The rear O sensors like to see around .5 or .6 volts, meaning the oxygen is pretty much used up by the cat, some residual is still left. When both the pre cat and post cat start moving between .2 and .8 volts at about 10 Hz it means the cat is history. The cat changes HC and CO to H2O and CO2 and uses up most of the O when catalyzing those two elements. When it loses its ability to store and release O its destroyed and rather expensive to replace the cats. Sine the rear O sensor has not authority for fuel control, I would try putting 5 volts on the sensor signal line. Not sure if a sensor simulator is set up to put a near steady voltage on the rear O, I have never used one. I have done this on a Mazda with a bad cat to turn the light out temporality. I did eventually replace the cat at a cost of $100. Not sure if it would work on an Audi.

I would like to know if that work for you not replacing the cat and tring to simulate a voltage so the car thinks it is there. Please respond if you can because I believe I have a similar problem.