02 Chevy Blazer knock sensor

I have a 2002 chevorlet Blazer 2door 4.3 V6 engine. I recently have a service engine light that comes on, the dealer hooked it up to the computer and the code read back to one of the knock sensors. He reset the code, and the light came back on after a few days. After about a week, it shut back off. First my questions is can it be bad gas? Two in the Haynes manual it says the knock sensors are in the left and right cyclinder head banks. I looked all over and could not find it, can you better describe where it is located? And How do I know which one to change if there are two. Also when it is changed does the code need to be reset to make the light go off or when the problem is fixed the service light shuts off? Thanks Jeff