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
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. Verify the cooling system is filled to the correct 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) Cooling System Draining and Filling (GE 47716).
• If the coolant level is not correct
Repair as necessary
• If the coolant level is correct
Note: A critical analysis of the operation of the thermostat is necessary to properly diagnose this DTC.
2. Verify the proper heat range and the operation of the thermostat. Refer to Thermostat Diagnosis Thermostat Diagnosis.
• If a condition is found
Replace the Thermostat.
• If a condition is not found
3. 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.
4. Verify the DTC does not set.
• If the DTC sets
Refer to Step 1.
• If the DTC does not set
5. All OK.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the diagnostic procedure.
* Engine Coolant Thermostat Replacement (L4) Service and RepairEngine Coolant Thermostat Replacement (LHU) Service and Repair
* Control Module References Control Module References for ECM replacement, setup and programming