I started getting the knock sensor malf. code about every 1,000 miles or so after clearing the code.  A few thousand miles past that and it started to stay on, the light only going off every once in a while.  By that time my mileage had decreased from 29-30 mpg highway to about 24-25.  I also noticed a distinct miss, sometimes at half-throttle, sometimes at full throttle.  Always when the a/c was on.  After about 30,000 miles w/ a knock sensor code, I finally replaced it.  Man the difference was night and day!  You can definitely feel it retard the timing a few seconds before that light comes on.