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 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 temperature sensor 1 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 thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the thermostat regulating temperature, diagnostics that use the ECT as enabling criteria, may not run when expected. If the engine coolant temperature fails to reach the thermostat regulating temperature, before a predetermined amount of air flow enters the engine, the DTC sets.

### Conditions for Running the DTC

• | DTCs P0112, P0113, P0116, P0117, and P0118 are not set.

• | The engine run time is between 20 seconds and 23 minutes.

• | The start-up ECT is between -20 to +75°C (-4 to +167°F).

• | The calculated airflow into the engine is greater than 11 g/s.

• | The thermostat commanded duty cycle is less than 50 percent.

• | The DTC runs once per ignition cycle when the above conditions are met.

### Conditions for Setting the DTC

The ECM detects that the amount of time it takes for the ECT to reach maximum temperature has exceeded a calibrated time.

### Action Taken When the DTC Sets

DTC P0128 is a Type B DTC.

### Conditions for Clearing the MIL/DTC

DTC P0128 is a Type B DTC.

### Diagnostic Aids

• | ECT sensor 1 is located in the engine and ECT sensor 2 is located in the radiator.

• | Insufficient vehicle interior heating is an indication of improper thermostat operation.

• | A resistance condition in the ECT sensor circuits may cause this DTC. This condition results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT.

• | The thermostat has a mechanical fail-safe in case of an electrical condition with the thermostat heater. The mechanical thermostat will open at approximately 104°C (220°F). The mechanical thermostat will cycle from approximately 104°C (220°F) to approximately 98°C (208°F).

### Reference Information

#### Schematic Reference

• | Engine Controls Schematics

• | Engine Cooling 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. Ignition OFF, inspect the cooling system surge tank for the proper coolant level. Refer to Loss of Coolant and Cooling System Draining and Filling.
2. Ignition ON, observe the DTC information with a scan tool. DTC P00B7, P0597, P0598, P0599, or P2181 should not be set.

⇒ | If a DTC is set, refer to Diagnostic Trouble Code (DTC) List - Vehicle.

3. Engine idling for 15 minutes with the A/C OFF.

Note: Depending on ambient temperature, it may take up to 4 minutes for the temperatures to equalize.

4. Command the Engine Coolant Thermostat Heater to 100 percent with a scan tool. Increase the engine speed to 3,000 RPM. Observe the scan tool RCT and the ECT sensor parameters. The RCT and the ECT sensor parameters should be within 20°C (68°F).
5. Command the Engine Coolant Thermostat Heater to 0 percent with a scan tool. Increase the engine speed to 3,000 RPM. Observe the scan tool ECT sensor parameter. The temperature should increase to greater than 102°C (215°F).

⇒ | If less than the specified value, replace the engine coolant thermostat heater.

6. Engine idling, observe the DTC information with a scan tool. DTC P0128 should not set.
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. Ignition OFF, disconnect the harness connector at the ECT sensor 1.
2. Ignition OFF for 1 minute, test for less than 5 Ω between the low reference circuit terminal 1 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 at -40°C (-40°F).

⇒ | If warmer than the specified value, test the signal circuit terminal 2 for a short to ground. If the circuit tests normal, replace the ECM.

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

4. Install a 1 A fused jumper wire between the signal circuit terminal 2 and ground. Verify the scan tool ECT sensor parameter is greater than 149°C (300°F).

⇒ | If less than 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 all circuits test normal, test or replace the ECT sensor 1.

### Component Testing

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

Note: A thermometer can be 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 table and verify that the resistance is within 5 percent of the specification.

⇒ | If not within the specified range, replace the ECT sensor 1.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Engine Coolant Thermostat Replacement

• | Engine Coolant Temperature Sensor Replacement

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