Just bought a used 2010 Prius. One key works, the other doesn't. On the non-working key, the red LED does not light when I push a button, but the battery measures 3 volts. Is it possible the battery needs replacement anyway? If not, what else could be wrong?
Who knows? The fob could've gone swimming in the ocean or a chlorinated pool, or through a wash cycle or two. You get the idea ... Sorry to be a downer, but if it isn't the battery, then it's all unknown as this was a preowned set.
quick check would be switch the battery from the working one to the other (mark it though so you do not get confused). this can quickly eliminate the battery. then you can go from there.... to replace the fob is not trivial you need some extra equipment. and dealer will charge quite a bit, but better replace the second while you have one working if you loose that it WILL be a lot more expensive.
