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P0128




DTC 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 Descriptor
DTC P0128
- Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature

Diagnostic Fault Information






Typical Scan Tool Data






Circuit/System Description

The engine control module (ECM) monitors the temperature of the engine coolant for engine control and as enabling criteria for some diagnostics.The ECM monitors the amount of time to reach a predetermined temperature based on the Start up ECT sensor. The ECM uses this temperature to determine if the engine has warmed up to the thermostat regulating temperature. If the engine 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 time, the DTC sets.

Conditions for Running the DTC

* DTCs P0101, P0102, P0103, P0111, P0112, P0113, P0114, P0116, P0117, P0118, P00B3, P00B4, P00B6 or P010C are not set.
* The start-up ECT sensor is between -10 to +59°C (14 to +138°F).
* The calculated airflow into the engine is greater than 1 g/s.
* The engine OFF time is greater than 30 min.
* The thermostat commanded duty cycle is less than 50%.
* 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 sensor to reach target temperature has exceeded a calibrated time.

Action Taken When the DTC Sets

* DTC P0128 is a Type B DTC.
* The engine is forced ON and Autostop is disabled.

Conditions for Clearing the DTC

DTC P0128 is a Type B DTC.

Reference Information
Schematic Reference

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

Component Connector End Views Hybrid/EV Battery Pack X1
Description and Operation

Hybrid Modes of Operation Description Hybrid Modes of Operation Description
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. Vehicle OFF, inspect the cooling system surge tank for the proper coolant level. Refer to Loss of Coolant Loss of Coolant and Cooling System Draining and Filling Cooling System Draining and Filling.
2. Vehicle in Service Mode, 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 Diagnostic Trouble Code (DTC) List - Vehicle.

3. Engine running for 15 min with the A/C OFF.

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

5. Command the engine coolant thermostat heater to 0% with a scan tool. Increase the engine speed to wide open throttle. Observe the scan tool ECT Sensor parameter. The temperature should increase to greater than 75°C (167°F).

• If less than the specified value, replace the E41 engine coolant thermostat heater.

6. Command the engine coolant thermostat heater to 100% with a scan tool. Increase the engine speed to wide open throttle. Observe the scan tool radiator coolant temperature sensor and the ECT Sensor parameters. The radiator coolant temperature sensor and the ECT Sensor parameters should be within 20°C (68°F).
7. Engine idling, observe the DTC information with a scan tool. DTC P0128 should not set.
8. 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. Vehicle OFF, disconnect the harness connector at the B34 engine coolant temperature sensor.
2. Vehicle OFF, all systems OFF. It may take up to 2 minutes for all vehicle systems to power down.Test for less than 5 ohm 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 K20 engine control module.

3. Vehicle in Service Mode, verify the scan tool ECT Sensor parameter is colder than -39°C (-38°F).

• If warmer than the specified range, test the signal circuit terminal 2 for a short to ground. If the circuit tests normal, replace the K20 engine control module.

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 3 A fused jumper wire between the signal circuit terminal 2 and the low reference circuit terminal 1. Verify the scan tool ECT Sensor parameter is greater than 130°C (266°F).

• If less 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 K20 engine control module.

5. If all circuits test normal, test or replace the B34 engine coolant temperature sensor.

Component Testing

Static Test
1. Vehicle OFF, remove the engine coolant temperature sensor.

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

2. Test the B34 engine coolant temperature 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% of the specification.

• If not within the specified range, replace the B34 engine coolant temperature sensor.

Repair Instructions

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

* Engine Coolant Temperature Sensor Replacement (Water Outlet) Engine Coolant Temperature Sensor Replacement (Water Outlet)Engine Coolant Temperature Sensor Replacement (Thermostat) Engine Coolant Temperature Sensor Replacement (Thermostat)
* Control Module References Programming and Relearning for ECM replacement, setup, and programming

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