Slider388

Hi everyone,
I have a 2000 A4 quattro 1.8t with 153k miles on it.
I'm getting the following codes:

P0411 Secondary Air Injection System Incorrect Flow Detected
P0300 Random/Multiple Cylinder Misfire Detected
P1113 Bank1-Sensor1 Internal Resistance too High
P1114 Bank1-Sensor2 Internal Resistant too High

I'll be checking the combi valve, hoses, and saip to fix the p0411, and I have a feeling that the P0300 is somehow related to one of the other problems.

My main question is about the B1S1 and B1S2 sensors...where are they located and which do I need to buy as replacements?
Any other help or advice is greatly appreciated.

Thanks a ton,
Alex