DTC P0126, P0128

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, the 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

The PCM monitors MAF, IAT, VSS and ECT signals and calculate radiator's heat radiation ratio while following monitoring conditions are met. If calculated value exceeds threshold, the 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 (Mode 2/Mode 12) is available.
The DTC is stored in the PCM memory.

POSSIBLE CAUSE

Coolant thermostat malfunction
ECT sensor 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 (engine cooling system 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 related PENDING CODE and stored DTC. Are any DTCs present?	Yes	Go to the applicable DTC inspection. (See DTC TABLE ).
3		No	Go to the next step.
4	INSPECT COOLANT THERMOSTAT FOR WHETHER STUCK OPEN Inspect the coolant thermostat. (See THERMOSTAT INSPECTION ). Is there any malfunction?	Yes	Replace the coolant thermostat, then go to the next step. (See THERMOSTAT REMOVAL/INSTALLATION ).
4		No	Inspect the ECT sensor. (See ENGINE COOLANT TEMPERATURE (ECT) SENSOR INSPECTION ). Replace the ECT sensor if necessary, then go to the next step. (See ENGINE COOLANT TEMPERATURE (ECT) SENSOR REMOVAL/INSTALLATION ).
5	VERIFY MONITORING CONDITION FOR REPAIR VERIFICATION Make sure to reconnect all disconnected connectors. Cool down the 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 the DTC from the PCM memory using the M-MDS. Access the 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 {43°F} Is there any PID that is out of specification?	Yes	Take corrective action (e.g. cool down engine), then repeat this step.
5		No	DTC P0126: Go to the next step. DTC P0128: Go to Step 7.
6	VERIFY TROUBLESHOOTING OF DTC P0126 COMPLETED Start the engine and turn off E/L and A/C. Access the DIAGNOSTIC MONITORING TEST RESULTS using the 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 the TEST #10:E1:81 (ECT) value. Is the value above minimum value?	Yes	Go to Step 8.
6		No	Replace the PCM, then go to Step 8. (See PCM REMOVAL/INSTALLATION ).
7	VERIFY TROUBLESHOOTING OF DTC P0128 COMPLETED Start the engine and turn off E/L and A/C. Access the DIAGNOSTIC MONITORING TEST RESULTS using the 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 the TEST #10:E1:80 (Heat radiation ratio) and #10:E1:81 (ECT) value. Are the 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.
7		No	Replace the PCM, then go to the next step. (See PCM REMOVAL/INSTALLATION ).
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.