s4_sp

Recently, my S4 (B6, 2005-early-year-model, 110k miles) gave me ECL code that said catalytic converter (bank-2) failed. So I went to an indy-shop to replace it at the cost of $1500.
After that pricey repair, now I have another ECL code that says "secondary air injection level (bank 1&2) has reached its lower limit." The mechanic at the indy-shop told me that I may ignore that error code because it is only about the engine cold start. He also said that error code is very common in many Audis'.

However, I really concern if it would be ok to drive with that, because it is a part of exhaust system of the engine. I think it has to be fixed eventually. But this time, I'd like to spend some time to do more research to root cause the issues that I am having or will potentially have soon.

So the question is, would it be ok to drive with this problem for a while? Or is it a critical issue that I should fix this immediately?

Many thanks to you in advance.