Hi there. I've just recently replaced the two thermostat housing sensors on my MK5b 1.6 Zetec, mainly because the temp gauge wasn't working and I didn't know which sensor was which so ordered both. The sender is on the bottom and is a pig to get at and can leak if you don't screw it in all the way. I had to remove the 'stat housing from the block and turn it upside down to get proper purchase on it. I ended up just replacing the 'stat, both sensors and renewing the coolant as I had drained the old stuff anyway. Use threadlock too.

Anyway, my fan is kicking in but not as often as I'd like so I'm on the hunt for this sensor too.