My bet is the coolant sensor. I have replaced a bunch of them. I think when open they default to -20*f.
Before you go dropping the tank, check at the harness near the tank for power and ground. It is very common for the connectors to fail and the grounds to rot off the frame. I have an old headlight I use to check for power, a test light does not draw enough current to show a bad connection all the time.
Artificial Intelligence is no match for Natural Stupidity