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 in order 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 Descriptors

This diagnostic procedure supports the following DTC. DTC P0128 engine coolant temperature (ECT) below thermostat regulating temperature.


Conditions for Running the DTC

- DTCs P0101, P0102, P0103, P0112, P0113, P0116, P0117, P0118, P0125, P1111, P1112, P1114, or P1115 are set.

- The coolant temperature at start up is less than 70°C (158°F).

- The minimum air temperature is less than 54°C (129°F) and more than -7°C (19°F).

- The engine has been running more than 2 minutes, but less than 22 minutes.

- The vehicle speed is more than 5 mph for more than 1.5 miles.

- The mass air flow average reading is more than 20 g/s and less than 75 g/s.

- DTC P0128 runs once per ignition cycle when the above conditions are met.


Conditions for Setting the DTC

The time for the engine coolant to reach a preset temperature has been exceeded.

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 3 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

- DTC P0128 is designed to detect a faulty thermostat.

- Excessive coolant flow through the bypass hose to the throttle body may cause this DTC to set.

- A skewed ECT sensor can cause a DTC P0128 to set.

- A skewed IAT sensor can cause this DTC to set.


Test Description

DTC P0128 Steps 1-7:

DTC P0128 Steps 8-10:

The numbers below refer to the step numbers on the diagnostic table.

3. This step tests for excessive resistance in the ECT sensor circuit.

4. This step determines whether the high resistance is in the ECT sensor signal or ground circuit.

7. This step tests for a skewed ECT sensor.