GM Service Manual Online
For 1990-2009 cars only

Circuit Description

The DTC P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature diagnostic monitors the relationship between engine temperature and engine running time. The ECT sensor signal is used to monitor the performance of the engine cooling system thermostat. A DTC P0128 may set if the ECT sensor voltage does not indicate that the engine coolant temperature has increased sufficiently during running of the diagnostic. When the ambient air temperature and engine operating conditions are right, the powertrain control module (PCM) starts the diagnostic using a water temperature counter. The water temperature counter times how long the cooling system takes to reach 75°C (167°F). Failure to reach 75°C (167°F) within the diagnostic window can indicate a malfunctioning thermostat.

DTC Descriptor

This diagnostic procedure supports the following DTC.

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

Conditions for Running the DTC

    • DTCs P0030, P0050, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0122, P0123, P0125, P0135, P0155, P0171, P0172, P0174, P0175, P0222, P0223, P0300-P0306, P0335, P0401, P0404, P0443, P0496, P0506, P0507, P1128, P1129, P2100, P2101, P2108, P2122, P2123, P2127, P2128, P2135, P2138, P2176, P2227, P2228, P2229, P2413, P2414, P2415, and U0107 are not set.
    • The engine speed is less than 5,000 RPM.
    • The engine coolant temperature (ECT) is between -7 and +35°C (20-95°F) at start-up.
    • The intake air temperature (IAT) is between -7 and +35°C (20-95°F) at start-up.
    • The difference between IAT temperature and ECT temperature at engine startup is less than 6°C (10°F).
    • DTC P0128 runs once per drive cycle when the above conditions are met.

Conditions for Setting the DTC

    • The engine coolant temperature failed to reach at least 70°C (159°F).
        OR
    • The difference between the actual ECT temperature and the estimated coolant temperature is 10°C (18°F) or more.
    • The above conditions exist when the engine has been operating for 20 minutes or less depending on the engine load.

Action Taken When the DTC Sets

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

Conditions for Clearing the MIL/DTC

    • The PCM turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
    • A last test failed, or current DTC, clears when the diagnostic runs and does not fail.
    • A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
    • Use a scan tool in order to clear the MIL and the DTC.

Diagnostic Aids

Check for the following conditions:

    • A faulty thermostat that stays open slightly can cause DTC P0128 to set in cold weather when the vehicle is started and allowed to sit while warming up. Replace any suspect thermostat. Refer to Thermostat Diagnosis in Engine Cooling.
    • An incorrect thermostat can cause the engine to warm up slowly. Replace any thermostat that does not meet OEM specifications. Refer to Engine Coolant Thermostat Replacement in Engine Cooling.
    • Use the Temperature vs Resistance table in order to evaluate the possibility of a shifted ECT sensor. A shifted sensor could result in DTC P0128. Refer to Temperature Versus Resistance - Engine Coolant Temperature Sensor .
    • Check for the proper operation of the engine cooling system, including the correct coolant level.

If DTC P0128 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining the vehicle operating conditions when the DTC was first set.

Test Description

The numbers below refer to the step numbers on the diagnostic table.

  1. The Diagnostic System Check - Engine Controls prompts the technician to complete some basic checks and store the Freeze Frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

  2. This step compares the ECT temperature to the IAT temperature. This inspection is to be performed when the vehicle has reached ambient room temperature. At that time both the surrounding air temperature and the engine coolant temperature are nearly equal. If the ECT sensor and circuit are OK, both sensors should indicate the same temperature.

  3. This step checks that the temperature of the engine coolant increases at least 60°C (140°F) within 20 minutes. If the temperature does not increase to the specified value there is a fault with the ECT sensor or the engine cooling system.

  4. This step checks for a correctly operating engine cooling fan. A cooling fan that comes ON too soon or stays ON all the time, can cause a DTC P0128 to set.

  5. Fault not present indicates the condition that caused DTC P0128 to set is intermittent and not currently present. If no other DTCs are stored, refer to Diagnostic Aids for additional information on diagnosing an intermittent DTC P0128.

  6. After replacing the PCM the relationship between the CKP sensor and the crankshaft must re-established. A new minimum throttle position and idle speed must also be established.

