P0128-THERMOSTAT RATIONALITY
For a complete wiring diagram Refer to Section 8W.
- 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. Two trip fault. Three good trips to turn off the MIL.
| Possible Causes |
| TEMPERATURE SENSOR WIRING OR CONNECTORS |
| ENGINE COOLANT LEVEL AND CONDITION |
| THERMOSTAT |
| TEMPERATURE SENSOR |
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE - STANDARD PROCEDURE).
Diagnostic Test
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.
Perform the diagnostic procedure for *CHECKING THE TEMPERATURE SENSORS.
Were any problems found?
Yes
- Repair as necessary.
- Perform the PCM Verification Test Ver. 1 (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No
- Go to 2
2. CHECKING THE ENGINE COOLANT TEMPERATURE SENSOR OPERATION.
Perform the diagnostic procedure for *CHECKING THE ENGINE COOLANT TEMPERATURE SENSOR OPERATION.
Were any problems found?
Yes
- Repair as necessary.
- Perform the PCM Verification Test Ver. 1 (Refer to 9 - ENGINE - STANDARD PROCEDURE)
No
- Refer to the *CHECKING FOR AN INTERMITTENT DTC Diagnostic Procedure. (Refer to 9 - ENGINE - DIAGNOSIS AND TESTING)