The Engine Coolant Temperature Sensor determines the exact temperature of the engine coolant by sending an analog signal to the ECM to sensure prpoer air/fuel ratio along with proper operation of the cooling fan(s) and temperature gauge. The Engine Coolant Temperature Sensor may be located on the cylinder head, engine block, thermostat housing or intake manifold. Reasons for replacement include: check engine light illumination, erratic or no temperature gauge operation, erratic or no cooling fan operation, and engine overheating.