P0128
DTC P0128
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions provides an overview of each diagnostic category.
DTC Descriptor
DTC P0128
- Engine Coolant Temperature (ECT) Below Thermostat Temperature
Diagnostic Fault Information
Typical Scan Tool Data
Circuit/System Description
The engine control module (ECM) uses this diagnostic to determine if the engine coolant temperature (ECT) has reached the minimum calibrated thermostat regulating temperature. The ECM supplies 5 volts to the ECT signal circuit, and a ground for the ECT low reference circuit. The ECM monitors the temperature of the coolant by using the input received from the ECT sensor. The ECM calculates the amount of fuel burned since start-up to determine if the vehicle has been driven within the conditions that would allow the engine coolant to heat up normally to the thermostat regulating temperature. If the coolant temperature does not increase normally, or does not reach the regulating temperature of the thermostat, the engine is considered not warm enough for stable, low emission operation.
The purpose of this rationality diagnostics is to use the ECT sensor to determine if the engine coolant will heat up at the correct rate, and also 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.
Conditions for Running the DTC
* DTC P0112, P0113, P0116, P0117, P0118 are not set.
* The engine is running.
* The IAT 1 is less than 10°C (50°F), low region.
* The IAT 1 is greater than 10°C (50°F), high region.
* The start-up engine coolant temperature is less than 65°C (149°F).
* DTC P0128 will only run once per ignition cycle until a Pass, Fail or Disable condition exists.
Conditions for Setting the DTC
The engine did not meet the ECT target temperatures of 50°C (120°F) low region or 72°C (162°F) high region during expected calibrated warm-up times and start-up temperatures.
Action Taken When the DTC Sets
DTC P0128 is a Type B DTC.
Conditions for Clearing the DTC
DTC P0128 is a Type B DTC.
Diagnostic Aids
DTC P0128 is designed to detect for a stuck open thermostat.
Circuit/System Verification
Caution: 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.
Important: A critical analysis of the operation of the thermostat is necessary to properly diagnose this DTC.
* Verify the proper heat range and for a stuck open thermostat. Refer to Thermostat Diagnosis. Component Tests and General Diagnostics
* Inspect the cooling system surge tank for the proper engine coolant level.
* Inspect the ECT sensor terminals for corrosion and for engine coolant leaking through the sensor.
* Verify the proper operation of the engine cooling fan, fan stuck ON/fully engaged all the time.
* Engine running, observe the DTC information with a scan tool. DTC P0128 should not set.
* 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 Records data.
Circuit/System Testing
1. Ignition OFF, disconnect the harness connector at the ECT sensor.
2. Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit terminal B 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 control module.
3. Ignition ON, verify the scan tool ECT parameter is less than -39°C (-40°F).
• If greater than the specified range, test the signal circuit for a short to ground. If the circuit tests normal, replace the control module.
4. Install a 3A fused jumper wire between the signal circuit terminal A and the low reference circuit terminal B. Verify the scan tool ECT parameter is greater than 149°C (300°F)
• If less 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 control module.
5. If all circuits test normal, test or replace the ECT sensor.
Component Testing
* Measure and record the resistance of the ECT sensor at various ambient temperatures and compare those measurements to the Temperature vs. Resistance table.
* Refer to Thermostat Diagnosis. Component Tests and General Diagnostics
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure.
* Thermostat Replacement (4.3L) Thermostat Replacement (6.6L (LLY, LBZ)) Thermostat Replacement (6.6L (LMM))
* Engine Coolant Temperature Sensor Replacement
* Control Module References for ECM replacement, setup, and programming Programming and Relearning Verification Tests