Diagnostic Instructions
| • | Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. |
| • | Review Strategy Based Diagnosis for an overview of the diagnostic approach. |
| • | Diagnostic Procedure Instructions provides an overview of each diagnostic category. |
DTC Descriptor
DTC P0128: Engine Coolant Temperature (ECT) Below Thermostat Regulating TemperatureDiagnostic Fault Information
Circuit | Short to Ground | Open/High Resistance | Short to Voltage | Signal Performance |
|---|---|---|---|---|
ECT Sensor Signal | P0117 | P0118 | P0118* | P0116, P0128 |
Low Reference | -- | P0118 | P0118* | P0128 |
*ECM or sensor damage may occur if the circuit is shorted to B+ | ||||
Typical Scan Tool Data
Circuit | Short to Ground | Open | Short to Voltage |
|---|---|---|---|
Operating Conditions: Engine Operating in Closed Loop Parameter Normal Range: Varies wit coolant temperature | |||
ECT Sensor Signal | 150°C (302°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 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 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.
Conditions for Running the DTC
| • | DTCs P0101, P0102, P0103, P0112, P0113, P0117, P0118 are not present. |
| • | The start-up ECT is less than 75°C (167°F). |
| • | The IAT parameter is more than -7°C (+19°F). |
| • | The engine run time is between 30-1,800 seconds. |
| • | The vehicle is driven more than 0.8 kilometer (0.5 mile) at more than 8 km/h (5 mph). |
| • | This diagnostic runs once per ignition cycle when the above conditions are met. |
Conditions for Setting the DTC
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 P0128 is a Type B DTC.
Conditions for Clearing the DTC
DTC P0128 is a Type B DTC.
Diagnostic Aids
| • | DTC P0128 occurring with insufficient vehicle interior heating is an indication of improper thermostat operation. |
| • | Inspect the ECT sensor terminals and the ECT harness connector for corrosion. This condition results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT. |
| • | This diagnostic runs in a specific range. Measure and record the resistance of the ECT sensor at various ambient temperatures between -7 to +80°C (+19 to +176°F), then compare those measurements to the Temperature vs Resistance table. Refer to Temperature Versus Resistance . |
| • | A slight to moderate resistance in the ECT sensor signal circuit or low reference circuit will affect this diagnostic. This condition results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT. |
Reference Information
Schematic Reference
Connector End View Reference
Electrical Information Reference
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions
Scan Tool Reference
Circuit/System Verification
- Turn OFF the ignition.
- Inspect the cooling system surge tank for the proper coolant level. Refer to Cooling System Leak Testing and Cooling System Draining and Filling .
- If the ignition has been OFF for 8 hours or more, the ECT and the IAT should be within 15°C (27°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.
- Use the scan tool to verify the proper operation of the engine cooling system fans.
- Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.
- Verify the proper heat range and the operation of the thermostat. Refer to Thermostat Diagnosis .
Important: A critical analysis of the operation of the thermostat is necessary to properly diagnose these DTCs.
Circuit/System Testing
- Ignition OFF, disconnect the harness connector at the ECT sensor.
- Ignition OFF for 90 seconds, test for less than 5 ohms of resistance between the low reference circuit terminal 2 and ground.
- Ignition ON, verify the scan tool ECT parameter is less than -39°C (-38°F).
- Install a 3A fused jumper wire between the signal circuit terminal 1 and the low reference circuit terminal 2. Verify the scan tool ECT parameter is greater than 149°C (300°F).
- If all circuits test normal, test the ECT sensor.
| ⇒ | If greater than the specified range, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the ECM. |
| ⇒ | If greater than the specified range, test the signal circuit terminal 1 for a short to ground. If the circuit tests normal, replace the ECM. |
| ⇒ | 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. |
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.
| • | Control Module References for ECM replacement, setup, and programming |
