Copied directly from the FAQ's 2 threads above this:
-Common Problems-
The following are a list of S4 problems that I've compiled after reading Audiworld for nearly 2 years. Some of these may be one time fixes that are corrected at the dealership under warranty, but may be something you want to look for in a potential purchase.
#1. Turbo failure: clearly this is a big problem, as the repair will run you >$6000 if you are not under warranty (not a typo!), but it is usually attributable to one of a few weak points in the system.
*** Here is a writeup from The Jerbel on the process of pulling the engine to replace your blown turbos***
Click Here!!
Throttle body boot (TBB). A small rubber boot that connects the two pipes that deliver air from the intercoolers to the throttle body. It is shaped like a truncated "Y", and frequently tears due to overtight clamps or simply a defective design (poor material). Several fixes are available on the aftermarket: Samco makes a silicon TBB, and it's available for about $60 when I checked last. Some individuals have complained that it does not fit quite properly, and you usually need to buy clamps separately, but it is usually a serviceable part. Another alternative is AMS' Kevlar-wrapped TBB. This is merely a stock TBB with a layer of Kevlar wrapped around it. I cannot recall seeing a failed one listed on the forum, and it looks and fits identically to the stock TBB. This is available for about $90. What most regard as the best replacement for the TBB, however, is a part called the Bipipe. Made by APR, it is a "Y" shaped pipe made of metal that replaces the TBB and the metal intercooler pipes feeding it. It runs about $350 from APR, and is generally regarded as a foolproof fix to the problem. Other options are available, but they will not be addressed here. Here is an AW tech article on TBB replacement:
http://www.audiworld.com/tech/eng19.shtml
Bypass valves (BPVs). Instead of using Blowoff valves, like many turbo cars, the S4 uses BPVs. I will not get into a technical description of what they do, because some knowledge of turbochargers is required. Suffice to say there are two BPVs (one for each turbo) that frequently fail because of a poor design by the manufacturer. Note that the BPVs have an exceptionally high probability of failure if the vehicle is modified to increase performance, but even without modifications, they frequently break. Several fixes are available here as well. The most common solution is to replace each one with the BPVs found in the Audi TT 225 (called 710Ns, by part #), which are a more robust design. Each will run you no more than $35, and can be replaced easily by yourself using the tech write-up on AW. These BPVs are known to perform well even with significant performance modification to the vehicle. Other BPV options exist through aftermarket manufacturers such as Bailey, Forge, Stratmosphere, and many more. The difference between these valves and the TT valves is that they are constructed of metal. They are considered very robust, but require internal lubrication from time to time. These typically run anywhere from $200-$350 for the pair. Here is an AW tech article on BPVs:
http://www.audiworld.com/tech/eng21.shtml (sorry, cut and paste)
F-Hose. This problem is exclusive to the 2000 and early 2001 model years (late 2001 year models had numerous changes, and are typically referred to as 2001.5). In the S4s where this is an issue, the F-hose is a little hose that, oddly enough, is shaped like an "F", and is made of rubber. It is prone to tearing. Alternatives available are one made by MTM, which is a metal pipe fashioned in the required shape with rubber connections; cost is about $80. Samco makes a silicon F hose that runs closer to $25. Both seem to work just fine. Note that the F-hose doesn't seem to be as common of a problem as the aforementioned parts.
These three trouble spots can create big problems for your turbo if malfunctioning. If you're interested in buying an S4, it's usually good if the seller at least has a knowledge of the potential problems, and even better if they have been remedied. Turbo failure rate does increase when performance modifications are made to the car. Naturally when you demand more activity of your turbo (to give you more power) it's going to see more wear, and probably premature failure.
#2. Driver's seat problem: apparently some seats likes to "rock" back and forth as if there is play in the track. This problem was usually fixed under warranty. I have personally never experienced it with my S4.
#3. Windshield washer fluid pump cracks. I am not aware of a particular cause for this failure, but most have gotten it taken care of under warranty.
#4. Transmission 1st-2nd gear slider problems (6sp only). This AW thread explains the phenomenon:
http://forums.audiworld.com/s4/msgs/691866.phtml. Coupled with weak synchros and a user-base that is frequently running significantly higher-than-stock performance powerplants, this problem *can* cause premature transmission failure. Typically if you treat your gearbox nicely, it will not have problems, so don't power shift through the gears and you should be okay. Transmission problems can also be caused by improper installation of a short-throw shift kit. Follow directions closely if installing one. Just pray that your transmission problems crop up while you are under warranty (if at all), because they will be a VERY expensive fix if you aren't. Do not become overly worried about this issue, many S4s have had no transmission issues at all (like mine).
#5. Mass air flow (MAF) sensor trouble. The mass air flow sensor detects the rate at which air is entering your intake system (to go to the engine). They frequently fail due to (you guessed it) poor design, and could cause gas mileage problems, or a rich or lean condition, even a loss of power. This is about a $350 part (and really easy to replace), so it's not crazy expensive if it goes, but just be aware. This problem appears to be generally limited to MY 2000 cars.
#6. Intercooler (IC) tank end caps leaking. This problem is (some say) due to the end caps on the IC tanks being made of plastic versus the metal of the IC. When they leak, you'll get a loss of power due to the pressurized air that's escaping. Not a terribly common problem.
#7. "Warped" front rotors. Due to their very large size (swept area) and inability to trasnmit heat to their surroundings efficiently, uneven brake pad deposits are frequently left on the rotors. This can result in poor braking performance, and can cause a sort of shuddering experience during hard braking. Many people simply buy inexpensive (in some cases cheaper than replacement OEM) aftermarket rotors which perform well with stock calipers. A set of aftermarket front rotors (good ones) can cost you as little as about $200.
#8. Fan internal to the climate control unit makes a buzzing noise. Replacing the fan fixes the problem.
Those are most of the most common problems that I have found in my experience with AW. Potential buyers worry not, however, as I can assure you they are anything but guaranteed. As they owner of a 2000 S4 with nearly 60k miles on it, I have not experienced one of these problems, and I have modified my car.
-Model Year Changes-
About half of the later 2001 model year S4s made many significant changes. These are the 2001.5 S4s referred to on AW. Among the differences were the following: larger oil inlet lines to the turbo to increase lubrication (made turbos more reliable); badges on front and rear were changed, and others. The first 2001 models received the following over 2000s: a longer warranty (4yr/50k instead of 3yr/50k), sport packages got chrome mirrors, got ESP (traction control), got different tires, avant became available, and others. Clearly, the ones you want are the 2001.5 or 2002s, but that's not to say you can't do well with a 2000.
Searching isn't that bad man, give it a shot next time.
Originally Posted by AudiSportB5S4
Dude, please STFA or at least read the stickys 
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