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 P0125: Engine Coolant Temperature (ECT) Insufficient for Closed Loop
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

#### P0125

• | DTCs P0112, P0113, P0117, P0118, P0480, P0481, P0691, P0692, P0693, and P0694 are not set.

• | The engine is operating.

• | DTC P0125 runs continuously once the above conditions have been met.

#### P0128

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

• | DTCs P0100, P0101, P0102, P0103, P0111, P0112, P0113, P0116, P0117, P0118, P0119, P0722, or P0723 are not set.

• | The engine speed is more than 960 RPM.

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

• | The calculated ambient air temperature is more than -11°C (-12°F) and less than 70°C (158°F).

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

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

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

### Conditions for Setting the DTC

#### P0125

The actual ECT is not within 10°C (18°F) of the calculated ECT after 2-5 minutes, which depends on the amount of air flow into the engine after start-up.

#### P0128

• | The ECM detects that the actual coolant temperature is 10°C (18°F) less than the calculated coolant temperature.

• | The condition exists for more than 4 seconds or for a cumulative of 50 seconds.

### Action Taken When the DTC Sets

DTC P0125 and P0128 are Type B DTCs.

### Conditions for Clearing the MIL/DTC

DTC P0125 and P0128 are 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 Calculated ECT - Thermostat parameter with a scan tool. If there is a condition, the calculated temperature will be 10°C (18°F) more then the actual engine coolant temperature.

### Reference Information

#### Schematic Reference

Engine Controls Schematics

#### Connector End View Reference

• | Engine Control Module Connector End Views

• | Engine Controls 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

• | Scan Tool Data List

• | Scan Tool Data Definitions

• | Scan Tool Output Controls

### 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 .

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 .

3. 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. DTC P0125 or P0128 should not set.
4. If the vehicle passes the Circuit/System Verification Test, then operate the vehicle within the conditions for running the DTC. You may also operate the vehicle within the conditions that are captured in the Freeze Frame/Failure Records List.

### Circuit/System Testing

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

Notice: 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.

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

2. Test for less than 5 ohms of resistance between the low reference circuit terminal 2 and ground.

⇒ | If greater than 5 ohms, test the low reference circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.

3. Ignition ON, using a scan tool verify that the ECT sensor parameter is at -40°C (-40°F).

⇒ | If greater than -40°C (-40°F), test the signal circuit for a short to ground. If the circuit/connections test normal, replace the ECM.

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

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

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

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

### Component Testing

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

Important: 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