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DTC P0128: Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature: Troubleshooting

TROUBLESHOOTING CHART

Step Action Value (s) Yes No
1 Perform an On-Board Diagnostic (OBD) System Check.
Is the system check complete?		 - Go to Step 2 Go to On-Board Diagnostic (OBD II) System Check
2
  1. Turn the ignition ON, with the engine OFF.
  2. Install a scan tool.

Does the scan tool display any additional DTCs set?		 - Go to applicable DTC table Go to Step 3
3
  1. Allow the engine to cool fully to ambient temperature.
  2. Compare the Engine Coolant Temperature (ECT) sensor reading to the Intake Air Temperature (IAT) sensor reading displayed on the scan tool.

Are the temperature readings close?		 - Go to Step 4 Go to Step 5
4 Check the thermostat for proper operation and replace as necessary. Is the repair complete? - Go to Step 13 -
5
  1. Turn the ignition OFF.
  2. Disconnect the ECT sensor connector.
  3. Turn the ignition ON, with the engine OFF.
  4. Using a Digital Voltmeter (DVM), measure the resistance across the ECT sensor terminals 1 and 2.
  5. Compare the ECT sensor value to the actual coolant temperature using the Temperature vs. Resistance table.

Does the ECT sensor accurately reflect the actual ECT?		 - Go to Step 6 Go to Step 8
6 With the ECT sensor connector still disconnected.
Is the ECT sensor value displayed on the scan tool less than the specified value?		 -30 ° C (-22 ° F) Go to Step 7 Go to Diagnostic Aids
7 Jumper the ECT sensor ground circuit terminal 2 and the sensor signal circuit terminal 1 together at the ECT sensor harness connector.
Is the ECT sensor value displayed on the scan tool greater than the specified value?		 130 ° C (266 ° F) Go to Step 13 Go to Step 9
8 Replace the ECT sensor.
Is the repair complete?		 - Go to Step 13 -
9
  1. Turn the ignition OFF.
  2. Disconnect the Engine Control Module (ECM) connector.
  3. Turn the ignition ON, with the engine OFF.
  4. Using a DVM, measure the voltage at ECM connector terminal E25.

Is the voltage within the specified value?		 4 - 5 V Go to Step 10 Go to Step 12
10 Check for an open or short to ground on the ECT signal circuit between the ECT harness connector terminal 1 and the ECM harness connector terminal E25 and repair as necessary.
Is the repair complete?		 - Go to Step 13 Go to Step 11
11
  1. Disconnect the ECM connector.
  2. Check the ECT ground circuit for an open between the ECT sensor harness connector terminal 2 and the ECM harness connector terminal E51 and repair as necessary.

Is the repair complete?		 - Go to Step 13 -
12
  1. Turn the engine OFF.
  2. Replace the ECM.

Is the repair complete?		 - Go to Step 13 -
13
  1. Using a scan tool, clear the Diagnostic Trouble Codes (DTCs).
  2. Start the engine.
  3. Operate the vehicle within the conditions for setting the DTC as specified in the supporting text.

Does the scan tool indicate that this diagnostic ran and passed?		 - Go to Step 14 Go to Step 2
14 Check if any additional DTCs are set.
Are any DTCs displayed that have not been diagnosed?		 - Go to applicable DTC table System OK
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