2000 powerstroke 4r100 transmission. It has 155k just had it rebuilt and the shop is having a night as am I. It was shifting hard and the o/d off light would start flashing after you drove it a while. Its still doing the same thing and the shop checked the whole wiring harnes and replaced the accumulator twice believing they got a bad remanufactured one. The only code I can pull is a 605 pcm read only memory error. Could the cause be a bad pcm? The shop dosent seem to have a clue at this point and I'm out 2500. IV taken it back twice now and Iv driven it a total of 35 miles since the rebuild

 

A P0605 won't flash the OD OFF light. You need a better tool to read the codes. There are transmission codes stored and without those codes you're not going to get this fixed. It is possible that it is a bad PCM, but that is very, very unlikely. Get the codes read by someone with the right tool and we can almost certainly point you to your problem.