P0341 Buick -

Question

i had a coustumer come in with a 2005 buick rondevous 3.4l . the eng. light was and eng. missing. scanned cpu and found that cyl. 6 and 3 missfire. performed compression test and had 0 compression. installed a used eng from my supplier LKQ. eng runns fine and strong but keep getting a p0341 dtc. ireplaced the crank and cam sensors with new ones. the car had no p0341 dtc prior to installing the used eng. i checked the harrness, and looked at live data with the snap-on modis scann tool. and seen there was no cam signal being read. costumer is very upset. any sugg. ?? i think its a bad cam. Please, any help will be greatfully appreiaciated.