The engine coolant temperature sensor (ECT) is usually made of a thermal resistor, which is covered by a resin and finally sealed by a metal or plastic housing. This sensor may be located near the thermostat or in the coolant passages around the intake manifold.
The thermal resistance of the Engine Coolant Temperature is low as the coolant heats up and passes through the system components. As a result, the voltage will increase in the ECT sensor circuit to the PCM.
When the Powertrain Control Module detects some out-of-range voltage levels at the Engine Coolant Temperature, which indicates low performance, the diagnostic code P00B7 OBDII will be set.