GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Buick 🢒 2008 🢒 LaCrosse 🢒 Safety and Security 🢒 Supplemental Inflatable Restraints 🢒 DTC 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 P0128:  Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature

Circuit Description

The engine control module (ECM) monitors the temperature of the engine coolant for engine control and as an enabling criteria for some diagnostics. The amount of air flow into an engine is proportional to the amount of heat an engine generates. The ECM monitors the amount of air flow into the engine to calculate engine coolant temperature (ECT). The ECM 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.

Conditions for Running the DTC

P0128

    • Before the ECM can report DTC P0128 failed, DTC P0117 must run and pass.
    • DTCs P0101, P0102, P0103, P0112, P0113, P0118, and P0700 are not set.
    • The engine speed is greater than 960 RPM.
    • The ECT is less than 71°C (160°F) at start up.
    • The calculated ambient air temperature is greater than -11°C (-12°F) and less than 70°C (158°F).
    • The vehicle speed is greater than 15 km/h (9 mph).
    • The air flow coming into the engine has accumulated to greater than 3,000 grams.
    • DTC P0128 runs continuously once the above conditions have been met for approximately 15 minutes.

Conditions for Setting the DTC

P0128

    • The actual ECT is not within 10°C (18°F) of the calculated ECT after approximately 8 minutes, which depends on the amount of air flow into the engine after start-up.
        OR
    • The ECM detects that the actual coolant temperature is 10°C (18°F) less than the calculated coolant temperature.
    • Either condition exists for greater than 4 seconds or for a cumulative of 50 seconds.

Action Taken When the DTC Sets

DTC P0128 is a Type B DTC.

Conditions for Clearing the MIL/DTC

DTC P0128 is a Type B DTC.

Diagnostic Aids

The following table illustrates the ECM connector locations:

ECM Connector Location

X2

X1

Front of Vehicle

An engine that soaks for about 8 hours will help diagnosis the condition. After the cold soak, operate the vehicle at highway speeds for 20 minutes while monitoring the Calculated ECT-Thermostat parameter with a scan tool. If there is a condition, the calculated temperature will be 10°C (18°F) greater than the actual engine coolant temperature.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component 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. Verify that the engine coolant level is correct.
    2. ⇒If the engine coolant is not at the correct level, refer to Loss of Coolant .
  2. Verify that the engine coolant temperature is at the normal operating temperature.
    3. ⇒If the engine coolant temperature is below normal operating temperature, refer to Thermostat Diagnosis .
  3. 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 are captured in the Freeze Frame/Failure Records Data List.

Circuit/System Testing
  1. Ignition OFF, disconnect the harness connector at the engine coolant temperature (ECT) sensor.
  2. Remove the fuse that supplies B+ to circuit terminal X2-36 of the engine control module (ECM).
  3. Test for less than 5 ohms between the low reference circuit terminal 2 and ground.
    4. ⇒If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM.
  4. Install the fuse that supplies B+ to circuit terminal X2-36 of the ECM.
  5. Ignition ON, verify the scan tool ECT sensor parameter is at -40°C (-40°F).
    6. ⇒If greater than the specified range, test the signal circuit for a short to ground. If the circuit tests normal, replace the ECM.

    Important: If the fuse in the jumper wire opens, the signal circuit is shorted to a voltage and the sensor may be damaged.

  6. Ignition ON, connect a 3A fused jumper wire between the signal circuit terminal 1 and the low reference circuit terminal 2. Verify the scan tool ECT sensor parameter is greater than 142°C (288°F).
    7. ⇒If less than the specified range, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.
  7. If all circuits test normal, test or replace the ECT sensor.

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

    2. Important: A thermometer can be used to the 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 - Engine Coolant Temperature Sensor 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 engine control module replacement, setup, and programming