So as the title says I have the dreaded 'Trans Fail Prog' error on my 2004 L322 4.4. I have read the many of the threads on the forum regarding this message and there has been some excellent help given. I think I have an idea what needs to be done to rectify the problem comparing my issue to others threads. I think it might be a problem with a split O-ring as explained by RRPhil back in 2015 https://www.rangerovers.net/forum/6-range-rover-mark-iii-l322/20824-transmission-fail-safe-program-slams-into-2.html however I would like a 2nd, 3rd, 4th........ opinion on this so let me explain what has been happening. During a long drive I suddenly got the 'Trans Fail Prog' error and the car went onto 4th gear and like a lot of other cars with these issues it went away when the car had been parked for a while. Initially I would get around 25 mins before the error would pop up on the dashboard and normally the error always came when i was slowing down and the transmission was coming down the gears now I only get 10 - 15 mins before the error shows. I checked for an error code and I saw it had a 'P0755 Shift Solenoid B' error. I contacted a local automatic transmission specialist and he said that it was probably better if i changed the solenoid and the wiring harness as the harness failing was a common issue on these transmissions so I agreed and paid him to change the solenoid, harness, filter and fluid. after the work had been done the problem is still present. The mechanic at the garage suggested that it might be the Transmission Control Module because I mentioned that I have sometimes received the error while stationary at the traffic lights and I wasn't putting any stress on the transmission. I bought a second hand TCM and swapped it with the one in the car and still the problem is there. In a nutshell i now get around 10 -15 minutes of normal operation before the error appears. Before the error appears the transmission work perfectly well, changes gear smoothly and all gears working. After the error appears the transmissions is locked in 4th gear, reverse still works although it makes a bit of a thud going from drive to reverse and back. At the weekend I did a little test and started the car on the driveway and just left it in 'Park' to see if the error would appear without driving and it did, after around 30 -35 mins i got the usual 'Ping' 'Trans Fail Prog' error and I hadn't moved the gear selector from 'P'.
Does this sound like it just needs the O-ring replacing or is it more? looking forward to any advice given by some of the ZF Transmission Jedi Masters that frequent this great forum.