The pcm has self diagnosing capabilities, and will return a specific error code if it finds a fault. I doubt that any scanner is more capable of finding a bad pcm than the pcm itself. As Tech suggested, you might want to run down the wiring to make sure it's not just some broken wire or corroded connector. If you continue to be stumped, you might want to get a diagnosis at a transmission shop. They will have both the electronic diagnosis equipment as well as the hydraulic and mechanical diagnosis equipment to narrow the problem down. Even if you are capable of pulling the transmission and repairing it yourself, it will be of great value to know exactly what to look for before you start. I would prefer a transmission shop to a dealership. Dealerships are okay for "pull and replace" jobs but if it involves tearing the transmission apart and digging into it, dealerships rarely do that and a shop is much more likely to do it better, and very likely to do it cheaper.