Check around the upper rad hose clamp area. The neck may be cracked.
I was losing about half gallon every two weeks. Radiator replacement fixed it.
The sensor is in the expansion tank, front and center of the firewall. Cheap and easy to replace.
Check the return line that runs from the radiator to the expansion tank for leaks or cracks. It's made of plastic that gets very brittle with age. Also check the hole where the return line flows into the expansion tank itself.
It can get clogged.
Ramon
1994 540iA Nikasil EAT Chip
Tampa Bay, Florida USA