Fault code P0491 secondary air system

This is the latest in a long line of small but very irritating faults with my 02 V6 Avant. The check engine light is on, the OBD scan gives P0491 and 0492, no air flow in either left or right bank. The pump is fine, the CEL did not come on until the engine was warm, by which time the system should have turned itself off anyway. I wonder if it might be a sensor fault, can anyone give me any idea where it's located and how to get to it? The pipework seems to run around the back of the engine and it's all a bit crowded in there and hard to see anything. If it is a faulty sensor, can I get a replacement from anyone other than Audi?