You have to reset it according to NCE's reset procedure (CV 30 to 2). CV 55 to 55 is for ProtoSound 3 only. If the decoder resets with no problem, then you're good. If the reset procedure doesn't work, your decoder is toasted.
There is a fellow club member at the club I belong to that has two of the MTH SD70ACes that are DCC ready. The club electrician installed the decoders for him. At first with the decoder plugged in, the locos caused a short. So, the club electrician ripped all the electronics out, and rewired the whole thing. He had to isolate the ditch lights from the headlight and flip the polarity (MTH uses a negative common instead of a positive common like the rest of the world). It worked for a while, but then it toasted the decoder.
So, the club electrician replaced the decoder, and the DCC system saw it as a short. As soon as he put the loco on the tracks, the decoder toasted. When he used a multimeter to check for shorts and high power draw, he found that the surface mount LEDs were operating at 200 milliamps (or so he told me) and said that the surface mount LEDs in the loco must not be standard (why am I not surprised?)
(The club electrician said that it was a nightmare and that if he owned the loco, it would be embedded in the wall by now)