The cabin air circulation system in the A4 includes an air filter (sometimes called a pollen filter) than helps keep your car from feeling "not-so-fresh". Part of scheduled maintenance is replacing the filter, but if your car is well out of warranty it's possible it hasn't been changed in many miles. Opinions vary about how often to change it, and depends on things like how dirty the air is outside and inside your car. Symptoms of an old filter include a smelly-socks odor.

TOOLS/SUPPLIES
No tools required.

You can get replacement filters at a variety of places including the dealership or ECS Tuning. There seem to be two types out there: regular folded paper, and activated carbon. As you can guess, plain is cheaper, carbon is more effective. So be aware of what you're getting.

SKILL LEVEL
I'd give this one wrench out of five. It'll take you longer to read this than to do it.

PROCEDURE
The cabin filter is hiding at the back part of the engine compartment up near the passenger-side firewall. You will need to remove a few plastic bits to get at it. Note it might help to park your wipers in mid-sweep, so they don't get bumped as you remove things, but this is not totally necessary.

Open the car's hood. Right near the back there's a long plastic shield that covers the battery area. This shield is held in place by weatherstriping material, which runs the entire width of the engine compartment. Note the plastic shield is a bit less extensive in '04+ cars.

The weatherstrip needs to be peeled away. Start pulling at one end and you'll see it all comes out fairly easily:


Next pull the plastic shield toward you. It slides out fairly easily, no need to use excessive force:


The filter itself is in a little plastic box all the way to your left. It's held shut with two little clips. Squeeze them and it pops open:


There's that dirty little thing. Clearly it's been neglected for quite a while!


Pull out the old filter, and pop in your new one. You shouldn't need to push down too hard, and be careful not to warp or bend the filter as you slide it in. Note the direction of airflow arrow, which should point DOWN.


A word of caution here: the blower motor is right below the filter. If you drop some crud down there, it can get caught in there, or even cause annoying buzzing noises that will haunt you forever. So be careful to clear away any debris in your engine compartment before removing the filter.

Reassembly is, as they say, the reverse. Before that, take a second and wipe off the old dirt from all the pieces in there, just so everything is nice and tidy for next time.

Note that getting the filter cover and shroud back in place can be tricky as they have to slide back into place "just so". Be patient and don't force anything. Also make sure the shroud cover is lying perfectly flat in place before you place the weatherstrip back on, otherwise it won't stay put.

For B5 owners: Note the procedure is very similar to the B6, with the exception that the cover near the firewall is held in place with screws, and it can be quite a bit trickier to jiggle everything in and out of place. Also because your car is older, the plastic's more brittle and prone to breakage. So try to do this on a warm day, and again, don't force anything.