01 S4 check engine light came on today. Scanned codes and got P1120, o2 Sensor Circ., Bank 2-Sensor 1 No Activity Detected. Also got PO154 o2 Sensor Circ., Bank 2 Sensor 1 open. Reset and car runs fine. Should I replace the sensors?? Is it something a back yard mechanic can do?? Thanks for your help.

 

Wait to see if the codes come back.

 

Yeah, if the codes come back its safe to say the sensors are bad. Replacing the sensors can prove to be difficult on an S4 though, even for a backyard mechanic.