P0128

P0128-THERMOSTAT RATIONALITY

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

- When Monitored:
Engine running, start up coolant temperature between -7° C (19.4° F) and 53.3° C (128° F).

- Set Condition:
The Powertrain Control Module (PCM) detects that the actual engine coolant temperature does not reach the predicted engine coolant temperature within a calibrated number of engine cycles. 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. DTC IS ACTIVE
1. Turn the ignition on.
2. With the scan tool, select View DTCs and review the DTC environmental information.

Is the DTC Active or Pending at this time?

Yes

- Go To 2

No

- Perform the CHECKING FOR AN INTERMITTENT DTC diagnostic procedure. Checking for an Intermittent DTC.

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.

1. 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.
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 PCM VERIFICATION TEST. PCM Verification Test.

No

- Perform the CHECKING FOR AN INTERMITTENT DTC diagnostic procedure. Checking for an Intermittent DTC.