The Coolant Temperature Sensor (ECT) has the function of measuring the temperature of the coolant that goes through the engine. This sensor is screwed to a coolant pipe and varies its resistance according to the temperature.
The Powertrain Control Module (PCM) gives the ECT Sensor a 5-volt reference and a ground signal. The voltage drop of this sensor is monitored to determine the coolant temperature. If the ECT Sensor indicates a lower freeze temperature at which the engine has been running for a few minutes, the PCM detects that condition as a circuit failure, so it proceeds to set the DTC code P2185 OBDII indicating that the sensor resistance is out of range.