Diagnostic Instructions

DTC Descriptors

DTC P0116: Engine Coolant Temperature (ECT) Sensor Performance

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

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

Diagnostic Fault Information

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

Typical Scan Tool Data

ECT Sensor Temperature

Circuit	Normal Range	Short to Ground	Open	Short to Voltage
ECT Sensor Signal	-39 to +120°C (-38 to +248°F)	150°C (303°F)	-40°C (-40°F)	-40°C (-40°F)
Low Reference	--	--	-40°C (-40°F)	-40°C (-40°F)

Circuit/System Description

P0116

The engine coolant temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The control module supplies 5 volts to the signal circuit and a ground for the ECT low reference circuit. When the ECT is low, the sensor resistance is high. When the ECT is high, the sensor resistance is low. The control module uses this High Side Coolant Rationality test to determine if the ECT input is skewed high. The internal clock of the control module will record the amount of time the ignition is OFF. At restart, the control module will compare the temperature difference between the ECT and the intake air temperature (IAT). Before failing this test, the control module will perform a calculation to determine the presence of an engine block heater. If the control module detects that the temperature difference between the ECT and the IAT is not within the calibrated range after the ignition OFF time, DTC P0116 sets.

P0125

An ECT sensor monitors the temperature of the coolant. This input is used by the control module for engine control and as an enabling criteria for some diagnostics. The air flow into the engine is accumulated and used to determine if the vehicle has been driven within the conditions that would allow the engine coolant to heat up normally to the Closed Loop temperature. If the coolant temperature does not increase normally or does not reach the Closed Loop temperature, the diagnostics that use engine coolant temperature as enabling criteria may not run when expected. If the control module detects the calibrated amount of air flow and engine run time have been met, and the engine coolant has not met the Closed Loop temperature, DTC P0125 sets.

P0128

An ECT sensor monitors the temperature of the coolant. This input is used by the control module for engine control and as an enabling criteria for some diagnostics. The air flow coming into the engine is accumulated and used to determine if the vehicle has been driven within the conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the regulating temperature of the thermostat, the diagnostics that use ECT as enabling criteria may not run when expected. If the control module detects the calibrated amount of air flow and engine run time have been met, and the ECT has not met the minimum thermostat regulating temperature, DTC P0128 sets.

Conditions for Running the DTC

P0116

P0125

P0128

Conditions for Setting the DTC

P0116

P0125

The control module detects that the actual amount of accumulated airflow is more than the predicted amount of accumulated air flow before the ECT reaches 15°C (59°F).

P0128

The control module detects that the actual amount of accumulated air flow is more than the predicted amount of accumulated air flow before the ECT reaches 80°C (176°F).

Action Taken When the DTC Sets

DTC P0116 P0125 and P0128 are Type B DTCs.

Conditions for Clearing the MIL/DTC

DTC P0116 P0125 and P0128 are Type B DTCs.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

Electrical Information Reference

Scan Tool Reference

Scan Tool Snapshot Procedure

Circuit/System Verification

1. If the ignition has been OFF for 8 hours or more, the ECT and the IAT, should be within 11°C (20°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. Inspect the ECT sensor terminals for corrosion, and for engine coolant leaking through the sensor.
3. 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.

4. Turn OFF the ignition.
5. Inspect the cooling system surge tank 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.

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

Circuit/System Testing

Important: 

•	For P0116 only, the vehicle should be OFF for at least 8 hours for the ECT and the IAT to be at ambient temperature. The vehicle should not have changed environments during this time.

•	The cooling fans are commanded ON when certain ECT DTCs are set.

1. Inspect for the following conditions:

•	The cooling system coolant level--Refer to Loss of Coolant .

•	Corrosion on the ECT terminals

•	Improper or corroded terminals at the ECT harness connector

•	Loose terminals in the ECT harness connector

•	The ECT sensor for leaking engine coolant internally through the sensor

2. With the key ON, engine OFF, measure for 4.8-5.2 volts from the signal circuit of the ECT sensor to a good ground with a DMM.

⇒	If not within 4.8-5.2 volts, test the ECT sensor signal circuit for high resistance, or for an intermittent or poor connection at the control module. If circuits test OK, then replace the control module.

3. Measure for 4.8-5.2 volts from the signal circuit of the ECT sensor to the low reference circuit of the ECT sensor with a DMM.

⇒	If not within 4.8-5.2 volts, test the ECT sensor low reference circuit for high resistance, or for an intermittent or poor connection at the control module. If circuits test OK, then replace the control module.

4. Measure for 4.8-5.2 volts from the IAT signal circuit to a good ground with a DMM.

⇒	If not within 4.8-5.2 volts, test the IAT signal circuit for high resistance, or for an intermittent or poor connection at the control module. If circuits test OK, then replace the control module.

5. Measure for 5 ohms of resistance from the low reference circuit of the IAT sensor to a good ground with a DMM.

⇒	If more than 5 ohms, test the IAT low reference circuit for high resistance or for an intermittent or poor connection at the control module. If circuits test OK, then replace the control module.

6. Observe and record the ambient air temperature of the vehicle environment using an accurate thermometer.
7. After the vehicle has been OFF for 8 hours, remove the MAF/IAT sensor, and the ECT sensor.
8. Place the sensors on a work surface away from any heat source. Allow the sensors to reach the ambient air temperature for 30-60 minutes.

Important: The IAT sensor will start to warm-up as soon as the ignition is turned ON.

9. Connect the MAF/IAT sensor and the ECT sensor to the electrical connector, but DO NOT install them. Insulate the sensors from any engine heat source.
10. Turn ON the ignition and take a snapshot of the Engine Data List with a scan tool. Refer to Scan Tool Snapshot Procedure .
11. Observe the recorded IAT sensor parameters from the scan tool. Compare the difference between the IAT sensor parameter, and the ambient air temperature.

⇒	If more than 8°C (14°F), replace the IAT sensor.

12. Review the snapshot data that was taken with the scan tool. Observe the ECT sensor and IAT sensor parameters with a scan tool for a difference of less than 15°C (27°F).

⇒	If more than 15°C (27°F), replace the ECT sensor.

Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.