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 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, P0128
Low Reference | -- | P0118 | P0118 | P0118 | 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 (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 signal circuit and supplies a ground to the low reference circuit.

The purpose of both of these rationality diagnostics are to use the ECT sensor to determine if the engine coolant will heat up 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 to determine if the ECT does increase normally, and reach the calibrated target temperatures.

### Conditions for Running the DTC

• | DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0171, P0172, P0174, P0175, P0502, P0503 are not set.

• | The start-up IAT is more than -7°C (+19°F).

• | The start-up ECT is less than 70°C (158°F), when the IAT is above 10°C (50°F).

• | The start-up ECT is less than 50°C (122°F), when the IAT is below 10°C (50°F).

• | The engine run time is between 90 seconds and 22 minutes.

• | The vehicle has traveled more than 2.4 kilometers (1.5 miles) at more than 8 km/h (5 mph).

• | The accumulated airflow is between 20-75 g/s, with the minimum average airflow more than 10 g/s.

• | The fuel ethanol percentage is less than 85 percent.

• | This DTC runs once per ignition cycle within the enabling conditions.

### Conditions for Setting the DTC

The ECM detects that the minimum ECT of 75°C (167°F) has not been met, when the IAT is more than 10°C (50°F).

OR

The ECM detects that the minimum ECT of 55°C (131°F) has not been met, when the IAT is less than 10°C (50°F).

### Action Taken when the DTC Sets

DTC P0128 is Type B DTC.

### Conditions for Clearing the MIL/DTC

DTC P0128 is 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 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

Engine Controls Schematics

#### Connector End View Reference

Component Connector End Views

#### Electrical Information Reference

• | Circuit Testing

• | Testing for Intermittent Conditions and Poor Connections

• | Wiring Repairs

#### Scan Tool Reference

Control Module References for scan tool information

### Circuit/System Verification

1. Ignition OFF, inspect the cooling system surge tank for proper coolant level. Refer to Cooling System Leak Testing and Cooling System Draining and Filling.
2. Ignition OFF for 8 hours or greater.
3. Ignition ON, observe the scan tool IAT Sensor and ECT Sensor parameters. The ECT, IAT, and ambient temperatures should be within 6°C (11°F) of each other.
4. Use the scan tool to verify the proper operation of the engine cooling system fans.

Note: A critical analysis of the operation of the thermostat is necessary, to properly diagnose these DTCs.

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

### Circuit/System Testing

1. Ignition OFF, disconnect the harness connector at the ECT sensor.
2. Ignition OFF for 90 seconds, test for less than 5 ohms between the low reference circuit terminal A 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 less than -39°C (-38°F).

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

4. Install a 3A fused jumper wire between the signal circuit terminal B and the low reference circuit terminal A. Verify the scan tool ECT Sensor parameter is greater than 149°C (300°F).

⇒ | If colder 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.

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

### Component Testing

Measure and record the resistance of the ECT sensor at different ambient temperatures, and then compare those measurements to the Temperature Versus Resistance Table. Refer to Temperature Versus Resistance.

### Repair Instructions

Perform the Diagnostic Repair Verification after completing the diagnostic procedure.

• | Engine Coolant Temperature Sensor Replacement

• | Engine Coolant Thermostat Replacement

• | Engine Control Module Replacement for ECM replacement, setup and programming.