Diagnostic Instructions

DTC Descriptors

DTC P0125: Engine Coolant Temperature (ECT) Insufficient For Closed Loop Fuel Control

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

Diagnostic Fault Information

Circuit	Short to Ground	High Resistance	Open	Short to Voltage	Signal Performance
ECT Sensor Signal	P0117	P0118	P0118	P0118	P0116, P0119, P0125, P0128
Low Reference	--	P0118	P0118	P0118	P0119, P0125, P0128

Typical Scan Tool Data

ECT Sensor Temperature

Circuit	Short to Ground	Open	Short to Voltage
Operating Conditions: Engine Running Parameter Normal Range: -39 to +120°C (-38 to +248°F)
ECT Sensor Signal	143°C (289°F)	-40°C (-40°F)	-40°C (-40°F)
Low Reference	--	-40°C (-40°F)	-40°C (-40°F)

Circuit/System Description

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 the P0125 diagnostic is to analyze the performance of the coolant temperature sensor, by using the ECT sensor to determine if the temperature of the engine coolant increases at the correct rate, by comparing it to a modeled ECT under various operating conditions.

The purpose of the P0128 diagnostic is to analyze the performance of the thermostat, by using the ECT sensor to determine if the temperature of the engine coolant increases at the correct rate, and then maintains that actual measured ECT within a calibrated range of the modeled ECT 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 then maintains that temperature within a calibrated range of the modeled ECT.

Conditions for Running the DTC

P0125

P0128

Conditions for Setting the DTC

P0125

The ECM detects that the actual measured ECT is 10°C (18°F) less than the modeled ECT.

P0128

The ECM detects that the actual measured ECT is 11°C (20°F) less than the modeled ECT.

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

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Electrical Information Reference

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

Circuit/System Verification

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.

1. Turn OFF the ignition.
2. Inspect the cooling system surge tank for the proper engine coolant level. Refer to Cooling System Leak Testing and Cooling System Draining and Filling .
3. If the ignition has been OFF for 8 hours or greater, the intake air temperature (IAT), the IAT 2 and the ECT should be within 9°C (16°F) of each other and also the ambient temperature. Ignition ON, observe the scan tool IAT Sensor, IAT 2 Sensor and the ECT Sensor parameters. Compare those sensor parameters to each other and also to the ambient temperature to determine if the ECT sensor is skewed colder.
4. Engine running, observe the ECT Sensor parameter. The reading should be between -39 to +120°C (-38 to +248°F) depending on the current ambient temperature and the vehicle operating conditions.
5. Use the scan tool to verify the proper operation of the engine cooling system fans.

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

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

Circuit/System Testing

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 and ground.

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

4. Install a 3A fused jumper wire between the signal circuit and the low reference circuit. Verify the scan tool ECT Sensor parameter is greater than 140°C (284°F).

⇒	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.

5. If the circuits test normal, test the ECT sensor.

Component Testing

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 .

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.