DTC Troubleshooting

Control System DTCs

DTC Troubleshooting: P0128

DTC P0128 : Cooling System Malfunction

NOTE: Before you troubleshoot, record all freeze data and any on-board snapshot, and review the general troubleshooting information How To Troubleshoot The Fuel And Emissions Systems.

1. Radiator fan control command check:

Radiator fan control command check
-1. Turn the ignition switch to ON (II).

-2. Clear the DTC with the HDS.

-3. Turn the blower switch off.

-4. Check the FAN HIGH CTRL and FAN LOW CTRL in the DATA LIST with the HDS.

Are they OFF?
-YES- Go to step 2.
-NO- Wait until the FAN HIGH CTRL and FAN LOW CTRL are off, then go to step 4.

2. Radiator fan operation check :

Radiator fan operation check
-1. Check the radiator fan operation.

Does the radiator fan keep running?
-YES- Go to step 3.
-NO- Wait until the FAN HIGH CTRL and FAN LOW CTRL are off, then go to step 4.

3. Radiator fan circuit and radiator fan relay check:

Radiator fan circuit and radiator fan relay check
-1. Check the radiator fan circuit, and the radiator fan relay.

Is it OK?
-YES- Update the ECM ECM/PCM Update if it does not have the latest software, or substitute a known-good ECM Service and Repair, then do the repair verification.
-NO- Repair the radiator fan circuit, and the radiator fan relay, then do the repair verification.

4. Problem verification:

Problem verification
-1. Let the engine cool until the coolant temperature is 104 °F (40 °C) or less.

-2. Note the value of ECT SENSOR 1 and ECT SENSOR 2 in the DATA LIST with the HDS.

-3. Start the engine, and let it idle.

-4. Let the engine idle until ECT SENSOR 1 goes up 40 °F (22 °C) or more from the recorded temperature.

-5. Check ECT SENSOR 2 in the DATA LIST with the HDS.

-6. Compare the recorded value of ECT SENSOR 2 and the present value of ECT SENSOR 2.

Did temperature rise 13 °F (7 °C) or more?
-YES- Test the thermostat Thermostat Removal, Installation, and Test and replace it if needed, then do the repair verification.
-NO- Intermittent failure, the system is OK at this time. Check for poor connections or loose terminals at ECT sensor 1, ECT sensor 2, and the ECM.

Repair verification (OBD STATUS check):

-1. Turn the ignition switch to ON (II).

-2. Reset the ECM with the HDS How To Troubleshoot The Fuel And Emissions Systems.

-3. Let the engine cool until the coolant temperature is between 21 °F (-6 °C) and 104 °F (40 °C).

-4. Do the ECM idle learn procedure. ECM/PCM Idle Learn Procedure

-5. Test-drive at a steady speed between 15-75 mph (25-120 km/h) for 10 minutes.

-6. Monitor the OBD STATUS for DTC P0128 in the DTCs MENU with the HDS.

Does the HDS indicate PASSED, FAILED, or NOT COMPLETED?
-PASSED- Troubleshooting is complete. If the ECM was substituted, replace the original ECM Service and Repair. If any other Pending or Confirmed DTCs are indicated, go to the indicated DTC's troubleshooting.
-FAILED- Go to step 1 and recheck. If the ECM was updated, substitute a known-good ECM Service and Repair, then check the OBD STATUS again.
-NOT COMPLETED- Meet the test condition, and check the OBD STATUS again.