Changing an air filter used to involve removing a wingnut, removing the top of the air cleaner, replacing the filter element, putting the top of the air cleaner back on, and tightening the wingnut. If it took more than a minute, you stopped for a drink of beer -- twice. Those days are gone.
The instructions below are for replacing the air filter on a 1.8T A4 AWM motor from the 2001 model. Older models are similar.
NOTE: This can be an easy or difficult job depending on how much slack is in the wires and hoses surrounding the upper air cleaner housing. Some users report that this is easier than changing the cabin air filter. Others say it's harder. I fell into the latter category and the instructions describe what worked for me, but I've included shortcuts mentioned by several users who were able to omit some steps.
Tools you'll need: medium cross tip (Phillips) screwdriver, medium flat tip screwdriver, short (4") cross tip, work light, 303 Aerospace protectant, vacuum cleaner with crevice tool.
1) Remove the protective cover from the air cleaner housing. It's held on by two spring clamps. Simply pull straight up.
2) Remove the air duct. The duct is actually two pieces that run from the air cleaner housing to an opening just over the radiator. Most manuals recommend that you wiggle the portion of the duct closest to the air cleaner to get just that piece out. Several B5 forum members have recommended that you remove the front portion of the duct that's held in place by two phillips head screws (see photo). I agree that you should do this. It prevents you breaking one or both pieces of the plastic duct while removing or installing it, and it gives you a bit more room while you're working. Use the opportunity to spif it up with your favorite vinyl protectant. To free the back end of the air duct from the air cleaner box, lift straight up while rocking it slightly side-to-side. If should come with moderate effort.
3) (OPTIONAL STEP) Remove the metal heat shield on the right of the air cleaner box. I know many B5 owners can get the air cleaner box cover off without doing this, or by just loosening the top of the heat shield, but I couldn't access the front right clamp with the heat shield on -- that is, I could undo the clamp, but couldn't get it back on. And the edges of the heat shield are sharp!! Try popping the clamps off with the heat shield on, then pop them back on. If you can do that, you won't need to remove the heat shield. However, removing and replacing the shield requires only a few minutes. I recommend that you do it the first time you change your air filter. If you don't think you need to do it after that, don't.
The shield is held on by four screws, two at the top and two in the middle. For the middle ones, you need the short cross tip screwdriver or you'll strip the head of the screws. You may find that someone has already done that. If so, install the stripped screws in the top holes when you put things back together.
4) Use a flat tip screwdriver to loosen the hose clamp that holds the rubber air duct exiting the upper air cleaner housing. Pull the air duct loose. Again, some may be able to get the box cover open without doing this, but I couldn't.
5) Pop the four clamps holding the upper air cleaner housing to the lower housing. There is one at each corner, but the front left and rear right clamps aren't on the sides. They're on the front and rear of the cover respectively. You'll probably need a work light to find the four clamps.
6) The upper part of the air cleaner housing should then lift off enough to get the old filter out and the new one in. That's the theory. It helps if you have an assistant to hold up the top while you wiggle the old filter out and the new one in. Because of wires/hoses on the front and on the left (facing the vehicle), you'll have to get the filters in and out at an angle from the bottom left corner. Some people say this is easy, but it wasn't for me working alone. It was a very tight fit with little slack in the wires and hoses.
Audi air filter part# XXXXX
7) Check that the new filter is completely seated. Fit the upper air cleaner housing back on the bottom and pop the clamps back on.
7a) If a clamp pops off the bottom air cleaner housing, don't despair. Hold it vertically and push it back into the slot from which it came out. It should pop right back in. It's really not a great idea to run with fewer than four clamps. Unfiltered air ingested by the system will harm your engine.
8) Use this opportunity to clean and protect the vinyl and rubber. I like 303 Aerospace Protectant for both, but it's especially good for rubber.
9) Reinstall the rubber air duct and the heat shield.
10) Use a vacuum with a crevice tool to remove leaves and gunk from the opening in the air cleaner over which the plastic air duct fits. Be careful not to damage the screen between that opening and the lower air cleaner housing.
11) Reinstall the plastic air duct and air cleaner cover. You're done.
I'm guessing this job would take about a half hour the first time you do it, depending on how picky you are cleaning and treating the rubber and vinyl. It will go a lot faster the next time you do it.