessible from the selected car via links, so may be relevant.

DTC P0421, P0431 [AJ] DIAGNOSTIC PROCEDURE

STEP	INSPECTION		ACTION
1	VERIFY FREEZE FRAME DATA AND DIAGNOSTIC MONITORING TEST RESULTS HAVE BEEN RECORDED Have FREEZE FRAME DATA and DIAGNOSTIC MONITORING TEST RESULTS (Catalyst related) been recorded?	Yes	Go to the next step.
1		No	Record FREEZE FRAME DATA and DIAGNOSTIC MONITORING TEST RESULTS on repair order, then go to the next step.
2	VERIFY RELATED REPAIR INFORMATION AVAILABILITY Check for related Service Bulletins and/or online repair information availability. Is any related repair information available?	Yes	Perform repair or diagnosis according to the available repair information. If the vehicle is not repaired, go to the next step.
2		No	Go to the next step.
3	VERIFY RELATED PENDING CODE OR STORED DTCs Turn ignition switch to OFF then ON (Engine off). Verify related pending code or stored DTCs. Are other DTCs present?	Yes	Go to appropriate DTC troubleshooting. (See DTC TABLE .)
3		No	Go to the next step.
4	INSPECT EXHAUST SYSTEM FOR EXHAUST GAS LEAKAGE Visually inspect for exhaust gas leakage in exhaust system. (See EXHAUST SYSTEM INSPECTION .) Is there any exhaust gas leakage?	Yes	Repair or replace faulty exhaust system parts, then go to Step 7. (See EXHAUST SYSTEM REMOVAL/INSTALLATION .)
4		No	Go to the next step.
5	INSPECT HO2S INSTALLATION Inspect for looseness of HO2S (RF, LF, RR, LR). Is HO2S installation normal?	Yes	Go to the next step.
5		No	Retighten HO2S, then go to Step 7. (See EXHAUST SYSTEM REMOVAL/INSTALLATION .)
6	INSPECT WU-TWC Clear the DTC from the PCM memory using the WDS or equivalent. Cycle ignition switch to OFF, then back to ON. Inspect WU-TWCs each bank. (See WARM-UP THREE-WAY CATALYTIC CONVERTER (WU-TWC) INSPECTION .) Is there any malfunction?	Yes	Replace malfunctioning WU-TWC, then go to the next step. (See EXHAUST SYSTEM REMOVAL/INSTALLATION .)
6		No	Go to the next step.
7	VERIFY TROUBLESHOOTING OF DTC P0421 OR P0431 COMPLETED Make sure to reconnect all disconnected connectors. Turn the ignition switch to the ON position. Clear the DTC from the memory using the WDS or equivalent. Start engine and perform DRIVE MODE except for DRIVE MODE 6. (See OBD-II DRIVE MODE .) Is PENDING CODE same as DTC present?	Yes	Replace PCM, then go to the next step. (See PCM REMOVAL/INSTALLATION .)
7		No	Go to the next step.
8	VERIFY AFTER REPAIR PROCEDURE Perform "AFTER REPAIR PROCEDURE". (See AFTER REPAIR PROCEDURE .) Are any DTCs present?	Yes	Go to the applicable DTC inspection. (See DTC TABLE .)
8		No	Troubleshooting completed.