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 00
- 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
* 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 P0102 00, P0103 00, P0111 00, P0112 00, P0113 00, P0114 00, P0116 00, P0117 00, P0118 00, P0119 00, P0121 00, P0122 00, P0123 00, P0221 00, P0222 00, P0223 00, P0335 00, P0336 00, or P0501 00 are not set.
* The ignition voltage is greater than 10 V.
* The start up ECT is between -10 to +75°C (+14 to +167°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 target 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 00 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 00 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 ([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)
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 (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 -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 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, LXV, 2H0, and LUW) (Thermostat Diagnosis)Thermostat 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 B34 Engine Coolant Temperature Sensor.
3. Inspect the B34 Engine Coolant Temperature Sensor terminals and the harness connector for corrosion.
4. Ignition OFF for 90 s, test for less than 5 ohm between the low reference circuit terminal 2 and ground.
• If the resistance measures greater than 5 ohm, test the low reference circuit for an open/high resistance. If the circuit tests normal, replace the K20 Engine Control Module.
5. Disconnect the K20 Engine Control Module.
6. Test for less than 5 ohm between B34 Engine Coolant Temperature Sensor terminal 1 and K20 Engine Control Module terminal 35 X1 for less than 5 ohm.
• If the resistance measures greater than 5 ohm, test the signal circuit for a high resistance.
7. If the circuits test normal, test or replace the B34 Engine Coolant Temperature Sensor.
Component Testing
Static Test
Measure and record the resistance of the K20 Engine Control Module Sensor at various ambient temperatures, then compare those measurements to the Temperature Versus Resistance (IAT) (Temperature Versus Resistance (IAT))Temperature Versus Resistance (ECT) (Temperature Versus Resistance (ECT)).
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Engine Coolant Temperature Sensor Replacement (Radiator) (Engine Coolant Temperature Sensor Replacement (Radiator))Engine Coolant Temperature Sensor Replacement (Thermostat) (Engine Coolant Temperature Sensor Replacement (Thermostat))
* Engine Coolant Thermostat Replacement (1.6L LDE, LXV, LLU, 1.8L 2H0, and LUW) (Service and Repair)
* Control Module References (Programming and Relearning) for engine control module replacement, setup, and programming