96 3.0 merc villager, knock sensor questions

Hi all, I am getting two codes, 325 and 400. 325 is knock sensor malfunction. Does anyone know how to test the sensor, it is very expensive and I would
rather not just replace it without being sure it is defective. Also, where is it located on the block and what does a code 400 mean? I know it is EGR related. Thanks for the help... Tony
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