IAC VALVE Cleaning

So I got the iac valve off of the throttle body and was able to clean all the build up on it, so much carbon that the valve was stuck in the open position. Now it rotates freely. one of the threads about checking to see if the valve is working properly before reinstalling says to "disconnect the ECT sensor then reconnect the IAC valve to its connector and switch the ignition on". Where is the ECT sensor, and why do I need to disconnect?