when I had a shift solenoid fail, I got an output shaft speed error. Both a dealer and 2 transmission shops said I needed to replace or rebuild it,thats what the fault tree in the manual led to (incorrectly) .

the fault tree was wrong in that it did not consider that the transmission was not in the gear it was told to be in because the solenoid failed, so thats why the output speed was wrong, not because it was slipping.