Rain causing computer errors

Are the drain holes under the windshield clear of debris? I am thinking that water fills up that tray and possibly gets some wiring wet. I have seen a few Passats with ABS modules (Blinking ABS/Brake Lights) that I could not connect to with my vag-com tool. :-( I suspect their modules need repair/replacement.

That Vacuum Loss in the fuel system has me curious though! AFAIK the fuel system is under pressure but the Emissions system can have vacuum issues. Vacuum hoses commonly need replacement on the 1.8t engines but I am not sure about the other Audi engines.

My son's '98 A4 V6 has been having a few vacuum issues lately most of which we have chased down. One was tough and was associated with the secondary air pump (under passenger side and injects air into the cat during warm-up to protect it from burning out). He called the local dealer and found out that there had been a recall on the vacuum lines for the pump. The dealer has had in for a week now while they try to find the parts but at least they have supplied him with a rental car at no charge. So, in short, the V6 engines have their share of vacuum issues.

Could one of the OPs codes been due to a loose gas cap?

fwiw, the secondary air system doesn't "protect" the cat during warm up - it adds oxygen to the cat to help burn up the excess hydrocarbons that occur during engine warm up, when the ECU is richening the fuel mix. A properly working system causes the cat to get hotter, quicker, to greatly cut emissions during warm up...

/daytripper '00 s4 6spd

Is probably the correct answer. IIRC the A6 and Passat are similar.

Your mechanic's estimate seems to be reasonable, the ABS module being the most expensive of the group. A regular problem, I could build a small hill with all the ABS modules I've had to replace over the years.