P0128

P0128 Thermostat Function

On Board Diagnosis Logic

NOTE:

If DTC P0128 is displayed with DTC P0300, P0301, P0302, P0303 or P0304, first perform the trouble diagnosis for DTC P0300, P0301, P0302, P0303, P0304. Refer to [P0300, P0301, P0302, P0303, P0304 Misfire] P0300.

Engine coolant temperature has not risen enough to open the thermostat even though the engine has run long enough.

This is due to a leak in the seal or the thermostat being stuck open.






DTC Confirmation Procedure

with SDT

NOTE:

If DTC Confirmation Procedure has been previously conducted, always turn ignition switch OFF and wait at least 10 seconds before conducting the next test.

TESTING CONDITION:

- For best results, perform at ambient temperature of -10 °C (14 °F) or higher.
- For best results, perform at engine coolant temperature of -10 °C (14 °F) to 56 °C (133 °F).
- Before performing the following procedure, do not add fuel.

1) Turn A/C switch OFF.
2) Turn blower fan switch OFF.
3) Turn ignition switch ON.
4) Select "COOLAN TEMP/S" in "Data list" mode with SDT.
5) Check the indication of "COOLAN TEMP/S".

If it is below 56 °C (133 °F), go to next step.

If it is above 56 °C (133 °F), cool down the engine to less than 56 °C (133 °F). then go to next steps.
6) Start engine and drive vehicle for 10 consecutive minutes under the following conditions.






CAUTION:
Always drive vehicle at a safe speed.

NOTE:
If "COOLAN TEMP/S" increases to more than 75 °C (167 °F) within 10 minutes, turn ignition switch OFF because the test result will be OK.

7) Check 1st trip DTC.
8) If 1st trip DTC is detected, go to "Diagnosis Procedure".

with GST

Follow the procedure "With SDT" above.

Diagnosis Procedure

1. CHECK ENGINE COOLANT TEMPERATURE SENSOR

Refer to "Component Inspection".

OK or NG
OK
GO TO 2.
NG
Replace engine coolant temperature sensor. Refer to [Thermostat and Thermostat Housing: Exploded View] Thermostat and Thermostat Housing: Exploded View.

2. CHECK THERMOSTAT

Refer to [Thermostat: Removal and Installation] Thermostat.

OK or NG
OK
INSPECTION END
NG
Replace thermostat. Refer to [Thermostat: Removal and Installation] Thermostat.

Component Inspection

Engine Coolant Temperature Sensor

1) Check resistance between engine coolant temperature sensor terminals 1 and 2 as shown in the figure.





[Reference data]











2) If NG, replace engine coolant temperature sensor.