Safety Warning

DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.

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HOMESERVICE MANUALSMAZDA20086 S SPORT, 4D HATCHBACK, STANDARDREPAIR AND DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 330 (ON-BOARD DIAGNOSTIC (ENGINE CONTROL SYSTEM (L3))DTC P0421DIAGNOSTIC PROCEDURE

2008 Mazda 6 s Sport, 4D Hatchback, Standard

Diagnostic procedure

2008 Mazda 6 s Sport, 4D Hatchback, StandardSECTION 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 [L3] 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 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 Step 7.
4		No	Go to the next step.
5	INSPECT INSTALLATION OF MIDDLE AND REAR HO2S Inspect the middle and rear HO2S for looseness. Is it normal?	Yes	Go to the next step.
5		No	Retighten the sensor, then go to Step 7. (See EXHAUST SYSTEM REMOVAL/INSTALLATION .)
6	INSPECT TWC Clear the DTC using the WDS or equivalent generic OBD function. Turn the ignition switch off then back to the ON position. Inspect the TWC. (See WARM-UP THREE-WAY CATALYTIC CONVERTER (WU-TWC) INSPECTION .) Is it normal?	Yes	Replace the heated oxygen sensor, then go to the next step.
6		No	Replace the TWC, then go to the next step.
7	VERIFY TROUBLESHOOTING OF DTC P0421 COMPLETED Make sure to reconnect all disconnected connectors. Turn the ignition switch to the ON position (Engine off). Clear the DTC from the memory using the WDS or equivalent. Perform the HO2S heater, HO2S, and TWC Repair Verification Drive Mode. (See OBD-II DRIVE MODE .) Is the PENDING CODE for this DTC present?	Yes	Replace the PCM, then go to the next step. (See PCM REMOVAL/INSTALLATION .)
7		No	Go to the next step.
8	VERIFY AFTER REPAIR PROCEDURE Perform the "AFTER REPAIR PROCEDURE". (See AFTER REPAIR PROCEDURE .) Are any DTC present?	Yes	Go to the applicable DTC troubleshooting. (See DTC TABLE .)
8		No	Troubleshooting completed.

RENDER: 1.0x

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When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.
• Brakes feel soft, pull hard to one side, or make grinding noises.
• The engine overheats, stalls repeatedly, or misfires under load.
• You are missing required tools, torque specs, or safe lifting equipment.
• You are not confident in the next step or safety outcome.