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 supplies a ground to the low reference circuit.
The purpose of this diagnostic is to analyze the performance of the thermostat by using the ECT sensor to determine if the engine coolant will increase at the correct rate, and also to meet the calibrated target temperatures under various operating conditions.
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, and engine run time are also factored in, to determine if the ECT does increase normally, and reach the calibrated target temperatures.
Conditions for Running the DTC
* DTCs P0101, P0102, P0103, P0106, P0107, P0108, P0112, P0113, P0116, P0117, P0118, P0120, P0121, P0122, P0123, P0128, P0171, P0172, P0174, P0175, P0220, P0221, P0223, P0502, P0503, P1516, P2101, P2119 are not set.
* The start-up IAT is warmer than -7°C (+19°F).
* The start-up ECT is colder than 70°C (158°F), when the IAT is above 10°C (50°F).
* The start-up ECT is colder than 50°C (122°F), when the IAT is below 10°C (50°F).
* The engine run time is between 90 s and 22 min.
* The vehicle has traveled more than 2.4 km (1.5 mi) at greater than 8 km/h (5 mph).
* The accumulated airflow is between 20-75 g/s, with the minimum average airflow greater than 10 g/s.
* The fuel ethanol percentage is less than 87%.
* This DTC runs once per ignition cycle within the enabling conditions.
Conditions for Setting the DTC
The ECM detects that the minimum ECT of 75°C (167°F) has not been met, when the IAT is warmer than 10°C (50°F).
OR
The ECM detects that the minimum ECT of 55°C (131°F) has not been met, when the IAT is colder than 10°C (50°F).
Action Taken When the DTC Sets
DTC P0128 is a Type B DTC.
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.
* Inspect the ECT sensor terminals and the ECT harness connector for corrosion. This condition results in a greater voltage on the ECT sensor signal circuit, which is interpreted by the ECM as a colder ECT.
* This diagnostic runs in a specific range. Measure and record the resistance of the ECT sensor at various ambient temperatures between -7 and +80°C (+19 and +176°F), then compare those measurements to the Temperature Versus Resistance (Temperature Versus Resistance).
* 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 ([1][2]Electrical Diagrams)
Connector End View Reference
Component Connector End Views (Connector End Views By Name)
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 (Programming and Relearning) for scan tool information
Circuit/System Verification
1. Ignition OFF, inspect the cooling system surge tank for the proper coolant level. Refer to Cooling System Leak Testing (Component Tests and General Diagnostics) and Cooling System Draining and Filling (LS3, L99 Static Fill) (Cooling System Draining and Filling (Static Fill))Cooling System Draining and Filling (LLT Static Fill) (Cooling System Draining and Filling (Static Fill))Cooling System Draining and Filling (GE 47716) (Cooling System Draining and Filling (GE 47716)).
Warning
Under pressure, the temperature of the solution in the radiator can be considerably higher, without boiling. Removing the radiator cap while the engine is hot (pressure is high), will cause the solution to boil instantaneously, with explosive force. The solution will spew out over the engine, fenders, and the person removing the cap. Serious bodily injury may result. Flammable antifreeze, such as alcohol, is not recommended for use at any time. Flammable antifreeze could cause a serious fire.
2. Ignition OFF for 8 h or greater.
3. Ignition ON, observe the scan tool IAT Sensor and ECT Sensor parameters. The ECT, IAT, and ambient temperatures should be within 6°C (11°F) of each other.
Note: A critical analysis of the operation of the thermostat is necessary to properly diagnose these DTCs.
4. Use the scan tool to verify the proper operation of the engine cooling system fans.
5. Verify the proper heat range and the operation of the thermostat. Refer to Thermostat Diagnosis (Thermostat Diagnosis).
6. 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
Note: All electrical components and accessories must be turned OFF and allowed to power down.
1. Ignition OFF, disconnect the harness connector at the B34 ECT sensor.
2. Ignition OFF for 90 s, test for less than 5 ohm between the low reference circuit terminal A and ground.
• If greater than the specified range, test the low reference circuit terminal B for an open/high resistance. If the circuit tests normal, replace the K20 ECM.
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 ECM.
4. Install a 3 A 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 ECM.
5. If all circuits test normal, test or replace the B34 ECT sensor.
Component Testing
Measure and record the resistance of the B34 ECT sensor at various ambient temperatures, then compare those measurements to the Temperature vs Resistance Table. Refer to Temperature Versus Resistance (Temperature Versus Resistance).
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 (LLT) (Service and Repair)Engine Coolant Thermostat Replacement (L99/LS3) (Service and Repair)
* Control Module References (Programming and Relearning) for ECM replacement, setup, and programming