The Engine Coolant Temperature (ECT) sensor is a variable resistor which the Engine Control Module (ECM) uses to measure coolant temperature. The sensor's resistance decreases as the coolant temperature increases. The sensor is usually located in or near the thermostat housing.
With ignition OFF , disconnect the ECT sensor. Measure the resistance across the sensor connector terminals. If the reading for a given temperature is about that shown in the table, the ECT sensor is okay.