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P0128




DTC P0128 (LUJ)

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 coolant temperature (ECT) sensor is a variable resistor that measures the temperature of the engine coolant. The engine control module (ECM) supplies 5 V to the engine coolant temperature sensor signal circuit and a ground for the low reference circuit.

The following table illustrates the difference between temperature, resistance, and voltage:






Conditions for Running the DTC

* DTC P0096, P0097, P0098, P00B6, P0101, P0102, P0103, P0111, P0112, P0113, P0114, P0116, P0117, P0118 or P0119 are not set.
* The start up ECT is between -20 to +73°C (-4 to +163.4°F).
* Average Airflow is greater than 11 g/s
* Engine not run time 30 min
* Engine run time between 22 s and 24 min
* 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 (LUJ) [1][2]Electrical Diagrams
Connector End View Reference

Component Connector End Views Sunshade - Left (D6I)
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 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 Loss of Coolant and Cooling System Draining and Filling (Static Fill) Cooling System Draining and Filling (Static Fill)Cooling System Draining and Filling (GE-47716 Fill) Cooling System Draining and Filling (GE-47716 Fill).
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 Diagnostic Trouble Code (DTC) List - Vehicle.

3. Engine idling 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.

4. Command the Engine Coolant Thermostat Heater to 0% 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 E41 Engine Coolant Thermostat Heater.

5. Command the Engine Coolant Thermostat Heater to 100% with a scan tool. Increase the engine speed to 3 000 RPM. 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).
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. 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 B34 Engine Coolant Temperature Sensor.
2. Ignition OFF for 1 min, 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. Ignition ON, 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.

Caution: If the fuse in the jumper wire opens, the signal circuit may have a short to a voltage or a short to ground and the sensor might 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 warmer than 149°C (300°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. Ignition off, disconnect the harness connector at the B34 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 (Radiator) Engine Coolant Temperature Sensor Replacement (Radiator)Engine Coolant Temperature Sensor Replacement (Thermostat) Engine Coolant Temperature Sensor Replacement (Thermostat)Engine Coolant Temperature Sensor Replacement (Water Outlet) Engine Coolant Temperature Sensor Replacement (Water Outlet)
* Engine Coolant Thermostat Housing Replacement (1.6L LDE, LXV, 1.8L 2H0, LUW, LFH) Service and RepairEngine Coolant Thermostat Housing Replacement (1.4L LUH and LUJ) Service and Repair
* Control Module References Control Module References for ECM replacement, programming and setup

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