Atf level too low?
 

Just as an update: Looks like all things are sorted out now, no returning codes. I will keep an eye on the ATF level and keep you guys updated.+++ Lets hope it'll stay that way.

Rebuild at Mackies or better at ZF in Germany (this is where I go).

Yes, that will cost a lot of money.
But it won't be more expensive than the money +++.
Get rid of the car now if you are not prepared for full rebuild.

If you're lucky, you can find donor gearbox, but that's lottery.

Rebuilt will always be cheaper if you do it immediately after you notice first symptoms. When done, change ATF and filter every year if it's daily driver. That's what I do.

Yeah, that's kinda what I am preparing myself for slowly. I would consider the rebuild if the car didn't have other things that will need doing in the future (the timing belt is up next year I believe). A shame really, as I cannot really see any alternative to the S8 for even close the money.:(

Hi Rico :)
I would give Mackies a ring and discuss those codes and see what they say, they have the most experience and any advice from experts is good free advice.

Good luck with sorting - from someone who has been through the pain!
+++

18223/P1815 - Pressure Control Valve 1 (N215): Short to Plus

Possible Causes
Wiring from/to Pressure Control Valve 1 (N215) Short to Plus
Pressure Control Valve 1 (N215) faulty

Possible Solutions
Check Wiring from/to Pressure Control Valve 1 (N215)
Check Pressure Control Valve 1 (N215)
Check Measuring Blocks

Special Notes
Pressure Control Valve 1 (N215) might be referred to as Solenoid Valve 4 (N91)
Start with checking connectors for corrosion or water damage, in case of 01V transmission pay special attention to 16-pin connector between valve body and wiring harness.

I've had a look at the TCM today to see if there is anything wrong with it. Everything looks fine, nice and dry. Definitely still the original unit. I also had a look underneath the car (as much as its possible with it being on the ground) to see if there is an ATF leak that could cause the slipping gear related codes, but everything is so oily that its impossible to tell where its coming from. I cleared the codes and took for a very short drive shifting through all gears as often as possible, still driving perfectly fine and the only code that came back is the MAF (16486/P0102 "Signal too Low"). I haven't checked the wires properly yet but will do that asap. I need notice this zip-tie around the connector though, is this normal?

https://c1.staticflickr.com/5/4414/3...e1f7a0b8_b.jpg

I guess that confirms that the gearbox codes above are intermittent. Maybe there's still a chance to the ATF level is just too low?

A bung MAF will give gearbox problems, as the ECU and TCU use the engine load/torque data that can be derived from the MAF for shifting purposes.
Buy a new MAF.

A quick and dirty test is to do a WOT run from standing start.
You will get a rough 1-2 shift with a dud MAF.

P.S. zip tie is normal