The sensor is a hydrocarbon gas detector. You dont need a full-on mass spectrometer for this, they are built pretty cheaply and inexpesively. I just went searching through my computer for the document that talked about the sensor on my e46 (it had one too) but I can't find it. It works pretty well though, the recirc "flap?" inside the dash makes it evident when it changes and pretty much like clockwork when I stop at a light amidst cars, I can hear it close me to outside air.
Some light googling found some tidbits too: If the AUC II sensor detects an increased level of pollutants in the environment from spark-ignition and diesel engines, the IHKA control unit will automatically switch to air recirculation mode.
Last edited by Drewsky; 10-04-2013 at 12:58 PM.