knock sensor on 2003 GMC Sierra 1500

Where is the knock sensor located, and is there more than 1?

Asked by for the 2003 GMC Sierra 1500
I have a 1999 4.8 vortec. There are two knock sensors under the intake manifold that are screwed into the lifter valley. The wires come out from under the rear of the intake manifold and connect the main wiring harness was at the rear of the intake manifold. Before you start taking off the intake check the wiring.
1 more answer
There is only one sensor. Located on the left rear top of engine block. Held on by a single bolt that goes through the middle of it.