Audi S4 Turbo ECU  4A0 907 551AA  551 AA  /  0 261 200 465
 Up for sale is a Working, in excellent condition Audi S4 ECU.  This was removed from a crash damaged 1993 US model Audi S4 that still ran.

As always your best bet is to match the part numbers with the original. This is located to the right of the passengers feet under the bulge. This is not an easy to find ECU, as this was a very expensive low production number car. These units now have to be special ordered from most dealers new for big $$ (often times over $2000!), and finding a good one second hand is nearly impossible. Though usually fairly reliable, these can be water damaged, or damaged by faulty sub systems (bad alternator causing voltage spikes, or getting a jump start for example). If you have a faulty ECU, you may have problems with no start, random stalling, rich or lean running, no start when hot or very cold, or boost pressure control problems.