Diagnostic Instructions
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 P0117 must run and pass. |
| • | DTCs P0100, P0101, P0102, P0103, P0111, P0112, P0113, P0116, P0117, P0118, P0119, or P0700 is not set. |
| • | The engine speed is greater than 960 RPM. |
| • | The ECT is less than 71°C (160°F), at start up, or less than 61°C (142°F) at start up for LLT. |
| • | The calculated ambient air temperature is greater than -30°C (-22°F) and less than 70°C (158°F), or greater than -11°C (-12°F) and less than 70°C (158°F) for LLT. |
| • | The vehicle speed is greater than 15 km/h (9 mph). |
| • | The air flow coming into the engine has accumulated to greater than 3,000 grams. |
| • | DTC P0128 runs continuously once the above conditions have been met for approximately 5 seconds, 20 seconds for LLT. |
| • | DTCs P0100, P0101, P0102, P0103, P0111, P0112, P0113, P0116, P0117, P0118, P0119, or P0700 is not set. |
| • | The ECT is less than 41°C (106°F) at start up. |
| • | The block heater is not detected. |
| • | The engine run time is between 120-300 seconds. |
| • | The calculated ECT is increasing based on air flow. |
| • | DTC P0128 runs continuously once the above conditions have been met for approximately 500 seconds. |
Conditions for Setting the DTC
P0128
| • | The ECM detects that the actual engine coolant temperature is 11°C (20°F) less than the calculated thermostat regulating temperature of 89°C. (192°F), for greater than 4 seconds or for a cumulative of 30 seconds. |
| • | The ECM detects that the actual engine coolant temperature is 10°C (18°F) less than the calculated ECT for greater than 1 second or a cumulative of 10 seconds, or 4 seconds or a cumulative of 30 seconds for LLT. |
Action Taken When the DTC Sets
DTC P0128 is a Type B DTCs.
Conditions for Clearing the MIL/DTC
DTC P0128 is a Type B DTCs.
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 10°C (18°F) more then the actual engine coolant temperature.
Reference Information
Schematic Reference
Engine Controls Schematics
Connector End View Reference
Component Connector End Views
Electrical Information Reference
DTC Type Reference
Powertrain Diagnostic Trouble Code (DTC) Type Definitions
Scan Tool Reference
Control Module References
for scan tool information
Circuit/System Verification
- Verify that the engine coolant level is correct.
| ⇒ | If the engine coolant is not at the correct level, refer to
Loss of Coolant
. |
- 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
. |
- 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
- Ignition OFF, disconnect the harness connector at the ECT sensor.
- Ignition OFF, remove the fuse that supplies B+ to circuit terminal X2-56 of the ECM.
Notice: 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.
Important: The control module or the sensor may be damaged if the circuit is shorted to battery positive voltage.
- Test for less than 5 ohms 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. |
- Install the fuse that supplies B+ to circuit terminal X2-56 of the ECM.
- Ignition ON, observe the scan tool the ECT sensor parameter. Verify that the ECT sensor parameter is at -40°C (-40°F).
| ⇒ | If greater than the specified value, test the signal circuit for a short to ground. If the circuit/connections test normal, replace the ECM. |
Important: If the fuse in the jumper wire opens, the signal circuit is shorted to a voltage and the ECT sensor may be damaged.
- Install a 3A fused jumper wire between the signal circuit terminal 1 and the low reference circuit terminal 2. Verify the scan tool ECT sensor parameter is greater than 142°C (288°F).
| ⇒ | If less 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. |
- If all circuits test normal, test or replace the ECT sensor.
Component Testing
- Ignition OFF, disconnect the harness connector at the ECT sensor.
Important: A thermometer can be used to the used to test the sensor off the vehicle.
- 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
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
after completing the diagnostic procedure.
