This section is intended to expose many of the more common problems with the B5 S4. Please do not add a problem unless you know that others have had the same problem as well, or this section could easily become far too cumbersome...

Clearly this is a big problem, as the repair will run you >$6000 if you are not under warranty, but it is usually attributable to one of a few weak points in the system (listed below). These 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.

The Throttle Body Boot - 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

Instead of using Blow Off Valves, like many turbo cars, the S4 uses Diverter Valves, or bypass valves. There are two DVs (one for each turbo) that frequently fail because of a poor design by the manufacturer. Note that the DVs have an exceptionally high probability of failure if the vehicle is modified to increase performance, but even without modifications, they frequently break.

The most common solution is to replace each one with the DVs 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 DVs are known to perform well even with significant performance modification to the vehicle. Other DV options exist through aftermarket manufacturers such as Bailey, Forge, Stratmosphere, and many more. More information about the B5 and Diverter Valves can be found here. 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 AudiWorld tech article on DVs: http://www.audiworld.com/tech/eng21.shtml

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.

the pump housing splits as a result of a defect. The design has since been changed.

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.

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.

Fan internal to the climate control unit makes a buzzing noise. Replacing the fan fixes the problem.

Audi S4 B5 Common Problems