On my 2002 A8 I have the following codes 491, 492. I cleared the codes and so far so good. I know I have a bad drivers side cat converter. Would this cause these codes.

Those aren't fault codes. A fault code would be P0300 or 16684 which is the DTC number for the same code.

P0491: Secondary Air Injection System Insufficient Flow Bank 1

P0492: Secondary Air Injection System Insufficient Flow Bank 2

Future reference: http://www.aboutautomobile.com/DTC/

Is this minor or major? The car still runs fantastic. Is this being caused by the bad cat converter?

Its minor, the secondary air injection system injects air into the exhaust to help heat up the cats upon cold start. Thats all it does. It won't affect engine performance in the least, so fixing it doesn't have to be done right away.

Thanks that is good to know. I just ordered the drivers side cat converter, and also want to get brakes done this month. I want everything to be perfect by winter.