DTC P0128: Cooling System Malfunction

NOTE: Before you troubleshoot, record all freeze data and any on-board snapshot, and review GENERAL TROUBLESHOOTING INFORMATION .

Turn the ignition switch to ON (II).
Clear the DTC with the HDS.
Make sure the blower switch is off.
Check the FAN CTRL in the DATA LIST with the HDS.
Is it OFF?

YES: Go to step 5.

NO: Wait until the FAN CTRL is off, then go to step 5.
Check the radiator fan operation.
Does the radiator fan keep running?

YES: Check the radiator fan circuit (see RADIATOR FAN CIRCUIT TROUBLESHOOTING ) and the radiator fan relay (see POWER RELAY TEST ). If the circuits and the relay are OK, go to step 19.

NO: Go to step 6.
Let the engine cool until the coolant temperature is 122°F (50°C) or less.
Note the value of ECT SENSOR 1 and ECT SENSOR 2 in the DATA LIST with the HDS.
Start the engine, and let it idle.
Let the engine idle until ECT SENSOR 1 is 162°F (72°C).
Check ECT SENSOR 2 in the DATA LIST with the HDS.
Compare the recorded value of ECT SENSOR 2 and the present value of ECT SENSOR 2.
Did the temperature rise 13°F (7.2°C) or more?

YES: 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/PCM.

NO: Test the thermostat (see FAN MOTOR TEST ), then go to step 12.
Turn the ignition switch to ON (II).
Reset the ECM/PCM with the HDS.
Let the engine cool until the coolant temperature is between 19°F (-7°C) and 122°F (50°C).
Do ECM/PCM IDLE LEARN PROCEDURE .
Test-drive at a steady speed between 15-75 mph (24-120 km/h) for 10 minutes.
Check for Pending or Confirmed DTCs with the HDS.
Is DTC P0128 indicated?

YES: Check the cooling system (see COMPONENT LOCATION INDEX ). If the cooling system is OK, go to step 19.

NO: Go to step 18.
Monitor the OBD STATUS for DTC P0128 in the DTCs MENU with the HDS.
Does the screen indicate PASSED?

YES: Troubleshooting is complete. If any other Pending or Confirmed DTCs were indicated in step 17, go to the indicated DTC's troubleshooting.

NO: If the HDS indicates FAILED, check the cooling system (see COMPONENT LOCATION INDEX ). If the cooling system is OK, go to step 19. If the HDS indicates NOT COMPLETED, go to step 14.
Update the ECM/PCM if it does not have the latest software (see ECM/PCM UPDATE ), or substitute a known-good ECM/PCM (see SUBSTITUTING THE ECM/PCM ).
Let the engine cool until the coolant temperature is between 19°F (-7°C) and 122°F (50°C).
Start the engine. Hold the engine speed at 3, 000 rpm without load (A/T in P or N, M/T in neutral) until the radiator fan comes on, then let it idle.
Test-drive at a steady speed between 15-75 mph (24-120 km/h) for 10 minutes.
Check for Pending or Confirmed DTCs with the HDS.
Is DTC P0128 indicated?

YES: Check for poor connections or loose terminals at ECT sensor 1, ECT sensor 2, and the ECM/PCM. If the ECM/PCM was updated, substitute a known-good ECM/PCM (see SUBSTITUTING THE ECM/PCM ), then go to step 20. If the ECM/PCM was substituted, go to step 1.

NO: Go to step 24.
Monitor the OBD STATUS for DTC P0128 in the DTCs MENU with the HDS.
Does the screen indicate PASSED?

YES: If the ECM/PCM was updated, troubleshooting is complete. If the ECM/PCM was substituted, replace the original ECM/PCM (see ECM/PCM REPLACEMENT ). If any other Pending or Confirmed DTCs were indicated in step 23, go to the indicated DTC's troubleshooting.

NO: If the HDS indicates FAILED, check for poor connections or loose terminals at ECT sensor 1, ECT sensor 2, and the ECM/PCM. If the ECM/PCM was updated, substitute a known-good ECM/PCM (see SUBSTITUTING THE ECM/PCM ), then go to step 20. If the ECM/PCM was substituted, go to step 1. If the HDS indicates NOT COMPLETED, go to step 20.