The Powertrain Control Module (PCM) constantly monitors the engine temperature through the cooling system while the engine is running. This process is done to determine the time in which the engine is cold, in that condition the air/fuel mixture is rich and allows the engine to run smoothly as it warms up. Low engine temperature is also detected which can affect the efficiency of the exhaust gases. And additionally, it is determined if the engine operating temperature is excessively high, which can result in overheating of the engine.
If the engine temperature reading changes faster than the PCM considers normal, the DTC code P2181 OBD2 is set and the Check Engine light illuminates.