Knock Sensor Chip for a 1995 Nissa Altima, 4 cyl? Price, expected labor?

My boyfriend is trying to get his 1995 Nissan Altima to p smog inspection. He was told by an auto shop that he needed to get the knock sensor chip replaced. He was quoted $250.00 for the part alone. Can anyone tell me specifically what the part does, how much it should cost, and maybe even expected labor costs? Also, he was told that the exhaust might have to be removed to get to the part. Any car fanatics out there that know this off the top of their head? Thanks!