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.

P0128

P0128

DTC P0128 is a Type B DTCs.

DTC P0128 is a Type B DTCs.

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 scan tool Calculated ECT - Thermostat parameter. If there is a condition, the calculated temperature will be 10°C (18°F) warmer than the actual engine coolant temperature.

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Component Connector End Views

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Control Module References for scan tool information

1. Verify that the engine coolant level is correct.

⇒	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.

⇒	If the engine coolant temperature is colder than the 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 you observed from the Freeze Frame/Failure Records data.

1. Ignition OFF, disconnect the harness connector at the ECT sensor.
2. Ignition OFF, remove the fuse that supplies B+ to circuit terminal X2-56 of the ECM.

Caution: Do NOT use a test lamp to test the continuity of the circuit. Damage to the control module may occur due to excessive current draw.

Note: The control module or the sensor may be damaged if the circuit is shorted to battery positive voltage.

3. Test for less than 5 Ω between the low reference circuit terminal 2 and ground.

⇒	If greater than specified value, test the circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.

4. Install the fuse that supplies B+ to circuit terminal X2-56 of the ECM.
5. Ignition ON, observe the scan tool the ECT sensor parameter. Verify that the ECT sensor parameter is at -40°C (-40°F).

⇒	If warmer than the specified value, test the signal circuit for a short to ground. If the circuit/connections test normal, replace the ECM.

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

6. Install a 3 A fused jumper wire between the signal circuit terminal 1 and the low reference circuit terminal 2. Verify the scan tool ECT sensor parameter is warmer than 142°C (288°F).

⇒	If colder than the specified value, test the signal circuit for an open/high resistance or for a short to voltage. If the circuit/connections test normal, replace the ECM.

7. If all circuits test normal, test or replace the ECT sensor.

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

Note: 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 % of the specification.

⇒	If not within the specified range, replace the ECT sensor.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.