P0128

P0128-THERMOSTAT RATIONALITY

For a complete wiring diagram, refer to the Wiring Information.

- 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 Powertrain Control Module (PCM) detects that the actual engine coolant temperature does not reach the predicted engine coolant temperature within a specific time. Two trip fault. Three good trips to turn off the MIL.





Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. Pre-Diagnostic Troubleshooting Procedure.

1. 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 crankshaft (CKP) or camshaft (CMP) sensor circuit or rationality DTCs before continuing with this test.

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

NOTE: Verify that the Pinion Factor has been programmed correctly into the PCM.

1. Perform the diagnostic procedure for CHECKING THE TEMPERATURE SENSORS. . Testing and Inspection

Were any problems found?

Yes

- Repair as necessary.
- Perform the POWERTRAIN VERIFICATION TEST. . Powertrain Verification Test

No

- Go To 2

2. CHECKING THE ENGINE COOLANT TEMPERATURE SENSOR OPERATION.
1. 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 POWERTRAIN VERIFICATION TEST. Powertrain Verification Test.

No

- Perform the INTERMITTENT CONDITION diagnostic procedure. Intermittent Condition Test.