For 1990-2009 cars

The engine coolant temperature (ECT) sensor is a thermistor that varies resistance according to changes in engine coolant temperature. When the sensor is cold it has a high resistance (high signal voltage at PCM). DTC P0128 diagnostic is based on driving distance, airflow, and startup ECT. After a minimum distance and accumulated airflow have been surpassed DTC P0128 will set if ECT has not reached normal operating temperature. The minimum accumulated airflow is calculated from startup intake air temperature.

### DTC Parameters

DTC P0128 will set if the ECT has not reached normal operating temperature when:

• | The minimum distance has been traveled.

• | The calculated minimum accumulated airflow has been reached (based on startup IAT)

• | No manifold absolute pressure (MAP), intake air temperature (IAT), ECT, or vehicle speed DTCs have been set.

DTC P0128 runs once per ignition cycle once the above conditions have been set.

DTC P0128 is a type B DTC.

### Diagnostic Aids

DTC P0128 indicates lower than normal engine coolant temperature which most likely was not caused by a wiring fault.

Possible causes of low engine coolant temperature:

• | Thermostat stuck open

• | Cooling fan always on

• | Skewed sensor

Refer to Engine Controls Component Descriptions for a Temperature versus Resistance chart.