I bought a 01 A4 1.8t quatro a few months ago and three days later I got the cel for secondary air injection, incorrect flow. Well I figured out that I could not hear the air pump running like my past two cars had done on cold start. So I take it to an audi shop and they tell me that the air pump is bad. I buy a used one from a junk yard and today plug it in and it doesnt work. I bench tested the new pump and it worked then my old pump and it worked so power is not getting to the pump why is this. I checked for broken wires and blown fuses and came up with nothing. Is there a relay that controls this somewhere or maybe the ecu is not sending the signal to turn it on?

 

Just curious what is the purpose of the air pump on cold start? I was wondering what that sound is when I start.

 

The fuse and the relay are under the dash on the drivers side, so check that.
As to what it does, it gets the cat heated up faster to the necessary temperature.