C6 estate auto box woes

[Mr Zap]

So my old 200k+ miles '05 reg 3ltr diesel estate lumbers on but has now thrown what looks like a gearbox problem and now won't select gears. The centre display went bold and diagnostics indicated fault numbers P0729, P0735 and P0783. Initial research suggests either the ECU or the transmission itself - obviously I'm hoping it's the ECU but I can't find conclusive info as to where it is? Some suggest it's under the carpet on the passenger side but is that l/h or r/h drive passenger? I'm going to try to have a look this weekend (if weather allows). I did have a problem a few years ago with the heater matrix leaking coolant into the cab so I'm wondering if the ECU is under the floor whether that might have had some adverse effects, but why has it waited all this time to show itself? Oh, one more clue, my diagnostic box could not communicate with the EMU and that's new so would gearbox ECU failure break the comms chain and cause that too?
Btw P0729 is "Gear 6 Incorrect ratio", P0735 is "Gear 5 Incorrect Ratio" and P0783 is "3-4 Shift".

[Crasher]

It is in the sump of the box itself. ZF 6HP-19

[Mr Zap]

Hmm, that does't sound like it makes things easier ..... Interesting write-up about that gearbox here: ZF 6HP transmission - Wikipedia maybe I'm lucky it's lasted 200k! So is there any hope or does this mean the old girl is heading for the breakers? I spoke to a local Audi chap who said he'd be disenclined to change the transmission fluid in such an old box, it nmight cause more problems than it solved, but that was of course before this catastrophe!

[Crasher]

Personally I would say it is terminal as you were selling it anyway, you would be looking at £3000 ish for a rebuild at a typical specialist or if you had really lost touch with reality, £6945.86 plus installation for a genuine Audi exchange unit or you could really lose the plot and buy a new one outright for £8993.18... but those prices do include VAT!

[Mr Zap]

Ok so assuming I don't have that money and haven't got a winning lottery ticket scrunched up in a pocket somewhere, would it be worth trying a fluid change? I can't see that there would be much to lose at this point.

[Crasher]

I think that would be a few more hundred pounds into the drain unit, at least it would make the job of stripping it down cleaner. This is going to be a problem that kills good useable cars in the future even more than now, £20k for a battery at 10 years old may concentrate the mind but few are listening and the consumers of the future are going to pay the price… oldies will blame Boris, the younger ones will say “who?”…