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 ambient air temperature to begin the diagnostic calculation. This diagnostic monitors the amount of energy going into and out of the engine cooling system during the warm up process 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 engine not run time is greater than 30 m.
* 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 ignition cycle when the above conditions are met.
Conditions for Setting the DTC
The ECM determines that the modeled ECT has met the calibrated target temperature of 92°C (198°F) and the measured ECT is less than 79°C (174°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.
* DTC P0128 and DTC 2181 both currently failed could indicate a stuck open thermostat.
Reference Information
Schematic Reference
Engine Controls Schematics [1][2]Electrical Diagrams
Connector End View Reference
Component Connector End Views Sunshade - Left (D61)
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. 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 Fill) Cooling System Draining and Filling (GE-47716 Fill).
• If the coolant level is not correct
Repair as necessary
• If the coolant level is correct
2. Verify the DTC P00B7, P0116, P0117, P0118, P0597, P0598, or P0599 is not set.
• If a DTC is set
Refer to Diagnostic Trouble Code (DTC) List - Vehicle Diagnostic Trouble Code (DTC) List - Vehicle
• If a DTC is not set
3. Engine running for 15 min with the A/C OFF, increase the engine speed to 3,000 RPM.
4. Verify the scan tool ECT Sensor parameter is greater than 79°C (174°F) when commanding the Engine Coolant Thermostat Heater to 0%.
• If less than 79°C (174°F)
Replace the E41 Engine Coolant Thermostat Heater.
• If greater than 79°C (174°F)
5. 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.
6. Verify the DTC does not set.
• If the DTC sets
Refer to Step 1.
• If the DTC does not set
7. All OK
Component Testing
1. Ignition OFF, disconnect the B34 Engine Coolant Temperature Sensor harness connector at the sensor.
2. Test for refer to Temperature Versus Resistance (ECT) Temperature Versus Resistance (ECT) Temperature Versus Resistance between the signal circuit terminal 1 and the low reference terminal 2.
• If the temperature versus resistance are not within the specified ranges
Replace the B34 Engine Coolant Temperature Sensor.
• If the temperature versus resistance are within the specified ranges
3. Test for infinite resistance between each terminal and the sensor housing.
• If less than infinite resistance
Replace the B34 Engine Coolant Temperature Sensor.
• If infinite resistance
4. All OK.
Repair Instructions
Perform the Diagnostic Repair Verification Verification Tests after completing the repair.
* Engine Coolant Temperature Sensor Replacement - Thermostat Side Engine Coolant Temperature Sensor Replacement - Thermostat Side
* Engine Coolant Thermostat Replacement Service and Repair
* Control Module References Control Module References for ECM replacement, programming, and setup.