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2008 - MX-5 - Engine

DTC P0126, P0128 [LF]



DTC P0126

DTC P0128

Coolant thermostat stuck open

DETECTION CONDITION

    DTC P0126
  • If the ECT sensor signal never exceeds 71 °C {160 °F} after engine start for specified period, PCM determines that the coolant thermostat is stuck open.

    • MONITORING CONDITIONS
    • IAT: above –10 °C {14 °F}

    • Vehicle speed: over 6 km/h {3.7 mph}

    DTC P0128
  • PCM monitors MAF, IAT, VSS and EAT signals and calculate radiator’s heat radiation ratio while following monitoring conditions are met. If calculated value exceeds threshold, PCM determines that the coolant thermostat is stuck open.

    • MONITORING CONDITIONS
    • IAT: above –10°C {14 °F}

    • ECT at engine start: Below 36 °C {97 °F}

    • Difference between ECT at engine start and minimum IAT: Below 6 °C {10.8 °F}

    • Vehicle speed: over 30 km/h {18.6 mph}

    Diagnostic support note
  • This is an intermittent monitor (Engine cooling system).

  • The MIL illuminates if the PCM detects above malfunction condition in two consecutive drive cycles or in one drive cycle while the DTC for the same malfunction has been stored in PCM.

  • PENDING CODE is available if the PCM detects the above malfunction condition during the first drive cycle.

  • FREEZE FRAME DATA is available.

  • The DTC is stored in the PCM memory.

  • DIAGNOSTIC MONITORING TEST RESULT is available.

POSSIBLE CAUSE

  • ECT sensor malfunction

  • Coolant thermostat malfunction

  • 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 (Engine cooling system 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

INSPECT FOR OTHER DTCS
  • Have other DTCs been stored?

Yes

Repair circuit malfunction for applicable DTCs.

No

Go to the next step.

4

INSPECT COOLANT THERMOSTAT FOR WHETHER STUCK OPEN

Yes

Inspect the ECT sensor.

Replace the ECT sensor if necessary, then go to the next step.

(See ENGINE COOLANT TEMPERATURE (ECT) SENSOR REMOVAL/INSTALLATION [LF].)

No

Replace the coolant thermostat, then go to the next step.

(See THERMOSTAT REMOVAL/INSTALLATION [LF].)

5

VERIFY MONITORING CONDITION FOR REPAIR VERIFICATION
  • Make sure to reconnect all disconnected connectors.

  • Cool down engine.

    NOTE:

    • If workshop inside and outside temperature difference is significant, PCM might not operate thermostat monitor. Therefore, it is recommended to

      cool down engine out of workshop.

  • Turn the ignition switch to the ON position (Engine off).

  • Clear DTC from PCM memory using M-MDS.

  • Access ECT, IAT PIDs and make sure that each value is within following specifications.

    • ECT: below 36 °C {97 °F} (for P0128 only)

    • Difference between ECT and IAT: Below 6 °C {10.8 °F}

  • Is there any PID that is out of specification?

Yes

Take corrective action (e.g. cool down engine), then repeat this step.

No

Go to the next step for DTC P0126 or go to Step 7 for DTC P0128.

6

VERIFY TROUBLESHOOTING OF DTC P0126 COMPLETED
  • Start the engine and turn off E/L and A/C.

  • Access DIAGNOSTIC MONITORING TEST RESULTS using M-MDS and monitor TEST #10:E1:81 (ECT).

    NOTE:

    • This test requires actual driving. Chassis roller cannot be used for this test.

    • During test drive, constant speed should be maintained, although 2 or 3 stops during every 5 minutes of driving time (e.g. for traffic signals) is acceptable. Stop-and-go (e.g. in case of traffic congestion) is not acceptable during the test period.

    • Test period depends on ECT at engine start. (e.g. if ECT is –10 °C {14 °F}, monitoring period is 20 minutes and ECT is 30 °C {86 °F}, monitoring period is 8 minutes)

  • Verify TEST #10:E1:81 (ECT) value.

  • Is value above minimum value?

Yes

Go to Step 8.

No

Replace the PCM, then go to Step 8.

(See PCM REMOVAL/INSTALLATION [LF].)

7

VERIFY TROUBLESHOOTING OF DTC P0128 COMPLETED
  • Start the engine and turn off E/L and A/C.

  • Access DIAGNOSTIC MONITORING TEST RESULTS using M-MDS and monitor TEST #10:E1:80 (Heat radiation ratio) or #10:E1:81 (ECT).

    NOTE:

    • This test requires actual driving. Chassis roller cannot be used for this test.

    • During test drive, constant speed should be maintained, although 2 or 3 stops (e.g. for traffic signals) is acceptable. Stop-and-go (e.g. in case of traffic congestion) is not acceptable during the test period.

  • Verify TEST #10:E1:80 (Heat radiation ratio) and #10:E1:81 (ECT) value.

  • Are value of TEST #10:E1:80 (Heat radiation ratio) below maximum value and value of TEST #10:E1:81 (ECT) above minimum value?

Yes

Go to the next step.

No

Replace the PCM, then go to the next step.

(See PCM REMOVAL/INSTALLATION [LF].)

8

VERIFY AFTER REPAIR PROCEDURE

Yes

Go to the applicable DTC inspection.

(See DTC TABLE [LF].)

No

Troubleshooting completed.




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