DTC P0421

Warm up catalyst system efficiency below threshold

DETECTION CONDITION

The PCM compares the number of A/F sensor and HO2S inversions for a predetermined time. The PCM monitors the number of inversions side performs while front side inverts for a specified number of times when the following monitoring conditions are met, the PCM detects the inversion ratio. If the inversion ratio is below threshold, the PCM determines that the catalyst has deteriorated.
MONITORING CONDITIONS
- Calculated TWC temperature: more than 400°C {752°F}
- Engine speed: 1, 500-3, 000 rpm
- LOAD: 15-48% (at engine speed 2, 000 rpm )

Diagnostic support note

This is an intermittent monitor (catalyst).
The MIL illuminates if the PCM detects the above malfunction condition in two consecutive drive cycles or in one drive cycle while the DTC for the same malfunction has been stored in the PCM.
PENDING CODE is available if the PCM detects the above malfunction condition during the first drive cycle.
FREEZE FRAME DATA (Mode 2/Mode 12) is available.
The DTC is stored in the PCM memory.

POSSIBLE CAUSE

Exhaust gas leakage
A/F sensor and/or HO2S looseness
TWC deterioration or malfunction
PCM malfunction

DIAGNOSTIC PROCEDURE

STEP	INSPECTION		ACTION
1	VERIFY FREEZE FRAME DATA (MODE 12) AND DIAGNOSTIC MONITORING TEST RESULTS HAVE BEEN RECORDED Have the FREEZE FRAME DATA (Mode 12) and DIAGNOSTIC MONITORING TEST RESULTS (catalyst related) been recorded?	Yes	Go to the next step.
1		No	Record the FREEZE FRAME DATA (Mode 12) and DIAGNOSTIC MONITORING TEST RESULTS on the repair order, then go to the next step.
2	VERIFY RELATED SERVICE BULLETINS AVAILABILITY Verify related Service Bulletins availability. Is any related Service Bulletins available?	Yes	Perform the repair or diagnosis according to the available Service Bulletins. If the vehicle is not repaired, go to the next step.
2		No	Go to the next step.
3	VERIFY RELATED PENDING CODE AND STORED DTC Turn the ignition switch off, then to the ON position (Engine off). Verify the related PENDING CODE and stored DTCs. (See ON-BOARD DIAGNOSTIC TEST ). Are any 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 according to the inspection results, then go to Step 7. (See EXHAUST SYSTEM REMOVAL/INSTALLATION ).
4		No	Go to the next step.
5	INSPECT INSTALLATION OF A/F SENSOR AND HO2S Inspect the A/F sensor and HO2S for looseness. Are the A/F sensor and HO2S installed securely?	Yes	Go to the next step.
5		No	Retighten the A/F sensor and/or HO2S, then go to Step 7. (See AIR FUEL RATIO (A/F) SENSOR REMOVAL/INSTALLATION ). (See HEATED OXYGEN SENSOR (HO2S) REMOVAL/INSTALLATION ).
6	INSPECT TWC Clear the DTC from the PCM memory using the M-MDS. Turn the ignition switch off then to the ON position. Inspect the TWC. (See WARM-UP THREE-WAY CATALYTIC CONVERTER (WU-TWC) INSPECTION ). Is there any malfunction?	Yes	Replace the TWC, then go to the next step. (See EXHAUST SYSTEM REMOVAL/INSTALLATION ).
6		No	Inspect the A/F sensor and HO2S. (See AIR FUEL RATIO (A/F) SENSOR INSPECTION ). (See HEATED OXYGEN SENSOR (HO2S) INSPECTION ). Replace the A/F sensor and/or HO2S, then go to the next step. (See AIR FUEL RATIO (A/F) SENSOR REMOVAL/INSTALLATION ). (See HEATED OXYGEN SENSOR (HO2S) REMOVAL/INSTALLATION ).
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 PCM memory using the M-MDS. Perform the Drive Mode 03 (PCM Adaptive Memory Production, EGR System, A/F Sensor Heater, HO2S Heater, A/F Sensor, 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 DTCs present?	Yes	Go to the applicable DTC inspection. (See DTC TABLE ).
8		No	DTC troubleshooting completed.