I recently was headed to emmisions and of course used the scanguage to check for codes just in case... and I had a knock sensor non-com code P0325. This didn't throw a CEL, mind you, but I figured it would mess with my emmisions test so I reset it. Of course I failed emmissions because my car failed readiness (car needs to log a few drive cycles after codes reset or battery disconnect). Anyway, it's a 2000 Nissan Altima and I was able to change the knock sensor with only a few tools, didn't have to remove anything, just had to get under the car and put the tools to it. It helped that I knew where it was because I had been down there recently replacing the intake manifold gasket. So the good news is, now my idle is back to normal and I'm sure that I'll see slightly better FE with a working knock sensor (for those who don't know, a knock sensor 'listens' to the engine to detect knock or ping, in which case it retards the timing and fuel mix)!