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 provide an overview of each diagnostic category.

### DTC Descriptors

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

### Circuit Description

The engine control module (ECM) monitors the temperature of the engine coolant for engine control and as an enabling criteria for some diagnostics. The amount of air flow into an engine is proportional to the amount of heat an engine generates. The ECM monitors the amount of air flow into the engine to calculate engine coolant temperature (ECT). The ECM uses the calculated temperature to determine if the engine has warmed up to the closed loop temperature or to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the closed loop temperature, diagnostics that use the ECT as enabling criteria, may not run when expected.

### Conditions for Running the DTC

• | Before the ECM can report DTC P0128 failed, DTC P0117 must run and pass.

• | DTC P0100, P0101, P0102, P0103, P0111, P0112, P0113, P0116, P0117, P0118, P0119, or P0700 is not set.

• | The engine speed is greater than 960 RPM.

• | The ECT is colder than 71°C (160°F) at start up.

• | The calculated ambient air temperature is between -30 to +70°C (-22 to +158°F).

• | The vehicle speed is greater than 15 km/h (9 mph).

• | The air flow coming into the engine has accumulated to greater than 3,000 grams.

• | DTC P0128 runs continuously once the above conditions have been met for approximately 15 minutes.

### Conditions for Setting the DTC

• | The ECM detects that the actual engine coolant temperature is 11°C (20°F) colder than the calculated thermostat regulating temperature of 89°C (192°F), for greater than 4 seconds or for a cumulative of 30 seconds.

| OR

• | The ECM detects that the actual coolant temperature is 10°C (18°F) colder than the calculated ECT, for greater than 4 seconds or for a cumulative of 30 seconds.

### Action Taken When the DTC Sets

DTC P0128 is a Type B DTCs.

### Conditions for Clearing the MIL/DTC

DTC P0128 is a Type B DTCs.

### Diagnostic Aids

An engine that soaks for about 8 hours will help diagnosis the condition. After the cold soak, operate the vehicle at highway speeds for 20 minutes while monitoring the scan tool Calculated ECT - Thermostat parameter. If there is a condition, the calculated temperature will be 10°C (18°F) greater than the actual engine coolant temperature.

### 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. Verify that the engine coolant level is correct.

⇒ | If the engine coolant is not at the correct level, refer to Loss of Coolant for further diagnosis.

2. Verify that the engine coolant temperature is at the normal operating temperature.

⇒ | If the engine coolant temperature is below normal operating temperature, refer to Thermostat Diagnosis for further diagnosis.

3. 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 are captured in the Freeze Frame/Failure Records Data List.

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the ECT sensor.

Caution: Do NOT use a test lamp to test the continuity of the circuit. Damage to the control module may occur due to excessive current draw.

Note: The control module or the sensor may be damaged if the circuit is shorted to battery positive voltage.

2. Remove the fuse that supplies B+ to circuit terminal X2-56 of the ECM.
3. 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.

4. Install the fuse that supplies B+ to circuit terminal X2-56 of the ECM.
5. Ignition ON, verify the scan tool ECT sensor parameter is at -40°C (-40°F).

⇒ | If warmer than the specified range, test the signal circuit for a short to ground. If the circuit/connections test normal, replace the ECM.

Note: If the fuse in the jumper wire opens, the signal circuit is shorted to a voltage and the ECT sensor may be damaged.

6. Install a 3A fused jumper wire between the signal circuit terminal 1 and the low reference circuit terminal 2. Verify the ECT sensor parameter is warmer than 142°C (288°F).

⇒ | If colder than the specified range, test the signal circuit for an open/high resistance or for a short to voltage. If the circuit/connections test normal, replace the ECM.

7. If all circuits test normal, test or replace the ECT sensor.

### Component Testing

1. Ignition OFF, disconnect the harness connector at the ECT sensor.

Note: A thermometer can be used to the used to test the sensor off the vehicle.

2. Test the ECT sensor by varying the sensor temperature while monitoring the sensor resistance. Compare the readings with the Temperature Versus Resistance - Engine Coolant Temperature Sensor table and verify that the resistance is within 5 percent of the specification.

⇒ | If the resistance vs. temperature is not within 5 percent, replace the ECT sensor.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Engine Coolant Temperature Sensor Replacement

• | Control Module References for engine control module replacement, setup, and programming