knock sensor

I took my 01, yukon to the dealership yesterday because of a rattling noise coming from the trans. The tech at the dealership tested it and said i had 2 knock sensors shorted. They want to charge me 630.00 to replace them. Does anybody know what could cause a knock sensor to short out? What is knock sensor anyway?

A knock sensor is used by the PCM to adjust spark timing to compensate for detonation while the engine is running.
If the PCM does not receive a KS signal, you should have a DTC set in the PCM and a SES light illuminated.
The KS is located under the intake manifold on that engine and must be torqued to the proper specification to operate correctly.