ecu code 61

yes i put a new skunk2 intake manifold on about a month ago. it stutters mid power band pretty bad now too. about 4k rpm it will lose power then build back up when i hit the gas. i though it was a vacuum leak but now that the ecu is saying its the o2 sensor and also my exhaust is extremely black with soot. i know exhausts are suppossed to be black but mine is really chuncky with soot. if ya have anymore input it would be appreciated