Hello,
I think I just solved a problem with my EVAP system in a 1998 A4 Avant Quattro 2.8 30V. I have the VAG-COM interface and got
4 Faults Found:
16839 - EVAP System: Large Leak Detected
P0455 - 35-00 - -
17546 - Fuel Trim: Bank 2 (Add): System too Lean
P1138 - 35-00 - -
17544 - Fuel Trim: Bank 1 (Add): System too Lean
P1136 - 35-00 - -
16825 - EVAP Emission Control Sys: Incorrect Flow
P0441 - 35-00 - -

I looked it all up on the Bentley manual (THAT's a load of fun, eh?) and also in my Haynes manual. The Haynes said to check the charcoal canister and leaks in hoses. I took of the badly corroded cap nuts to view it, and nothing looked wrong. I took out the canister, basically cutting off the two crimped fittings. The line to the gas tank has a large aluminum adapter, and this was badly corroded. The inside of the hose had a lot of corrosion, which I removed. I also sanded to adapter. I pressurized the fuel tank with two breaths of air (no I do not smoke) and held my thumb over it to see if it lost pressure. It did not. I them used gasket sealer on all three tubes, and clamped them in tight with wide hose clamps. I ran the car for 30 minutes, starting it 4-5 times. I took it out and went just a tiny bit above the posted speed limit, then shut it off and on 3-4 more times. It says that it will throw a code right after startup if the pressurizer pump keeps running. No code. Did I fix it in my 20 minute and $0.00 effort? Just posting for the "might be simple" concept.