The temperature gauge sending unit controls the gauge or warning light. Some models use two temperature sensor (one for the gauge or warning light and one as a PCM input) which may be located adjacent to each other. Refer to the illustrations.
The Engine Coolant Temperature (ECT) is located on or near the thermostat housing.
The ECT sensor's resistance changes in response to engine coolant temperature. The sensor resistance decreases as the coolant temperature increases, and vice-versa. This provides a reference signal to the PCM, which indicates engine coolant temperature. The signal sent to the PCM by the ECT sensor helps the PCM to determine spark advance, EGR flow rate, and air/fuel ratio.
Some vehicles may have two ECTs fitted: one for input to the PCM and the other for input to the coolant temperature gauge or warning light.