The "trac off" light comes on, and the tachometer quits working

well i guess that one is so difficult, even the best car techs dont have a clue. that figures

Sometimes those things are tough to spot unless the failure occurs while the tech is actually working on the car. What sort of trouble codes is it giving you?

Having said that, I always go to the battery first. Have it load tested and make sure the connections are clean and tight. I won't bore you with my Cobalt story, but I have learned that modern electrical systems are extremely sensitive and something as seemingly insignificant as a weak battery can cause all kinds of problems.

Same thing happen to me check Crankshaft position sensor