Does your Audi UrS4/S6 sometimes fail to start but once running seems to run fine? Your G40 Cam Position Sensor (CPS) may be starting to fail.
On the S4/S6 (and many other 90's era Audis) the cam position sensor is only used during startup to tell the engine when the compression stroke occurs. from then on out, the speed sensor (flywheel tooth counter) and reference pin sensors take over. The result is that if you have a flaky cam position sensor, the car will sometimes not start until a signal is sent from the G40 CPS.
This tool plugs in-line with the factory wiring harness. It has a built-in LED that will blink every time the CPS sends a signal as the camshafts complete a revolution. This is great for diagnostic purposes, because it allows you to see if your CPS is functioning properly (its not the easiest sensor in the world to replace, since the cam gear has to come off).
Want to build your own? Read this article for pinout information.