trany sensor red light comes on during idle. when car drive for a while light goes out

Call your local parts stores, (unless you have an Autozone or Advance Auto parts store around, which I know do this) and see if they check for error codes for free. Bring a en and paper and write down all codes that show up (both the number & description) & in the same order that they come up in. Post results here. Some codes may show as intermittent, or history as opposed to current. It could be as simple as a switch ontop of the tranny that could be out of adjustment, but can't say for sure w/o the codes because not familiar with your exact car.