Let me just clear up the misinformation as I seem to have upset people...
1. The firmware needs down grading to accept the code. What model mk2 rnse is it? Cheers Mike
Not true, the right pin code will work regardless
2. Other possible causes are that the CAN High and Low wires are swapped or disconnected.
Again, this is not true
3. A 'G' unit won't work fully with the A2's electronics unless a 'CAN-gate' is used
Probably a typo, I'm sure you meant "cangate"
4. and nor will it communicate correctly with the DIS.
It will communicate in A4 and A6 modes, but not correctly in A3/TT/R8 modes as they use a can gateway to talk to the cluster
As for the post about the code and swapping the unit.
"What are your thoughts on why it won't take a code ... the code is correct going by whats been written on and what Audi tell me it should be......
Also which led display units will fit without much messing around and can still work with the Dis , phone module and mfsw already installed.."
My A3 unit (Rev B) quite happily accepted it's code in all modes and on the bench, but it might be the swapping to A3 mode and can compatibility. Only way to be sure is downgrade and change back again. I'm a bit surprised you had a A3 unit coded for A6 mode, who did that? was it bought that way?
I have some suspicions that you might have an older unit, where was the G confirmation? on the lid? or could you read it in VCDS? have you checked the hardware revision in VCDS? that is crucial as the H02 or H03 units do have the pins for the speed pulses (not impulses).
AFAIK there are no units which work with the audi DIS/MFSW fully. Most have there own phone kits built in and such, like DAB radio or TV interface. Most good units seem to be able to read a can bus or resistor ladder style MFSW, but with limited control, probably volume and channel,, but it maybe programmable, I've never looked at this option fully.
I have often wanted to swap to a 3rd party unit, better navigation, USB etc integrated DAB, but there is just nothing that works well.