Secondary Air System Flow

My check engine light came on and I ran the codes. I have a 328i convertible with 63k miles. The chart states: "Secondary Air System Flow Too Low." Please explain what the seconday air system is, and what I can do to fix the problem. I will avoid the dealership at all costs, they are terribly expensive. Thanks for any help,
Philip Schmalz

the secondary air system injects fresh air from the big round black pump mounted behind the R/F shock tower. Replace it and the non-return valve on the front exhaust manifold. Make sure to get all the water out of that plastic intake pipe that ends behind the R/F headlamps...the pumps burn out and the valves can leak. have fun.