The engine coolant temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 volts to the ECT sensor signal circuit and supplies a ground to the low reference circuit.

The purpose of this diagnostic is to analyze the performance of the thermostat by using the ECT sensor to determine if the engine coolant will increase at the correct rate, and also to meet the calibrated target temperatures under various operating conditions.

The ECM uses the start-up ECT and the start-up intake air temperature (IAT) to begin the diagnostic calculation. The air flow into the engine is accumulated, and vehicle speed, distance, and engine run time are also factored in, to determine if the ECT does increase normally, and reach the calibrated target temperatures.

DTC P0128 is a Type B DTC.

DTC P0128 is a Type B DTC.

Inspect the ECT sensor terminals for corrosion, and for engine coolant leaking through the sensor.

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. If the ignition has been OFF for 8 hours or more, the ECT and the IAT, should be within 6°C (11°F) of each other and also the ambient temperature. Turn ON the ignition, with the engine OFF, and use a scan tool to observe the IAT and the ECT sensor parameters.
2. Use the scan tool to verify the proper operation of the engine cooling system fans.

Warning: Under pressure, the temperature of the solution in the radiator can be considerably higher, without boiling. Removing the radiator cap while the engine is hot (pressure is high), will cause the solution to boil instantaneously, with explosive force. The solution will spew out over the engine, fenders, and the person removing the cap. Serious bodily injury may result. Flammable antifreeze, such as alcohol, is not recommended for use at any time. Flammable antifreeze could cause a serious fire.

3. Turn OFF the ignition.
4. Inspect the cooling system surge tank for the proper engine coolant level. Refer to Cooling System Leak Testing and Cooling System Draining and Filling.

Note: A critical analysis of the operation of the thermostat is necessary to properly diagnose these DTCs.

5. Verify the proper heat range, and the operation of the thermostat. Refer to Thermostat Diagnosis.

Note: All electrical components and accessories must be turned OFF and allowed to power down.

1. Ignition OFF, disconnect the ECT harness connector at the ECT sensor.
2. Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit pin A and ground.

⇒	If greater than the specified range, test the low reference circuit pin A for an open/high resistance. If the circuit tests normal, replace the ECM.

3. Ignition ON, verify the scan tool ECT Sensor parameter is less than -39°C (-38°F).

⇒	If greater than the specified range, test the signal circuit pin B for a short to ground. If the circuit tests normal, replace the ECM.

4. Install a 3A fused jumper wire between the signal circuit pin B and the low reference circuit pin A. Verify the scan tool ECT Sensor parameter is greater than 149°C (300°F).

⇒	If less than the specified range, test the signal circuit pin B for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.

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

Measure and record the resistance of the ECT sensor at various ambient temperatures, then compare those measurements to the Temperature vs Resistance Table. Refer to Temperature Versus Resistance.

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.