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 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 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 ECT to a modeled ECT using information from the following inputs:
* ECT
* Engine load
* Intake air temperature (IAT)
* Mass air flow (MAF)
* Vehicle speed (VS)
The ECM uses the start-up ECT and the start-up intake air temperature (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 P0102, P0103, P0111, P0112, P0113, P0114, P0116, P0117, P0118, P0119, P0121, P0122, P0123, P0221, P0222, P0223, P0335, P0336, or P0501 is not set.
* The decrease in ECT after engine start is within a calibrated range.
* The decrease in IAT after engine start is within a calibrated range.
* The engine idle time is less than 40 %.
* The engine run time is greater than 8 s.
* The engine run time at minimum load is less than 50 %.
* The engine run time at maximum load is less than 90 %.
* The ignition voltage is greater than 10 V.
* The start-up ECT is between -10 and +75°C (14-167°F).
* The start-up IAT is warmer than -10°C (14°F).
* The DTC runs once per drive cycle within the enabling conditions.
Conditions for Setting the DTC
The ECM determines that the modeled ECT has met the calibrated target of 92°C (198°F) and the measured ECT 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 Engine Coolant Temperature Gauge is inoperative.
* The cooling fans will be commanded ON.
* The AC compressor will be commanded OFF.
Conditions for Clearing the MIL/DTC
DTC P0128 is a Type B DTC.
Diagnostic Aids
* DTC P0128 occurring with insufficient vehicle interior heating is an indication of improper thermostat operation.
* Corrosion on the ECT sensor terminals or the ECT harness connector results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT.
* A slight to moderate resistance in the ECT sensor signal circuit or low reference circuit will affect this diagnostic. This condition results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT.
Reference Information
Schematic Reference
Engine Controls Schematics (LEA) Electrical Diagrams
Connector End View Reference
Component Connector End Views Outside Rearview Mirror - Driver
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. 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 (GE 47716) Cooling System Draining and Filling (GE 47716).
3. If the ignition has been OFF for 8 h or greater, the IAT, the ambient temperature, and the ECT should be within 9°C (16°F) of each other. Ignition ON, observe the scan tool IAT Sensor, Ambient Temperature, and the ECT Sensor parameters. Compare those 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 ECT Sensor parameter. The reading should be between -39 to +120°C (-38 to +248°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 fans.
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 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. Ignition 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 ohms between the low reference circuit terminal A 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 B for a short to ground. If the circuit tests normal, replace the K20 engine control module.
4. Install a 3A fused jumper wire between the signal circuit terminal B and the low reference circuit terminal A. Verify the scan tool ECT Sensor parameter is warmer than 149°C (300°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.
5. If the circuits test normal, test or replace the B34 engine coolant temperature sensor.
Component Testing
* Measure and record the resistance of the ECT sensor at various ambient temperatures, then compare those measurements to the Temperature Versus Resistance (Engine Coolant Temperature Sensor) Temperature Versus Resistance (Engine Coolant Temperature Sensor) table.
* Measure and record the resistance of the IAT sensor at various ambient temperatures, then compare those measurements to the Temperature Versus Resistance (Engine Coolant Temperature Sensor) Temperature Versus Resistance (Engine Coolant Temperature Sensor) table.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.
* Engine Coolant Temperature Sensor Replacement Service and Repair
* Engine Coolant Thermostat Replacement Service and Repair
* Control Module References Control Module References for ECM replacement, setup and programming