In the keys there is a small clear pill shaped gizmo with what appears to be a copper coil inside super glued into the head of the key. Does the A4 have a "key code" type key? If you take the key apart, you can see where there is a black section of plastic that extendeds into the key near the head is where the code would be read. According to Graham: Do your US models have a key like symbol light up when you start the ignition?? The immobilizer warning light is in the tacho just above the 1/1000 RPM text and between the spindle and the 7 (000) mark. Check your dash, you should be able to see if you look at the correct angle if there is an empty light position there. If so then this is part of the immobilizer. The keys have a code in them and the immobilizer part of the ECU reads the key each time you start the car to make sure it is a valid key. Also, the codes tumble. That is, each time you take a key out it will have a different code in it. The ECU remembers each code that it has written to a key and not yet used - hence your spare and emergency keys work. As soon as a key is inserted and the engine fired up, that key gets a new code and that is what the ECU remembers. This should certainly help with the reduction in the midnight renters, especially as the number of available codes kept by the ECU is in the millions. Graham is in Australia where all A4's have the immobilizer. The sales literature in Australia makes a big point of the immobilizer feature. The U.S. A4's appear to have the immobilizer-type keys, but not the hardware in the car to run the system! Update (Aug/98) - According to David the immobilizer key is no longer standard on all Australian A4's.