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 signal circuit and supplies a ground to the low reference circuit.

The purpose of both of this rationality diagnostic is to use the ECT sensor to determine if the engine coolant will heat up at the correct rate and also 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 to determine if the ECT does increase normally, and reach the calibrated target temperatures.

The ECM detects that the minimum ECT of 75°C (167°F) has not been met, when the IAT is more than 10°C (50°F).

OR

The ECM detects that the minimum ECT of 55°C (131°F) has not been met, when the IAT is less than 10°C (50°F).

DTC P0128 is type B DTC.

DTC P0128 is 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

Scan Tool Reference

Control Module References

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. Ignition ON, 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.

Caution: 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. Ignition OFF.
4. Inspect the cooling system for the proper engine coolant level. Refer to Cooling System Leak Testing and Cooling System Draining and Filling .

Important: 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 .

1. Ignition OFF, and all electrical accessories OFF. Allow sufficient time for all of the control modules to power down before taking a resistance measurement.
2. Disconnect the ECT sensor.
3. Measure for a proper value of less than 10 ohms of resistance between the ECT low reference circuit and a known ground.

⇒	If the resistance is more than 10 ohms, test the low reference circuit for a open/high resistance.

⇒	If the ECT low reference circuit tests normal, then replace the ECM.

4. Disconnect the ECM.
5. Measure the ECT sensor signal for a proper value of less than 5 ohms of resistance between the ECT sensor harness and the ECM harness connectors.

⇒	If the resistance is more than 5 ohms, repair the high resistance in the ECT sensor signal circuit.

6. Remove and test the ECT sensor. Observer the component testing procedure below.

⇒	If the resistance measurements of the ECT sensor are out of range, replace the sensor.

⇒	If the ECT sensor tests normal and the DTC continues to set, then replace the ECM.

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

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.