P0128
DTC P0128 (LUW)
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 (ECT) sensor signal circuit and a ground for the low reference circuit. The purpose of this diagnostic is to analyze the performance of the thermostat by comparing the measured engine coolant temperature (ECT) sensor to a modeled engine coolant temperature using information from the following inputs:
* Engine coolant temperature (ECT) sensor
* Radiator coolant temperature (RCT) sensor
* Engine load
* Intake air temperature (IAT)
* Manifold absolute pressure (MAP)
* Vehicle speed
The ECM uses the start up ECT and the start up IAT to begin the diagnostic calculation. The air flow into the engine is accumulated, and vehicle speed, distance, engine load, and engine run time are also factored in to determine if the ECT does increase normally and then maintains that temperature within a calibrated range of the modeled ECT.
Conditions for Running the DTC
* DTC P0101, P0102, P0103, P0111, P0112, P0113, P0116, P0117, P0118, P00B6 are not set.
* The ignition voltage is greater than 10 V.
* The start up ECT is between -20 to +73°C (-4 to +163.4°F).
* The start up IAT is warmer than -10°C (+14°F).
* The engine run time at minimum load is less than 50%.
* The engine run time at maximum load is less than 90%.
* The engine idle time is less than 40%.
Conditions for Setting the DTC
The ECM determines that the modeled engine coolant temperature has met the calibrated object of +92°C (+198°F) and the measured engine coolant temperature (ECT) sensor is less than +81°C (+178°F) for greater than 2 s.
Action Taken When the DTC Sets
* DTC P0128 is a type B DTC.
* The cooling fan will be commanded ON.
* The A/C compressor will be commanded OFF.
Conditions for Clearing the DTC
DTC P0128 00 is a Type B DTC.
Diagnostic Aids
The following conditions can also set the DTC:
* DTC P0128 occurring with insufficient vehicle interior heating is an indication of improper thermostat operation.
* Corrosion on the engine coolant temperature (ECT) sensor terminals or the engine coolant temperature (ECT) sensor harness connector results in a greater voltage on the engine coolant temperature (ECT) sensor signal circuit, which is interpreted by the ECM as a colder engine coolant temperature (ECT) sensor.
* A slight to moderate resistance in the engine coolant temperature (ECT) sensor signal circuit or low reference circuit will affect this diagnostic. This condition results in a greater voltage on the engine coolant temperature (ECT) sensor signal circuit, which is interpreted by the ECM as a colder engine coolant temperature (ECT) sensor.
Reference Information
Schematic Reference
Engine Controls Schematics (LUW) [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
Circuit/System Verification
Note: You must perform the Circuit/System Verification before proceeding with Circuit/System Testing.
1. Turn OFF the ignition.
2. Inspect the cooling system surge tank for the proper engine coolant level. Refer to Cooling System Leak Testing Component Tests and General Diagnostics 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).
3. If the ignition has been OFF for 8 h or greater, the intake air temperature, the ambient temperature, and the engine coolant temperature should be within +9°C (+16°F) of each other. Ignition ON, observe the scan tool IAT Sensor, Ambient Air Temperature, ECT Sensor parameters. Compare those scan tool sensor parameters to each other and also to the ambient temperature to determine if the ECT Sensor is skewed colder.
4. Engine running, observe the scan tool ECT Sensor parameter. The reading should be between -40 to +140°C (-40 to +284°F) depending on the current ambient temperature and the vehicle operating conditions.
5. Use the scan tool to verify the proper operation of the engine cooling system fan.
Note: A critical analysis of the operation of the thermostat is necessary to properly diagnose this DTC.
6. Verify the proper heat range and the operation of the thermostat. Refer to Thermostat Diagnosis (LDE, L2W, LXT, LXV, 2H0, LUW and LFH) Thermostat DiagnosisThermostat Diagnosis (1.4L LDD and LUJ) Thermostat Diagnosis.
7. 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. Inspect the B74 Manifold Absolute Pressure Sensor and the B66 Intake Air Temperature Sensor elements for contamination and debris.
2. Ignition OFF, disconnect the harness connector at the B34A Engine Coolant Temperature Sensor 1.
3. Ignition OFF for 90 s, test for less than 5 ohm between the low reference circuit terminal 2 and ground.
• If greater than the specified range, test the ground circuit for an open/high resistance.
4. Ignition ON, verify the scan tool ECT Sensor parameter is at -40°C (-40°F).
• If warmer than -40°C (-40°F), test the signal circuit terminal 1 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.
5. Install a 3 A fused jumper wire between the signal circuit terminal 1 and the low reference circuit terminal 2. Verify the scan tool ECT Sensor parameter is at 140°C (284°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 K20 Engine Control Module.
6. If all circuits/connections test normal, replace the B34A Engine Coolant Temperature Sensor 1.
Component Testing
Static Test
1. Ignition OFF, disconnect the harness connector at the B34A Engine Coolant Temperature Sensor 1.
Note: A thermometer can be used to the used to test the sensor off the vehicle.
2. Test the B34A Engine Coolant Temperature Sensor 1 by varying the sensor temperature while monitoring the sensor resistance. Compare the readings with the Temperature Versus Resistance (IAT) Temperature Versus Resistance (IAT) 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 - Thermostat Side (1.6L LDE, LXV, and 1.8L 2H0) Engine Coolant Temperature Sensor Replacement - Thermostat Side
* Engine Coolant Thermostat Replacement (1.6L LDE, LXV, 1.8L LUW, 2H0 and LFH) Service and Repair
* Control Module References Control Module References for Engine Control Module replacement, programming and setup