The Engine Coolant Temperature Sensor is used by the engine control unit (ECU) to monitor the engine's coolant temperature. The ECU uses the signals from the engine coolant temperature sensor to determine adjustments required to the engine's timing and fuel calculations for peak performance. Temperature sensors fail due to simple wear and tear, or the sensor connector and wiring harness can lose connectivity over time. Symptoms of failure include poor fuel economy & reduced engine power, rough idle, surging & stalling.