E53 3.0D Trans Fail Safe (GM Box) - Scratching my head (SOLVED)

[PixelBarnX5]

Hi all,

This has been covered a million times I know... for the last few months I've been reading this forum and sure I've covered every single thread and still not found my issue! Anyway, here goes...

A few months back I was crusing along the motorway in 5th gear at a bit of a rate. Lets just say 70mph for the sake of argument although there may have been a bit of tax... The gearbox was happily in 5th and then out of nowhere I got 'Trans Failsafe Prog', the car violently shifted down to third and then thats where it stayed. Took it for diagnosis at my local BMW Specialist which just gave me a P0734 code which I understand to be incorrect ratio at 4th gear. Now when I drive I have 1st, 2nd and 3rd and once it tries changing to 4th I get the Trans Failsafe Prog.

Since then I have done a lot of research and have even tried to buy a replacement box (no luck there... they seem to be gold dust).

Last weekend I decided to make a serious attempt at fixing this and did a oil / filter change in the gearbox. There was a bit of a leak from the gasket, although this is now fixed. There were no alarming signs inside the oil pan, no chunks of metal and just the usual fine metal dust as one would expect with high mileage (155k miles). Mechatronic looked all good too so I'm pretty sure the box itself is fine.

Since then I've got myself a K+DCAN cable and scanned the codes myself using INPA. This has come up with:

Gangueberwachung 4 with notable figures such as 66c oil temp and 13.7 volts of power.

Now because the box appeared in good condition I was thinking that the valve body was probably the issue. Especially considering one of the original fault codes was "EGS Signal Line Disturbed" - A friend of mine looked at the wiring diagram for the mechatronic and seems to think that the solenoids are wired in series so if the 3-4th solenoid is dead then it won't change beyond there - this would make sense if the signal gets disturbed, dead solenoid means no signal...

However, today in INPA I have activated all 3 solenoids, they all make a noise and show 'Solenoid Valve #X is being correctly activated'. So this would suggest that the valve body is actually fine and the problem is elsewhere...

So before I rip the box out is there anything else I should be looking for as I'm not totally convinced the box is the issue, and I want to troubleshoot absolutely everything else before going to the expense of a used / rebuild box...

Apologies for the long post, I assure you all I have been reading here for a while, just never posted before now!

Thanks in advance!