Circuit Description

The DTC P0115 Engine Coolant Temperature (ECT) sensor circuit diagnostic detects an out of range sensor signal. ECT sensor is a thermistor. A thermistor is a resistor whose value varies with temperature. Resistance of ECT sensor is high when coolant temperature is cold, and resistance is low when coolant temperature is warm. ECT sensor is wired in series with a fixed resistor in the Powertrain Control Module (PCM). PCM applies 5 volts to the ECT sensor. PCM monitors voltage across ECT sensor and converts voltage into a temperature reading. Voltage measured by PCM will be high when coolant temperature is cold, and low when coolant temperature is warm. See IAT RESISTANCE/SIGNAL VOLTAGE  table.