Wont insert or eject discs

I've been working in a repair shop for a while and seen a number of these units come in. At first its suspected that the motor that senses when a disc is in the drive is defective, in this case the kld-002. After checking everything with the current drive, and replacing the sensor motor, they still dont accept discs. We usually opt to replace the entire drive and all cables, with the exception of the drive board, same results. I'm thinking there is something failing with these drive boards and wondering if anyone knows any tips or tricks to get these boards to work again?