A\C and knock sensor

I have a 97 Legacy Outback. It needs a new knock sensor. What is a knock sensor and what does it do? Car is a bit sluggish now. Also, the AC hasn't worked since the head gasket was replaced. Doesn't seem to have any power going to it. Fuses are fine and so is belt and alternator. Both belt and alternator are new. Any ideas anyone?

Knock sensor is essentially a microphone. It listens to the engine. When knocking (pinging) is detected, it retards the timing. If yours is bad, the engine computer will retard the timing. This is likely the sluggishness you notice.

Recent thread on them here: http://www.ultimatesubaru.net/forum/showthread.php?t=62310

a/c: most common failure is low refrigerent charge. a/c system is not simple, nor is the electrical system that helps it run. I recommend a pro.

Nick