The Cam Position Sensor is used by the ECU (Engine Control Unit, i.e. the Motronic Computer) to figure out where the pistons are early in the starting cycle rather than waiting for the G4 crank position sensor to come around. If the ECU does not get a cam position sensor signal, it will not fire the J17 fuel pump relay. Once the engine is running, the ECU will use the G4 crank sensor position sensor to keep track of things. If the cam sensor fails, a "2113" blink code will be thrown (into memory) and check engine light will come on. If it fails while running, you can keep driving but as soon as you turn the engine off, you are done. The engine will NOT re-fire until you fix the cam sensor. You might get lucky once or twice by cooling the cam sender down (try poring water down the back of the cam belt cover) Location photo and diagrams in the URL links below.
Marc S. recently (March 09) added:
"that if your CPS has failed (or is on the way out) and you need a way to drive the car disconnect the 3 pin connector at the CPS (front of the fuel rail) and shove a piece of wire or a paperclip or whatever between pins 2 and 3 (you can read the pin numbers by pulling back the rubber boot) on the car-harness end, leaving the sensor itself disconnected.
I had posted about this earlier but mistakenly suggested that you hold the signal pin up at 5v (pin 1), but in fact it needs to be held to ground (pin 3). Just came across this with a customer and thought I'd share / correct my early post.
Note: by doing this you have a 50% chance of starting the car. If it doesn't start right up, stop, try again. repeat. Probability has it that you'll get it fired up after 3 or 4 tries max, but I suppose it depends on how unlucky you are
This might come in handy sometime.<ul><li><a href="http://forums.audiworld.com/s4s6/msgs/101711.phtml">Link to a CPS location diagram and info</a></li></ul>