96 Deville Knock Sensor

My Check Engine Lite Is Always On . I Went To Auto Zone To Have It Checked..they Said I Need A Knock Sensor. What Is A Knock Sensor, Were Is It . And Is It Ez To Change?????
Codes Are;pc325, Pc1602 Pc2042 Pc1571 Ac1347.
Thank You..

I ALSO HAVE A 98 DEVILLE THAT NEEDS HEAD GASKETS ARE THEY HARD TO DO?? I HAVE CHANGED HEAD GASKETS ABOUT 35 YEARS AGO WHEN I WAS INTO RACEING....IS IT STILL THE SAME?? I HEARD NORTHSTAR ARE HARD TO DO

A knock sensor detects pre ignition or knock, (due to poor quality gas or carbon build-up, etc). It sends a signal to the engine management computer which retards the ignition timing until the knock goes away.

Head gaskets on the northstar is generally the same as 35 years ago, but all the overhead cam valve train components and the electronic components complicate matters.

However, almost always the head gaskets have blown because the head bolts have stripped the threads in the block. Northstar engines are notorious for this failure.
The bolts thread right into the aluminum block. If the cooling system is not maintained properly the bolts will shift and come loose, so the head is no longer sufficiently clamped to the block.
In this case, the engine must come out, the heads removed and "Time-Serts" are installed to replace the stripped threads. Do not use HeliCoils. They will not do the job.

Its a BIG job. Mechanics usually bill for 19 hours labour plus materials. Figure on $2000 if someone else does the work.
Look here for more info: http://caddyinfo.onedgesolutions.com/howto/nsrepair.htm

Were Is The Knock Sensor, And Can I Change It??????