For 1990-2009 cars

### 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 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, P0128
Low Reference | -- | P0118 | P0118 | P0118 | P0119, P0128

### Typical Scan Tool Data

#### ECT Sensor

Circuit | Short to Ground | Open | Short to Voltage
--- | --- | --- | ---
Operating Conditions: The ignition is ON, or the engine is running Parameter Normal Range: Varies with ambient temperature, within a range of -39 to +120°C (-38 to +248°F)
ECT Sensor Signal | 140°C (284°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 a ground for the low reference circuit.

The purpose of this diagnostic is to analyze the performance of the thermostat by comparing the measured ECT to a modeled ECT using information from the following inputs:

• | ECT

• | Engine load

• | Intake air temperature (IAT)

• | Mass air flow (MAF)

• | Vehicle speed (VS)

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, engine load, 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

• | DTC P0102, P0103, P0111, P0112, P0113, P0114, P0116, P0117, P0118, P0119, P0121, P0122, P0123, P0221, P0222, P0223, P0335, P0336, or P0501 is not set.

• | The decrease in ECT after engine start is within a calibrated range.

• | The decrease in IAT after engine start is within a calibrated range.

• | The engine idle time is less than 40 percent.

• | The engine run time is greater than 8 seconds.

• | The engine run time at minimum load is less than 50 percent.

• | The engine run time at maximum load is less than 90 percent.

• | The ignition voltage is greater than 10 volts.

• | The start-up ECT is between -10 and +75°C (14-167°F).

• | The start-up IAT is warmer than -10°C (14°F).

• | The DTC runs once per drive cycle within the enabling conditions.

### Conditions for Setting the DTC

The ECM determines that the modeled ECT has met the calibrated target of 92°C (198°F) and the measured ECT is less than 81°C (178°F) for greater than 2 seconds.

### Action Taken When the DTC Sets

• | DTC P0128 is a Type B DTC.

• | The cooling fans will be commanded ON.

• | The AC compressor will be commanded OFF.

### Conditions for Clearing the MIL/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.

• | Corrosion on the ECT sensor terminals or the ECT harness connector results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT.

• | 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

Engine Controls Schematics

#### Connector End View Reference

Component Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Connector Repairs

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

#### Scan Tool Reference

Control Module References for scan tool information.

### Circuit/System Verification

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 IAT, the ambient temperature, and the ECT should be within 9°C (16°F) of each other. Ignition ON, observe the scan tool IAT Sensor, Ambient Temperature, 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 this DTC.

6. Verify the proper heat range, and the operation of the thermostat. Refer to Thermostat Diagnosis.
7. Operate the vehicle within the Conditions for Running the DTC to verify the DTC does not reset. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records data.

### Circuit/System Testing

1. Inspect the MAF/IAT sensor elements for contamination, and debris.
2. Ignition OFF for 90 seconds, disconnect the harness connector at the ECT sensor.
3. Inspect the ECT sensor terminals and the harness connector for corrosion.
4. Test for less than 5 ohms between the low reference circuit terminal 2 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.

5. Disconnect the ECM.
6. Test the ECT signal circuit between terminal 1 and ECM connector X 1 terminal 60 for less than 5 ohms.

⇒ | If greater than the specified range, test the signal circuit for a high resistance.

7. If the circuits test normal, test or replace 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 Versus Resistance table.

• | Measure and record the resistance of the IAT sensor at various ambient temperatures, then compare those measurements to the Temperature Versus Resistance table.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Engine Coolant Temperature Sensor Replacement

• | Engine Coolant Thermostat Replacement

• | Control Module References for ECM replacement, setup, and programming