P0128

P0128-THERMOSTAT RATIONALITY

For a complete wiring diagram Consult Diagrams/Electrical.

- When Monitored:
With the engine running, start up coolant temperature between -8° C (17.6° F) and 50° C (122° F), the difference between ambient temperature and coolant temperature less than 10° C (50° F) and average vehicle speed greater than 16 kph (10 mph) for more than 18% of vehicle run time.

- Set Condition:
The PCM detects that the actual engine coolant temperature does not reach the predicted engine coolant temperature within a specific time.





Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding.Initial Inspection and Diagnostic Overview

Diagnostic Test

1. DTC IS ACTIVE
Turn the ignition on.
With the scan tool, select View DTCs and review the DTC environmental information.
Is the good trip counter greater than 0?

Yes

- The DTC is not active at this time. Refer to the *CHECKING FOR AN INTERMITTENT DTC Diagnostic Procedure.Checking for an Intermittent DTC

No

- Go to 2

2. CHECKING THE TEMPERATURE SENSORS.

NOTE: Diagnose and repair any ambient air temperature (AAT), intake air temperature (IAT), or engine coolant temperature (ECT) sensor circuit or rationality DTCs before continuing with this test.

NOTE: Diagnose and repair any communication DTCs before continuing with this test.

Perform the diagnostic procedure for *CHECKING THE TEMPERATURE SENSORS.Checking the Temperature Sensors
Were any problems found?

Yes

- Repair as necessary.
- Perform the PCM Verification Test.PCM Verification Test

No

- Go to 3

3. CHECKING THE ENGINE COOLANT TEMPERATURE SENSOR OPERATION.
Perform the diagnostic procedure for *CHECKING THE ENGINE COOLANT TEMPERATURE SENSOR OPERATION.Checking the Engine Coolant Temperature Sensor Operation
Were any problems found?

Yes

- Repair as necessary.
- Perform the PCM Verification Test.PCM Verification Test

No

- Refer to the *CHECKING FOR AN INTERMITTENT DTC Diagnostic Procedure.Checking for an Intermittent DTC