first off what year ????
if mkII ??? is this the brown connector down on the airbox/fuel distributor
i had troubles with it once, my Jetta GLI would idle up and down continuously and i was told it was that sensor, i didnt replace it right away, but did fix it one day.... took a 130 mph run down this back highway tryin to bury the speedo at 140, but ran out of room, when i got back home it wasnt doing it's idle dance, and never did it again
my other experience with most sensors are when they are bad, the car will run full rich, so no harm but very sh*tty fuel milage