Ran over a curb lately?  The sensor is bad. It's usually in the plastic
under the front of the right fender. It may have gone missing entirely
if you got up on a curb..

Fix is a new one. Cost is about $40 for the part. Figure 1 hour labor
for a dealer to install, 15 minutes for an independent, 5 minutes for a
competent DIYer.

The sensor is located in the left side air ducting that directs air onto the
front brake assembly.

The sensor should be more reliable than what you expereinced, my guess is
that it has come unplugged or the wires are otherwise damaged.

Iscraped the sensor on my E39 & got the same readings.  When I soldered the
wires, my readings became a bit whacky.  Seems to be off by 5-10 degrees, at
least  &not consistant.  My guess is a change in capacitance from the solder
job.  As such, I suggest replacing the unit & wires instead of trying what I
did.