P0128

DTC P0128

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 thermostat regulating temperature. If the coolant temperature does not increase normally or does not reach regulating temperature of the thermostat, diagnostics that use the ECT as enabling criteria, may not run when expected. If the engine coolant temperature fails to reach the thermostat regulating temperature, before a predetermined amount of air flow enters the engine, this DTC sets.

DTC Descriptor

This diagnostic procedure supports the following DTC:

DTC P0128 Engine Coolant Temperature (ECT) Below Thermostat Regulating Temperature

Conditions for Running the DTC

* Before the ECM can report DTC P0128 failed, DTC P0117 must run and pass.
* DTCs P0101, P0102, P0103, P0112, P0113, P0118, P0722, or P0723 are not set.
* The engine speed is more than 960 RPM.
* The ECT is less than 71°C (160°F) at start up.
* The calculated ambient air temperature is more than -11°C (-12°F) and less than 45°C (113°F).
* The vehicle speed is more than 15 km/h (9 mph).
* The air flow coming into the engine has accumulated to more than 2,000 grams.
* DTC P0128 runs continuously once the above conditions have been met for approximately 15 minutes.

Conditions for Setting the DTC

* The ECM detects that the actual coolant temperature is 10°C (18F) less than the calculated coolant temperature.
* The condition exists for more than 4 seconds.

Action Taken When the DTC Sets

* The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
* The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.

Conditions for Clearing the MIL/DTC

* The control module turns OFF the malfunction indicator lamp (MIL) after 4 consecutive ignition cycles that the diagnostic runs and does not fail.
* A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
* A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
* Clear the MIL and the DTC with a scan tool.

Diagnostic Aids

* Use the J 35616 Connector Test Adapter Kit for any test that requires probing the ECM harness connector or a component harness connector.
* The lower connector of the ECM is connector C1 and the upper connector of the ECM is connector C2. Refer to Engine Controls Component Views.
* 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) more then the actual engine coolant temperature.
* For an intermittent condition, refer to Intermittent Conditions. Intermittent Conditions

Test Description

Step 1 - Step 9:

Step 10 - Step 16:

The numbers below refer to the step numbers on the diagnostic table.
7. This step tests for excessive resistance in the ECT circuit.
8. This step isolates the condition. If the temperature changes, test for a condition in the low reference circuit. If the temperature remains the same test the signal circuit for a condition.