check engine light

[cavf150]

Hey,
I pulled the codes and they are 21 and 52. I changed the O2 sensor for 21 , and cleared the code. Code 52 remains, and it is the knock sensor circuit. I plan to reset the valve clearances/timing.
Is this codes the sensor itself or , is the sensor triggering because of a problem with the valves or related to a engine malfunction? The engine has been rebuilt as with the head. I am slightly worried that it might be something that is big! The light comes on after I rev the engine slightly 2-3 times. I haved tried to clear the code 4 times and it remains! The manuals say it could be a short in the sensor circuit. What does this mean? Thanks cavf150