Diagnostic procedure

WARNING: This page is about a different car, the 2005 Mazda RX-8. However, it is still accessible from the selected car via links, so may be relevant.

DTC P0420 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 the FREEZE FRAME DATA and DIAGNOSTIC MONITORING TEST RESULTS on the repair order, then go to the next step.
2	VERIFY RELATED REPAIR INFORMATION AVAILABILITY Verify related Service Bulletins and/or on-line 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 DTC Turn the ignition switch off, then to the ON position (Engine off). Verify the related PENDING CODE or stored DTCs. Are other DTCs present?	Yes	Go to the applicable DTC inspection. (See DTC TABLE .)
3		No	Go to the next step.
4	INSPECT INSTALLATION OF FRONT HO2S Inspect the front HO2S for looseness. Is it normal?	Yes	Go to the next step.
4		No	Tighten the front HO2S, then go to Step 11. (See FRONT HEATED OXYGEN SENSOR (HO2S) REMOVAL/INSTALLATION .)
5	INSPECT EXHAUST SYSTEM FOR EXHAUST GAS LEAKAGE Visually inspect exhaust gas leakage in the exhaust system. Is there exhaust gas leakage?	Yes	Repair or replace the malfunctioning part, then go to the next step.
5		No	Go to the next step.
6	INSPECT PURGE CONTROL SYSTEM OPERATION Perform the "Purge Control System Operation Inspection". (See PURGE CONTROL SYSTEM INSPECTION .) Does purge control system operate properly?	Yes	Go to the next step.
6		No	Repair or replace malfunctioning part according to the inspection results. Then go to Step 11.
7	VERIFY CURRENT INPUT SIGNAL STATUS OF VSS Connect the WDS or equivalent to the DLC-2. Start the engine. Access the VSS PID. Inspect the VSS PID. (See PCM INSPECTION .) Is the PID normal?	Yes	Go to the next step.
7		No	Inspect the malfunctioning part according to the inspection results. Then go to Step 11.
8	INSPECT ECT SENSOR Inspect the ECT sensor. (See ENGINE COOLANT TEMPERATURE (ECT) SENSOR INSPECTION .) Is there any malfunction?	Yes	Replace the ECT sensor, then go to Step 11. (See ENGINE COOLANT TEMPERATURE (ECT) SENSOR REMOVAL/INSTALLATION .)
8		No	Go to the next step.
9	INSPECT ECCENTRIC SHAFT POSITION SENSOR Inspect the eccentric shaft position sensor. (See ECCENTRIC SHAFT POSITION SENSOR INSPECTION .) Is there any malfunction?	Yes	Replace the eccentric shaft position sensor, then go to the next step (See ECCENTRIC SHAFT POSITION SENSOR REMOVAL/INSTALLATION .)
9		No	Go to the next step.
10	INSPECT THREE-WAY CATALYTIC CONVERTER (TWC) Inspect the TWC. (See THREE-WAY CATALYTIC CONVERTER (TWC) INSPECTION .) Is there any malfunction?	Yes	Replace the TWC. (See EXHAUST SYSTEM REMOVAL/INSTALLATION .)
10		No	Go to the next step.
11	VERIFY TROUBLESHOOTING OF DTC P0420 COMPLETED Make sure to reconnect all disconnected connectors. Clear the DTC from the PCM memory using the WDS or equivalent. Perform the DRIVE MODE 1 and 3. (See OBD-II DRIVE MODE .) Is the PENDING CODE same as the DTC present?	Yes	Replace the PCM, then go to the next step. (See PCM REMOVAL/INSTALLATION .)
11		No	Go to the next step.
12	VERIFY AFTER REPAIR PROCEDURE Perform the "AFTER REPAIR PROCEDURE". (See AFTER REPAIR PROCEDURE .) Are any DTCs present?	Yes	Go to the applicable DTC inspection. (See DTC TABLE .)
12		No	DTC troubleshooting completed.