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.