DTC P0128: ECT Below Thermostat Regulating Temperature: Description

Engine Coolant Temperature (ECT) sensor is a thermistor that varies resistance according to changes in engine coolant temperature. PCM supplies a 5-volt reference voltage through a pull-up resistor to sensor, which is connected to ground. ECT sensor resistance is high when coolant temperature is cold. Resistance drops when coolant temperature increases. PCM reads voltage drop on signal line to determine coolant temperature. DTC diagnostic is based on driving distance, airflow, and start-up ECT. After a minimum distance and accumulated airflow have been surpassed, DTC will set if ECT has not reached normal operating temperature (minimum accumulated airflow is calculated from start-up intake air temperature).