It most likely is a burned out diode or there is no power going to the magnet which controls the needle,the same happened to mine,I called a few places that deal with these tachs,they couldn`t give an exact price on how much to fix it,but the guys I talked to told me it would be cheaper to buy a new one as they could not guarantee that the tach would work,and that it would cost about half as much as the new one,and that a new tach has a one year warranty.