[Captain Slow IRL]

2002 Audi A4 needs an ECU, is it a straight swap between cars assuming the ECU is coming from an identical car? Is there any coding or mileage issues that would arise?

[Slidey]

I would imagine there is coding to be done.

The key is probably matched to the ECU

[Nissan doctor]

You'll need the codes from the old ECU so you can programme the new one to all the cars other systems.

[crosshair1]

You need to change the skc (pin) of the replacement ecu to match that of the vehicle,
To do this you will need existing vehicle skc and skc of the replacement unit.
Vw dealers do this through geko and cannot see the skc (or supply it to you)
Vag commander and autologic can do this or vag tacho if you have access to the donor car instrument.
There are other options such as immo removal available