DTC P0125: ECT Insufficient For Closed Loop Fuel Control: 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 P0125 will set the if engine coolant temperature is a certain amount below a modeled temperature after a certain amount of air has gone through the engine. This indicates that the engine is running cooler than normal.