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2008 - MX-5 - Engine

DTC P0421 [LF]



DTC P0421

Warm up catalyst system efficiency below threshold

DETECTION CONDITION

  • PCM compares number of front HO2S and rear HO2S inversions for a predetermined time. PCM monitors number of inversions rear side performs while front side inverts for a specified number of times when the following monitoring conditions are met, PCM detects inversion ratio. If inversion ratio is below threshold, PCM determines that catalyst has deteriorated.

    • MONITORING CONDITION
    • 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.

  • DIAGNOSTIC MONITORING TEST RESULTS is available.

  • FREEZE FRAME DATA is available.

  • The DTC is stored in the PCM memory.

POSSIBLE CAUSE

  • TWC deterioration or malfunction

  • Exhaust gas leakage

  • Loose front HO2S

  • Loose rear HO2S

  • PCM malfunction


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.

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.

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 [LF].)

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.

(See EXHAUST SYSTEM REMOVAL/INSTALLATION [LF].)

No

Go to the next step.

5

INSPECT INSTALLATION OF REAR HO2S
  • Inspect the rear HO2S for looseness.

  • Is it normal?

Yes

Go to the next step.

No

Retighten the sensor, then go to Step 7.

(See REAR HEATED OXYGEN SENSOR (HO2S) REMOVAL/INSTALLATION [LF].)

6

INSPECT TWC

Yes

Replace the HO2S, then go to the next step.

(See FRONT HEATED OXYGEN SENSOR (HO2S) REMOVAL/INSTALLATION [LF].)

(See REAR HEATED OXYGEN SENSOR (HO2S) REMOVAL/INSTALLATION [LF].)

No

Replace the TWC, then go to the next step.

(See EXHAUST SYSTEM REMOVAL/INSTALLATION [LF].)

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 M-MDS.

  • Perform the HO2S heater, HO2S, and TWC Repair Verification Drive Mode.

    (See OBD-II DRIVE MODE [LF].)

  • Is the PENDING CODE for this DTC present?

Yes

Replace the PCM, then go to the next step.

(See PCM REMOVAL/INSTALLATION [LF].)

No

Go to the next step.

8

VERIFY AFTER REPAIR PROCEDURE

Yes

Go to the applicable DTC troubleshooting.

(See DTC TABLE [LF].)

No

Troubleshooting completed.




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