Hey all, I have a 2000 A6 Audi Quattro with a 2.8 (Great car great condition, which I bought new in 1999)

This is the second engine, with the first engine going, because I didn't change the timing belt when I should have (stupid me)

With the first engine the CHECK ENGINE light wouldn't go out even after numerous trips to the dealership and this included the main computer board being replaced because some mice decided to build a nest under the engine cover to stay warm in the winter and fed on the engine main wiring harness.

Anyway the main computer in the engine bay was replaced, but the check engine light would not go off or reset at the dealer... the dealer told me that it was an air sensor in the head and it would have to be replace when the heads were rebuilt... so I decided to live with it.

About 4 months later the timing belt let loose taking out the valves and at that point it was cheeper putting in a used 2.8 engine with 35,000 miles on in stead of having my 80,000+ mile motor rebuilt.

So with the new used motor I thought the CHECK ENGINE light would go off... AND NO IT DIDN'T. (So no it wasn't what the dealer said it was)

Recently I had the engine codes checked and the two error codes that are coming are...
P0411 (Secondary Air Injection System Incorrect Flow Detected)
and
P0306 (Cylinder 6 Misfire Detected) (Which is strange because the engine runs real smooth)

The question I have is what might be causing the above codes?

I also tried reseting the CHECK ENGINE light by disconnecting the battery cable for 10 minutes but the light is still on.

Any thoughts from someone in the know?