P0128

DTC P0125 or P0128

Diagnostic Instructions

* Perform the Diagnostic System Check - Vehicle (Diagnostic System Check - Vehicle) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Strategy Based Diagnosis) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Diagnostic Procedure Instructions) provides 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 powertrain control module (PCM) uses the temperature of the engine coolant for engine control and as an enabling criteria for some diagnostics. The PCM monitors the starting engine coolant temperature, engine loads, engine speed, and elapsed time to calculate engine coolant temperature (ECT). The PCM 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. If the engine coolant temperature fails to reach the closed loop temperature or the thermostat regulating temperature, before a predetermined amount of air flow enters the engine, the applicable DTC sets.

Conditions for Running the DTC

P0125
* DTCs P0100, P0102, P0103, P0110, P0112, P0113, P0115, P0116, P0117, P0118 are not set.
* The engine coolant temperature is at a specified value at engine start up.
* The engine is running.
* The DTC runs continuously when the above conditions are met.

P0128
* DTCs P0010, P0011, P0012, P0031, P0032, P0100, P0102, P0103, P0110, P0112, P0113, P0115, P0116, P0117, P0118, P0125, P0171, P0172, P0300-P0304, P0335, P0339, P0340, P0341, P0351, P0352, P0353, P0354, P0500, P2196, P2238, P2239, P2252, P2253, P2A00 are not set.
* The engine is running.
* The battery voltage is greater than 11 volts.
* The DTC runs once per ignition cycle when the above conditions are met.

Conditions for Setting the DTC

P0125

The PCM detects one of the following conditions:

Condition 1
* The ECT is colder than -10°C (+14°F) at engine start up.
* The increase in ECT is insufficient for closed loop fuel control after running for 20 minutes

Condition 2
* The ECT is between -10°C and -2°C (+14°F and +28°F) at engine start up.
* The increase in ECT is insufficient for closed loop fuel control after running for 5 minutes

Condition 3
* The ECT is warmer than -2°C (+28°F) at engine start up.
* The increase in ECT is insufficient for closed loop fuel control after running for 2 minutes

P0128

The PCM detects that the amount of time it takes for the ECT to reach 75°C (167°F) has exceeded a calibrated time.

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

If the customer reports a concern that the engine does not reach operating temperature or that the heater output is low, refer to Thermostat Diagnosis (Thermostat Diagnosis).

Reference Information
Schematic Reference

Engine Controls Schematics ([1][2]Electrical Diagrams)
Connector End View Reference

Component Connector End Views (Connector Views)
Electrical Information Reference

* Circuit Testing (Circuit Testing)
* Connector Repairs (Connector Repairs)
* Testing for Intermittent Conditions and Poor Connections (Testing for Intermittent Conditions and Poor Connections)
* Wiring Repairs (Wiring Repairs)

DTC Type Reference

Powertrain Diagnostic Trouble Code (DTC) Type Definitions (Powertrain Diagnostic Trouble Code (DTC) Type Definitions)
Scan Tool Reference

Control Module References (Programming and Relearning) for scan tool information

Circuit/System Verification

1. Perform the Loss of Coolant (Loss of Coolant) procedure to verify correct fluid level.
2. Perform the Thermostat Diagnosis (Thermostat Diagnosis) procedure to verify correct normal operating temperature.
3. Engine running, observe the DTC information with a scan tool. DTCs P0125 or P0128 should not set.
4. 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 ECT sensor.
2. Test for less than 10 ohm 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, or for a short to voltage. If the circuit tests normal, replace the PCM.

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

• If warmer than the specified value, test the signal circuit for a short to ground. If the circuit tests normal, replace the PCM.

4. Install a 3 A fused jumper wire between the signal circuit terminal 2 and ground. Verify the scan tool ECT sensor parameter is greater than 139°C (280°F).

• If less than the specified value, test the signal circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the PCM.

5. 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 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 (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.

Repair Instructions

Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.

* Engine Coolant Temperature Sensor Replacement (Service and Repair)
* Control Module References (Programming and Relearning) for PCM replacement, setup, and programming