correct me if i am wrong but aux fan should only turn on on when a sensor tells it to... don't know why worry if it doesn't turn on .....why i am saying this is i recently removed my clutch fan and drove without it for a week...no problems and my aux fan never went on that i know of and my temp gauge never went past half way mark..today installed 14" electrical fan on my 2001 3.0i 15min of work with a help of friend..plus i think that sensor is inside a thermostat if that sensor fails thermostat will open up manually..and i would only guess u would get some sort of error message on a dash board display...?