Nissan Repair: 03 Xterra Knock Sensor, broken rings, knock sensor

Question
I am looking to buy an '03 Xterra Supercharged with 41K miles.  I heard a noise while test driving.  Dealer took it to Nissan who said it was the knock sensor...to my knowledge - the check engine light was not on.  What does this mean and would I be heading into more significant mechanical problems?

Any idea what average gas mileage is with the Supercharged model?
Thank you.

Answer
Julie,

A knock sensor is what tells the computer to retard and advance timing.  If it is defective it should be replaced and I would have this done before purchasing the vehicle.  I would make this part of the deal if I was going to purchase it.  The knock sensor has to be replaced because if the engine keeps knocking it will eventually hurt the engine by detonation.  This will lead to rod bearing failure or a cracked piston or broken rings and even a blown headgasket.  The knock sensor is important so have it done or do not make the deal.  Also, if it were me I would use only premium to get the most from the supercharged engine.