GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The engine control module (ECM) uses this diagnostic to determine if the engine coolant temperature (ECT) has reached the minimum calibrated thermostat regulating temperature. The ECM supplies 5 volts to the ECT signal circuit and a ground for the ECT low reference circuit. The ECM monitors the temperature of the coolant by using the input received from the ECT sensor. The ECM calculates the amount of fuel burned since start-up to determine if the vehicle has been driven within the conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally, or does not reach the regulating temperature of the thermostat, the engine is considered not warm enough for stable low emission operation. Additionally other diagnostics that use engine coolant temperature as enabling criteria, may not run when expected.

If the ECM detects that the calibrated amount of fuel has been burnt and the ECT has not reached the minimum calibrated temperature DTC P0128 sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

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

Conditions for Running the DTC

    • DTCs P0112, P0113, P0116, P0117, P0118, P0201, P0202, P0203, P0204, P0205, P0206, P0207, P0208, P0611, P0612, P1223, P1226, P1229, P1232, P1235, P1238, P1241, or P1244 are not set.
    • The engine coolant startup temperature is less than 51°C (124°F).
    • The ambient air temperature is more than -7°C (+19°F).
    • The engine run time is more than 925 seconds.
    • The fuel burned since startup is more than 8.5 million cubic millimeters.
    • The total idle time is less than 300 seconds.
    • This DTC will only run once per ignition cycle until a Pass, Fail or Disable condition exists.
        OR
    • DTCs P0112, P0113, P0116, P0117, P0118, P0201, P0202, P0203, P0204, P0205, P0206, P0207, P0208, P0611, P0612, P1223, P1226, P1229, P1232, P1235, P1238, P1241, or P1244 are not set.
    • The engine coolant startup temperature is between 51-65°C (124-149°F).
    • The ambient air temperature is more than -7°C (+19°F).
    • The engine run time is more than 500 seconds.
    • The fuel burned since startup is more than 5 million cubic millimeters.
    • The total idle time is less than 150 seconds.
    • DTC P0128 will only run once per ignition cycle until a Pass, Fail or Disable condition exists.

Conditions for Setting the DTC

The ECM detects the following:

    • The calibrated amount of engine run time has been met.
    • The calibrated amount of fuel has been burnt.
    • The calibrated ECT of 72°C (162°F) has not been met.

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.

Diagnostic Aids

    • DTC P0128 is designed to detect a faulty thermostat.
    • A skewed low ECT sensor can cause a DTC P0128 to set.
    • If an intermittent condition is suspected refer to Inducing Intermittent Fault Conditions in Wiring Systems and Intermittent Conditions .

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 - Engine Controls?

--

Go to Step 2

Go to Diagnostic System Check - Engine Controls

2

Is the cooling system coolant low?

--

Go to Cooling System Draining and Filling in Engine Cooling

Go to Step 3

3

Perform the following actions:

    • Test and verify the proper operation and the heat range of the thermostat .
    • Inspect for a faulty turbocharger coolant bypass valve. Refer to Engine Fails To Reach Normal Operating Temperature in Engine Cooling.

Did you find and correct the condition?

--

Go to Step 14

Go to Step 4

4

  1. Disconnect the engine coolant temperature (ECT) sensor.
  2. Inspect for the following conditions:
  3. • Corrosion on the ECT sensor terminals
    • Improper or corroded terminals at the ECT harness connector
    • Spread or loose terminals in the ECT harness connector

Did you find and correct the condition?

--

Go to Step 14

Go to Step 5

5

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 6

6

Measure the resistance of the ECT sensor signal circuit between the sensor and the ECM with a DMM. Refer to Circuit Testing in Wiring Systems.

Is the resistance within the specified range?

0-10 ohms

Go to Step 7

Go to Step 10

7

Measure the resistance of the ECT sensor low reference circuit between the sensor and the ECM with a DMM. Refer to Circuit Testing in Wiring Systems.

Is the resistance within the specified range?

0-10 ohms

Go to Step 8

Go to Step 11

8

  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 30-60 minutes to reach the ambient air temperature.
  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. Refer to Circuit Testing in Wiring Systems.
  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?

--

Go to Step 9

Go to Step 12

9

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

Is the action complete?

--

Go to Step 13

--

10

Repair the high resistance in the ECT sensor signal circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 14

--

11

Repair the high resistance in the ECT sensor low reference circuit. Refer to Wiring Repairs in Wiring Systems.

Did you complete the repair?

--

Go to Step 14

--

12

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

Did you complete the replacement?

--

Go to Step 14

--

13

Replace the ECM. Refer to Engine Control Module Replacement .

Did you complete the replacement?

--

Go to Step 14

--

14

  1. Clear the DTCs with a scan tool.
  2. Turn OFF the ignition for 30 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

System OK