DTC P0128: ECT Below Thermostat Regulating Temperature: Description
WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.
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 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 engine coolant temperature. DTC P0128 will set when the engine coolant temperature is 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.