If your site was working fine, and all of a sudden you navigate to the category page ( ShopbyVehicle.epc?sup=WHD&mode=PGA ) and nothing appears or somethign is wrong, you need to do the following.
1) A Clear the catalog cache table (ask support for help if necessary)
2) A Delete the file templates/WHDPGPS.XML via your File Manager or FTP login
3) A Try again, your done.