P0128
DTC P0128
Diagnostic Instructions
* Perform the Diagnostic System Check - Vehicle (Initial Inspection and Diagnostic Overview) prior to using this diagnostic procedure.
* Review Strategy Based Diagnosis (Initial Inspection and Diagnostic Overview) for an overview of the diagnostic approach.
* Diagnostic Procedure Instructions (Initial Inspection and Diagnostic Overview) provides an overview of each diagnostic category.
DTC Descriptors
DTC P0128
- Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature
Circuit Description
The engine control module (ECM) monitors the temperature of the engine coolant for engine control and as an enabling criteria for some diagnostics. The amount of air flow into an engine is proportional to the amount of heat an engine generates. The ECM monitors the amount of air flow into the engine to calculate engine coolant temperature (ECT). The ECM uses the calculated temperature to determine if the engine has warmed up to the closed loop temperature or to the thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach the closed loop temperature, diagnostics that use the ECT as enabling criteria, may not run when expected.
Conditions for Running the DTC
P0128
* Before the ECM can report DTC P0128 failed, DTC P0100, P0101, P0102, P0103, P0117 P0118, P0119, P0335, and P0336 must run and pass.
* The ECT is colder than 89°C (192°F).
* The vehicle speed is greater than 15 km/h (9 mph).
* The engine speed is greater than 920 RPM.
* The air flow coming into the engine has accumulated to greater than 2,002 grams.
* The startup ECT is colder than 70°C (158°F).
* The intake air temperature (IAT) is warmer than -30°C (-22°F) and colder than 70°C (158°F).
* DTC P0128 runs once per ignition when the above conditions have been met.
Conditions for Setting the DTC
* The ECM detects that the actual engine coolant temperature is 11°C (20°F) colder than the calculated thermostat regulating temperature of 89°C (192°F) for greater than 10 seconds.
OR
* The ECM detects that the engine coolant temperature is 11°C (20°F) colder than the calculated ECT for greater than 10 seconds.
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
An engine that soaks for about 8 hours will help diagnosis the condition. After the cold soak, operate the vehicle at highway speeds for 20 minutes while monitoring the scan tool Calculated ECT - Thermostat parameter. If there is a condition, the calculated temperature will be 11°C (20°F) warmer than the actual engine coolant temperature.
Reference Information
Schematic Reference
Engine Controls Schematics (Electrical Diagrams)
Connector End View Reference
Component Connector End Views (Connector Views)
Electrical Information Reference
* Circuit Testing (Component Tests and General Diagnostics)
* Connector Repairs (Component Tests and General Diagnostics)
* Testing for Intermittent Conditions and Poor Connections (Component Tests and General Diagnostics)
* Wiring Repairs (Component Tests and General Diagnostics)
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions (Diagnostic Trouble Code Descriptions)
Scan Tool Reference
Control Module References (Programming and Relearning) for scan tool information
Circuit/System Verification
1. Ignition OFF, open the hood to disable the Auto-Stop function.
2. Ignition ON, verify the scan tool hood position parameter displays Open.
• If the hood position does not display Open, refer to DTC P254F (P254F).
3. Verify that the engine coolant level is correct.
• If the engine coolant is not at the correct level, refer to Loss of Coolant (With HP5) (Loss of Coolant)Loss of Coolant (Without HP5) (Loss of Coolant).
4. Engine idling, verify that the engine coolant temperature is at the normal operating temperature.
• If the engine coolant temperature is below normal operating temperature, refer to Thermostat Diagnosis (Component Tests and General Diagnostics).
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.
Circuit/System Testing
1. Ignition OFF, open the hood to disable the Auto-Stop function.
2. Ignition ON, verify the scan tool hood position parameter displays Open.
• If the hood position does not display Open, refer to DTC P254F (P254F).
3. Ignition OFF, disconnect the harness connector at the ECT sensor.
4. Ignition OFF, remove the fuse that supplies B+ to circuit terminal X2-56 of the ECM.
Caution: Do NOT use a test lamp to test the continuity of the circuit. Damage to the control module may occur due to excessive current draw.
5. Test for less than 5 ohm between the low reference circuit terminal 2 and ground.
• If greater than specified value, test the circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM.
6. Install the fuse that supplies B+ to circuit terminal X2-56 of the ECM.
7. Ignition ON, observe the scan tool the ECT sensor parameter. Verify that the ECT sensor parameter is at -40°C (-40°F).
• If warmer than the specified value, test the signal circuit for a short to ground. If the circuit/connections test normal, replace the ECM.
Note: If the fuse in the jumper wire opens, the signal circuit is shorted to a voltage and the ECT sensor may be damaged.
8. Install a 3 A fused jumper wire between the signal circuit terminal 1 and ground. Verify the scan tool ECT sensor parameter is warmer than 142°C (288°F).
• If colder than the specified value, test the signal circuit for an open/high resistance or for a short to voltage. If the circuit/connections test normal, replace the ECM.
9. If all circuits test normal, test or replace the ECT sensor.
Component Testing
1. Ignition OFF, disconnect the harness connector at the ECT sensor.
Note: A thermometer can be used to the used to test the sensor off the vehicle.
2. Test the ECT sensor by varying the sensor temperature while monitoring the sensor resistance. Compare the readings with the Temperature Versus Resistance - Engine Coolant Temperature Sensor (Temperature Versus Resistance - Engine Coolant Temperature Sensor) table and verify that the resistance is within 5 percent of the specification.
• If the resistance vs. temperature is not within specified value, replace the ECT sensor.
Repair Instructions
Perform the Diagnostic Repair Verification (Verification Tests) after completing the diagnostic procedure.
* Engine Coolant Temperature Sensor Replacement (Service and Repair)
* Control Module References (Programming and Relearning) for engine control module replacement, setup, and programming