where is the knock sensor located?

location of knock sensor is one of two places. It screws into the side of the engine block and best to look for it from under the engine to visually see it. It screws into the side of the engine block at the mid-point length of the block.

I'm not sure where it is, but here is a pic of the knock sensor, this might help you to identify the sensor: http://www.rockauto.com/catalog/raframecatalog.php?carcode=1215324&parttype=5108 Here is the code info too: http://repairpal.com/gm-code-42