Step

Action

Values

Yes

No

Schematic Reference: Engine Controls Schematics

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

  1. Allow the engine to cool to the ambient air temperature.
  2. Turn ON the ignition, with the engine OFF.
  3. Compare the ECT sensor temperature reading to the IAT sensor temperature reading on a scan tool.

Are the ECT and the IAT readings within 2 or 3 degrees of each other?

--

Go to Step 3

Go to Step 6

3

  1. Start the engine and immediately observe the ECT on the scan tool.
  2. Continue to observe the ECT while idling the engine for 10 minutes.

Does the ECT indicated on the scan tool increase to at least the specified value within the 20 minutes?

60°C (140°F)

Go to Step 5

Go to Step 4

4

Test for the correct operation of the engine cooling fan. Refer to Cooling Fan Always On in Engine Cooling.

Did you find and repair a condition with engine cooling fan operation?

--

Go to Step 17

Go to Step 6

5

The fault is not present.

Are there any DTCs stored that have not been diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

Go to Diagnostic Aids

6

  1. Disconnect the ECT sensor electrical connector.
  2. Measure the resistance of the ECT sensor with a DMM.
  3. Compare the measured resistance of the ECT sensor with the resistance values in the Temperature vs Resistance table. Refer to Temperature Versus Resistance - Engine Coolant Temperature Sensor .

Is the resistance of the ECT sensor within specifications?

--

Go to Step 7

Go to Step 14

7

  1. Disconnect the ECT sensor electrical connector.
  2. Turn ON the ignition, with the engine OFF.
  3. Observe the ECT sensor temperature with a scan tool.

Does the scan tool indicate that the ECT sensor temperature is at the specified value?

-40°C (-40°F)

Go to Step 8

Go to Step 12

8

  1. Turn OFF the ignition.
  2. Connect a 3-amp fused jumper wire between the signal circuit and the sensor ground circuit of the ECT sensor.
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the ECT sensor temperature with a scan tool.

Does the scan tool indicate that the ECT sensor temperature is at the specified value?

215°C (419°F)

Go to Step 13

Go to Step 9

9

  1. Turn OFF the ignition.
  2. Connect a 3-amp fused jumper wire between the signal circuit and a known good ground.
  3. Turn ON the ignition, with the engine OFF.
  4. Observe the ECT sensor temperature with a scan tool.

Does the scan tool indicate that the ECT sensor temperature is at the specified value?

215°C (419°F)

Go to Step 10

Go to Step 11

10

  1. Test the ECT sensor ground circuit for high resistance. Refer to Circuit Testing in Wiring Systems.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 15

11

  1. Test the ECT sensor signal circuit for high resistance. Refer to Circuit Testing in Wiring Systems.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 15

12

  1. Test the ECT sensor signal circuit for a short to ground.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 15

13

  1. Inspect the engine cooling system for all of the following:
  2. • The correct coolant level -- Refer to Cooling System Draining and Filling in Engine Cooling.
    • The correct operation of the cooling system -- Refer to Engine Fails To Reach Normal Operating Temperature in Engine Cooling.
    • The correct operation of the thermostat -- Refer to Thermostat Diagnosis in Engine Cooling.
    • The correct operation of the engine cooling fan-- Refer to Cooling Fan Always On in Engine Cooling.
  3. Repair as necessary.

Did you complete the action?

--

Go to Step 17

--

14

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

Did you complete the replacement?

--

Go to Step 17

--

15

  1. Inspect the PCM and the ECT sensor electrical connectors for poor connections. Refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
  2. Repair as necessary. Refer to Wiring Repairs in Wiring Systems.

Did you find and correct the condition?

--

Go to Step 17

Go to Step 16

16

  1. Replace the PCM. Refer to Powertrain Control Module Replacement .
  2. Perform the idle learn procedure. Refer to Idle Learn .

Did you complete the replacement?

--

Go to Step 17

--

17

  1. Use the scan tool in order to clear the DTCs.
  2. Turn OFF the ignition for 30 seconds.
  3. Start the engine.
  4. Operate the vehicle within the Conditions for Running the DTC as specified in the supporting text or until the DTC P0128 diagnostic test has run.

Does the DTC run and pass?

--

Go to Step 18

Go to Step 2

18

With a scan tool, observe the stored information, Capture Info.

Does the scan tool display any DTCs that you have not diagnosed?

--

Go to Diagnostic Trouble Code (DTC) List

System OK