The truck was sold to me with the issue and the original owner never could fix it. The tranny would lose reverse after warm up and would show the 42 code. He changed the fluid and swapped out solenoids. I bought a known good tranny and converter and tranny harness and swapped it mostly because it seemed like a mechanical issue to me with the gear going out once it was warm. It had the same problem so i bought a TCU. It seemed to give me some different responses but would still lose reverse. If I unplugged the tcu reverse was there but the other gears got screwed up. After a couple of weeks I decided to try out a known good ECU. Bam! problem gone. No FSM info took me to the ECU. Hope it helps. Moral of this story, unplug the battery for a few minutes, verify solenoids, do TCU its cheap, do ECU cheaper than a tranny, then do the tranny.
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