Diagnostic Instructions
DTC Descriptor
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
Condition 1
| • | DTC P0116, P0117, P0118, or P0119 is not set. |
| • | The engine speed is greater than 25 RPM. |
| • | DTC P0128 runs continuously when the above conditions have been met for approximately 8 minutes. |
Condition 2
| • | Before the ECM can report DTC P0128 failed, DTC P0118 must run and pass. |
| • | DTC 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. |
| • | The calculated ambient air temperature is between -30 to +70°C (-22 to +158°F). |
| • | 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 when the above conditions have been met for approximately 15 minutes. |
Conditions for Setting the DTC
Condition 1
The ECM detects that the actual coolant temperature is 10°C (18°F) colder than the calculated ECT for greater than 1 second, or for a cumulative of 10 seconds.
Condition 2
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 4 seconds, or for a cumulative of 30 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 Calculated ECT-Thermostat parameter with a scan tool. If there
is a condition, the calculated temperature will be 10°C (18°F) warmer than 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.
Caution: Do NOT use the low reference circuit in the component harness connector for this test. Damage to the control module may occur due to excessive current draw.
Note: The control module or the sensor may be damaged if the circuit is shorted to battery positive voltage.
- Test for less than 5 Ω between the low reference circuit terminal 2 and ground.
| ⇒ | If greater than the specified range, test the low reference circuit for a short to voltage or an open/high resistance. If the circuit tests normal, replace the ECM. |
- Ignition ON, verify the scan tool ECT sensor parameter is at -40°C (-40°F).
| ⇒ | If warmer than the specified value, test the signal circuit terminal 1 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.
- Install a 3 A fused jumper wire between the signal circuit terminal 1 and the low reference circuit terminal 2. 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. |
- If all circuits test normal, test or replace the ECT sensor.
Component Testing
- Ignition OFF, disconnect the harness connector of the ECT sensor.
Note: 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 % of the specification.
| ⇒ | If not within the specified range, replace the ECT sensor. |
Repair Instructions
Perform the
Diagnostic Repair Verification after completing the diagnostic procedure.
