GM Service Manual Online
For 1990-2009 cars only

Circuit Description

An engine coolant temperature (ECT) sensor monitors the temperature of the coolant. This input is used by the engine control module (ECM) for engine control and as an enabling criteria for other diagnostics. This input also determines if the ECT is insufficient for thermostat regulating of the coolant.

This engine coolant thermostat monitoring diagnostic is designed to detect a stuck open thermostat. When the engine is started the ECM uses a timer and a modeled ECT table that is based on engine coolant temperature at start-up, intake air temperature (IAT), engine speed (RPM) and engine load to determine when the calibrated threshold of 90°C (194°F) has been met. When this threshold is met, the modeled ECT is compared to the actual measured ECT.

If the ECM detects that the modeled ECT has met the threshold of 90°C (194°F) and the measured ECT has not met the minimum calibrated thermostat regulating temperature of 72°C (161°F) for more than 10 seconds, DTC P0128 sets.

Conditions for Running the DTC

    • DTCs, P0068, P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0125, P1101, P1258 are not set.
    • The ECT Sensor Start-Up parameter is between -45 and +42°C (-49 and +107°F)
    • The Intake Air Temperature Sensor parameter is more than -8.25°C (+17°F).
    • The engine idle time is less than 95 percent.
    • The fuel cut off time is less than 50 percent.
    • The engine metal overtemperature protection (EMOP) is not active.
    • This diagnostic will run under normal driving conditions within the enabling conditions.

Conditions for Setting the DTC

If the ECM detects that the modeled ECT has met the threshold of 90°C (194°F) and the measured ECT has not met the minimum calibrated thermostat regulating temperature of 72°C (161°F) for more than 10 seconds, DTC P0128 sets.

Action Taken When the DTC Sets

    • The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
    • The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

    • The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Clear the MIL and the DTC with a scan tool.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

Connector End View Reference: Engine Controls Connector End Views or Engine Control Module Connector End Views

1

Did you perform the Diagnostic System Check-Vehicle?

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Go to Step 2

Go to Diagnostic System Check - Vehicle in Vehicle DTC Information

2

Important: The cooling fans, if equipped, are commanded ON when certain engine coolant temperature (ECT) DTCs are set.

Is the cooling system coolant low?

--

Go to Cooling System Draining and Filling in Engine Cooling

Go to Step 3

3

Test and verify the proper operation of the thermostat. Refer to Thermostat Diagnosis in Engine Cooling.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 4

4

  1. Disconnect the ECT sensor.
  2. Inspect for the following conditions:
  3. • An ECT sensor that is leaking coolant internally
    • Corrosion on the ECT sensor terminals
    • Improper or corroded terminals at the ECT harness connector
    • Loose terminals in the ECT harness connector

Did you find and correct the condition?

--

Go to Step 14

Go to Step 5

5

Measure the voltage from the signal circuit of the ECT sensor to a good ground with a DMM. Refer to Circuit Testing in Wiring Systems.

Is the voltage within the specified range?

4.8-5.2 V

Go to Step 6

Go to Step 8

6

Measure the voltage from the signal circuit of the ECT sensor to the low reference circuit of the ECT sensor with a DMM. Refer to Circuit Testing in Wiring Systems.

Is the voltage within the specified range?

4.8-5.2 V

Go to Step 9

Go to Step 7

7

Test the ECT sensor low reference circuit for high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 11

8

Test the ECT sensor signal circuit for high resistance. Refer to Circuit Testing and Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 11

9

  1. Turn OFF the ignition.
  2. Remove the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement .
  3. Place the sensor on a work surface away from any heat source.
  4. Allow the sensor to reach the ambient air temperature for 30-60 minutes.
  5. Observe and record the ambient air temperature of the vehicle environment using an accurate thermometer.
  6. Important: Do not hold the ECT sensor by the probe.

  7. Measure the resistance of the ECT sensor and record the value.
  8. Compare the resistance measurement of the ECT sensor to the ambient air temperature on the Temperature vs. Resistance table. Refer to Temperature Versus Resistance .

Is the resistance measurement of the ECT sensor within the specified range for that temperature?

--

Go to Step 10

Go to Step 12

10

Install the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement .

Is the action complete?

--

Go to Intermittent Conditions

--

11

Test for an intermittent and for a poor connection at the engine control module (ECM). Refer to Testing for Intermittent Conditions and Poor Connections , Repairing Connector Terminals and Connector Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 13

12

Replace the ECT sensor. Refer to Engine Coolant Temperature Sensor Replacement .

Did you complete the replacement?

--

Go to Step 14

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13

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 14

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14

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 60 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records.

Did the DTC fail this ignition?

--

Go to Step 2

Go to Step 15

15

Observe the Capture Info with a scan tool.

Are there any DTCs that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information

System OK