The knock sensor on the GMC 1500 is used to sense excess vibrations in the engine caused by the wrong air-to-fuel mixture. When the knock sensor picks up the vibrations, it sends a signal to the computer. The computer then tries to adjust the air-to-fuel mixture to get rid of the knocking. The knocking could be light -- this is known as pinging. A bad knock sounds like marbles inside the engine.
Open the hood.
Look on either side of the engine, depending on the year of the 1500, for a round sensor with one wire.
Find the round sensor with one wire going to it, and at the top of the round part, a hexagon-shaped piece. The knock sensor is screwed into the block. The hexagon-shaped piece is there so you can unscrew the knock sensor with a wrench to replace it.
