Have you recently had any work done with the stereo or the AC unit that requires the center of the dash to be removed? There is a computer behind this section that, when it is removed, can affect the Tachometer from working. When I was fixing this issue myself, I was told that there are occasions where a coin or a random metal item can fall into the vent cracks of the module and short it out. That module behind the dash could be the problem.