Does your locomotive have a plug in decoder with an p in NMRA socket? If so, test your locomotive by removing the decoder and inserting the dummy plug in the socket. If I'm not mistaken there should have been a dummy plug included with the locomotive. This will convert the locomotive back to DC and is a good way to see if the decoder is faulty, or if the problem lies elsewhere. If it's not a decoder related problem, then replacing the decoder with something else will not get you running again.