From what i can gather from the vag-com printout, an intermittent "implausable signal" from the temp sender g62 in the thermostat housing triggers a fault. The computer reads water temp as -40 deg C, which is an open circuit, or short to B+ , according to my bentley dvd.
this could be a loose connection for a fraction of a second, anywhere in the harness between the sender and the ecu. a simple ground wire, or loose connector, or a break in the wire somewhere buried deep,.
Once the software reads this fault, it deems all data from the sender to be less than trustworthy, and disables the gauge in the cluster, for spite, and the engine management runs on a default map for temperature based on what i dont know. after some period of time, or miles, or trips, the computer finally gets pissed off and lights up the check engine light. This has happened to me twice, despite replacing the senders each time, and replacing the g83 sender (in the radiator) for extra measure.
the sequence goes like this:
temp gauge stops working one day for no reason.
after a few days, check engine light comes on.
after a few more days, take car to mechanic w/vag com, "implausable signal g62 -40 deg C etc.. reset fault.
now gauge works.
two days later, cycle repeats.
mechanic checks wiring, finds "bad connection" at instrument cluster (which makes no sense according to wiring diagram). clears fault. gauge works.
cycle repeats again. going back to mechanic w/vag com next week to keep digging. Its getting to be an expensive little electrical gremlin. I was hoping audi fixed its electrical gremlin problems since my last audi, but i guess not.