GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Pontiac 🢒 2008 🢒 Vibe - FWD 🢒 Safety and Security 🢒 Supplemental Inflatable Restraints 🢒 DTC P0125 or P0128

Diagnostic Instructions
    • Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
    • Review Strategy Based Diagnosis for an overview of the diagnostic approach.
    •  Diagnostic Procedure Instructions provides an overview of each diagnostic category.

DTC Descriptors

DTC P0125:  Engine Coolant Temperature (ECT) Insufficient for Closed Loop

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

Circuit Description

The powertrain control module (PCM) uses the temperature of the engine coolant for engine control and as an enabling criteria for some diagnostics. The PCM monitors the starting engine coolant temperature, engine loads, engine speed, and elapsed time to calculate engine coolant temperature (ECT). The PCM uses the calculated temperature to determine if the engine has warmed up to the closed loop temperature or to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the closed loop temperature, diagnostics that use the ECT as enabling criteria, may not run when expected. If the engine coolant temperature fails to reach the closed loop temperature or the thermostat regulating temperature, before a predetermined amount of air flow enters the engine, the applicable DTC sets.

Conditions for Running the DTC

P0125

    • DTCs P0100, P0102, P0103, P0110, P0112, P0113, P0115, P0116, P0117, P0118 are not set.
    • The engine coolant temperature is at a specified value at engine start up.
    • The engine is running.
    • The DTC runs continuously when the above conditions are met.

P0128

    • DTCs P0010, P0011, P0012, P0031, P0032, P0100, P0102, P0103, P0110, P0112, P0113, P0115, P0116, P0117, P0118, P0125, P0300-P0304, P0335, P0340, P0341, P0351, P0352, P0353, P0354, P0500, P2196, P2238, P2239, P2252, P2253, P2A00 are not set.
    • The engine is running.
    • The battery voltage is greater than 11 volts.
    • The DTC runs once per ignition cycle when the above conditions are met.

Conditions for Setting the DTC

P0125

The PCM detects one of the following conditions:

Condition 1

    • The ECT is colder than -10°C (+14°F) at engine start up.
    •  The increase in ECT is insufficient for closed loop fuel control after running for 20 minutes

Condition 2

    • The ECT is between -10°C and -2°C (+14°F and +28°F) at engine start up.
    •  The increase in ECT is insufficient for closed loop fuel control after running for 5 minutes

Condition 3

    • The ECT is warmer than -2°C (+28°F) at engine start up.
    •  The increase in ECT is insufficient for closed loop fuel control after running for 2 minutes

P0128

The PCM detects that the amount of time it takes for the ECT to reach 75°C (167°F) has exceeded a calibrated time.

Action Taken When the DTC Sets

DTC P0125 and P0128 are Type B DTCs.

Conditions for Clearing the MIL/DTC

DTC P0125 and P0128 are Type B DTCs.

Diagnostic Aids

If the customer reports a concern that the engine does not reach operating temperature or that the heater output is low, refer to Thermostat Diagnosis.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

    •  Powertrain Control Module Connector End Views
    •  Engine Controls Connector End Views

Electrical Information Reference

    •  Circuit Testing
    •  Connector Repairs
    •  Testing for Intermittent Conditions and Poor Connections
    •  Wiring Repairs

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

Circuit/System Verification
  1. Perform the Loss of Coolant procedure to verify correct fluid level.
  2. Perform the Thermostat Diagnosis procedure to verify correct normal operating temperature.
  3. Engine running, observe the DTC information with a scan tool. DTCs P0125 or P0128 should not set.
  4. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

Circuit/System Testing

Warning: An electric fan under the hood can start up even when the engine is not running and can injure you. Keep hands, clothing and tools away from any underhood electric fan.

  1. Allow the engine to cool to ambient air temperature.
  2. Ignition ON, observe the scan tool ECT sensor parameter and the scan tool intake air temperature (IAT) sensor parameter. The ECT and the IAT readings should be within 2 or 3 degrees of each other.
    3. ⇒If not within the specified range, test the ECT sensor. If the ECT sensor tests normal, test for high resistance in the ECT sensor signal circuit terminal 2 and the ECT sensor low reference circuit terminal 1. If the circuits test normal, replace the PCM.
  3. Start the engine, immediately observe the scan tool ECT sensor parameter. Observe the ECT sensor reading for 10 minutes. The ECT reading should increase by greater than 20°C (36°F).
    4. ⇒If less than the specified range, test for the following conditions:
    • The correct operation of the cooling system--Refer to Engine Fails To Reach Normal Operating Temperature.
    • The correct operation of the thermostat--Refer to Thermostat Diagnosis.
    • The correct temperature specification and correct design thermostat for the vehicle application.
    ⇒If you find any of the above conditions, repair as necessary.

Component Testing
  1. Ignition OFF, disconnect the harness connector at the ECT sensor.

    2. Important: A thermometer can be used to test the sensor off the vehicle.

  2. Test the ECT sensor by varying the sensor temperature while monitoring the sensor resistance. Compare the readings with the Temperature Versus Resistance table and verify that the resistance is within 5 percent of the specification.
    3. ⇒If not within the specified range, replace the ECT sensor.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

    •  Engine Coolant Temperature Sensor Replacement
    •  Control Module References for control module replacement, setup, and programming