P0128
DTC P0128
DTC Descriptor
DTC P0128
- Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature
Diagnostic Fault Information
Perform the Diagnostic System Check - Vehicle prior to using this diagnostic procedure. Initial Inspection and Diagnostic Overview
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 volts 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, P0171, P0172, P0174, P0175, P0502, P0503 are not set.
* The start-up IAT is more than -7°C (+19°F).
* The start-up ECT is less than 70°C (158°F), when the IAT is above 10°C (50°F).
* The start-up ECT is less than 50°C (122°F), when the IAT is below 10°C (50°F).
* The engine run time is between 90 seconds and 22 minutes.
* The vehicle has traveled more than 2.4 kilometers (1.5 miles) at more than 8 km/h (5 mph).
* The accumulated airflow is between 20-75 g/s, with the minimum average airflow more than 10 g/s.
* The fuel ethanol percentage is less than 85 percent.
* 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 more 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 less 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
Inspect the ECT sensor terminals for corrosion, and for engine coolant leaking through the sensor.
Circuit/System Verification
1. If the ignition has been OFF for 8 hours or more, the ECT and the IAT, should be within 6°C (11°F) of each other and also the ambient temperature. Turn ON the ignition, with the engine OFF, and use a scan tool to observe the IAT and the ECT sensor parameters.
2. Use the scan tool to verify the proper operation of the engine cooling system fans.
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.
3. Turn OFF the ignition.
4. Inspect the cooling system surge tank for the proper engine coolant level.
Important: A critical analysis of the operation of the thermostat is necessary to properly diagnose these DTCs.
5. Verify the proper heat range, and the operation of the thermostat. Refer to Thermostat Diagnosis. Component Tests and General Diagnostics
Circuit/System Testing
Important: All electrical components and accessories must be turned OFF and allowed to power down.
1. Ignition OFF, disconnect the ECT harness connector at the ECT sensor.
2. Ignition OFF, test for less than 5 ohms of resistance between the low reference circuit 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 ECM.
3. Ignition ON, verify the scan tool ECT Sensor parameter is less than -39°C (-38°F).
^ If greater than the specified range, test the signal circuit for a short to ground. If the circuit tests normal, replace the ECM.
4. Install a 3A fused jumper wire between the signal circuit and the low reference circuit. Verify the scan tool ECT Sensor 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 ECM.
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, then compare those measurements to the Temperature vs Resistance Table.
Repair Instructions
Perform the Diagnostic Repair Verification after completing the diagnostic procedure. Verification Tests
* Engine Coolant Temperature Sensor Replacement
* Thermostat Replacement (RPO LS4) Thermostat Replacement (RPOs LZE, LZ4, LZ8)
* Programming and Relearning Programming and Relearning