The good news is that the maintenance will likely have been performed on schedule or even more often. If parts are known to fail prematurely (such as the timing belt/tensioner), the tuner will likely have replaced it earlier than Audi recommends. The owner will probably have used top quality fluids and parts. The car will likely have been detailed more than a typical car, and the paint will probably be in better condition than average.

The bad news is that the vehicle may have been driven very hard, autocrossed, dragged, or tracked. While this can be true of any used car, it's somewhat more likely with a car that has significant performance modifications. One useful thing to check is the integrity of the fuel line. If it's been patched, that's a likely sign that the previous owner used a nitrous oxide injection system. That system has only one purpose.

Also, the vehicle may come with some mods that are illegal:

1) Sometimes tint that is too dark has been installed. If the tint level is illegal for your state, you'll have to remove it -- or be content being a magnet for police attention.

2) The catalytic converter (cat) may have been replaced by a straight pipe (test pipe) or been hollowed out. This makes the vehicle illegal for street use anywhere in the United States, and it is a violation of federal law to make such a modification. Depending on the state's inspection system, or lack thereof, this modification may not have been detected, and the vehicle may have passed inspection. However, if you're caught, at the very minimum you will be paying for an expensive new cat -- think about $1000 with installation. It is illegal for a salvage yard to sell used cats.

It's conventional wisdom among tuners that other modifications will not add value to the sale price of the car unless the owner is lucky enough to find a buyer who is also a tuner. As a result, owners will often remove their modifications prior to the sale and sell them to fellow tuners separately. While you'd like to think that the modified part is replaced with the original part, the part that gets put back on may be a used part from another tuner or from a salvage yard. So while your new-to-you vehicle may have only 50,000 miles on it, the springs and shocks might have 80,000 miles.

The best thing to do is get to the owner before the vehicle is de-modded and make an offer for those mods you want. A good offer is 30-50% of the original parts cost, which is what the tuner expects to get selling the part separately. The tuner saves the time needed to remove the mod and find a replacement. You get a mod at a greatly reduced price, already installed. Audi mods are usually high quality; many come with a lifetime warranty, at least to the original purchaser. And most manufacturers go to great lengths to ensure that their parts do not affect the legality of the vehicle for street use.

Also note that there are some really EVIL tuners out there, if only in the sense that they do modifications that ruin the car. If you're looking at an Audi that has a wing, or a hood scoop, or an engine compartment that was "simplified" by throwing out some engine components, or flame decals on the sides, or a coffee can exhaust, or a blow-off valve, these are signs of evil tuning. If you get a great deal on the car, all of these modifications can be removed, of course. The problem is that you don't know what else was done. Things such as hacked up electrical harnesses can be extremely difficult to troubleshoot, as can ECUs with amateur modifications, which may be why the car is for sale in the first place. Unless you have a lot of patience and a lot of money, you're better off avoiding cars like this.

Problems from a modified wiring harness?

Rather than list good mods, I'll suggest you take a look at the signatures of any 25 members of this forum and make a list of the mods you see in those sigs. Some may not be aesthetically pleasing to you, but they're all pretty much tried and true mods that improve the car.

Finally, even if the previous owner of the car is technically knowledgeable and completely honest, it's still a good idea to get an independent mechanic who knows Audi to inspect the car for you.

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2001 A4TQM
GIAC X-Chip
710N BPV
APR catback exhaust
H-Sport front and rear anti-sways
Koni Adjustable Yellows
A8 big brake kit
Carbotech Bobcat pads
Kumho Ecsta Sports
AWE Tuning Center Vent Boost Gauge
VW Taschenlampe

Link to the A4 B5 FAQ