For 1990-2009 cars

The engine coolant temperature (ECT) sensor is a thermistor that varies resistance according to changes in engine coolant temperature. The ECM supplies a 5-volt reference through a pull-up resistor to the sensor, which is connected to a ground. When the sensor is cold it has a high resistance (high signal voltage at ECM). As the sensor temperature increases, its resistance decreases (lower signal voltage at ECM). The ECM uses the signal voltage to determine the engine coolant temperature. DTC P0128 will set when the engine coolant temperature is consistently a certain temperature lower than the predicated engine coolant temperature during certain engine accumulated airflow stages. This DTC indicates that the engine is running cooler than normal.

### DTC Parameters

DTC P0128 will set if the ECT is 10.5°C (51°F) lower than normal based on predicted measurements after a set of calculated accumulated engine airflow stages have passed when:

• | Startup ECT is between -7.5 and 50°C (19 and 122°F)

• | Startup IAT is between -30 and 45°C (-22 and 113°F)

• | ECT is less than 84°C (183°F)

• | At least 3 kg of air has passed through the engine

• | Vehicle speed is present

DTC P0128 runs continuously once the above conditions have been met.

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 short/open or intermittent short/open in the ECT wiring.

Possible causes of low engine coolant temperature:

• | The thermostat is stuck open.

• | The cooling fan(s) is always On.

• | The ambient temperatures are very low.

• | There is corrosion in the connector or resistance in the wiring.

• | The ECT sensor is skewed.

Refer to ECT General Description for the Temperature vs. Resistance chart.