"Feedback System (poss O2 sensor)"
this came up with my check engine codes, still got popping during idle after replacing trailing coil and intake engine thermosensor. appreciate the help very much!

 

Problems with the ACV can cause this during idle. Port air injection would normally be on during idle and if the ECU see's activity from the O2 sensor during this time, it will throw a code after 20 seconds and start ignoring the O2 sensor.

Not really a big deal honestly as the ECU will resume normal function once everything normalizes. If the check engine light goes off once you start moving again, the ECU is going back to reading the O2 sensor.

A bad relief solenoid can cause this issue. If you are just being annoyed by the check engine light, this code is not really a big deal if it only shows up during idle.

 

Not sure if this is your solution but I use to get that code with Bosch O2 sensor installed. Switched to NTK and fixed the code.

 

the feedback system refers to the car being in closed loop. you're looking for the closed loop stuff, i'm sure there is a page in the FSM that actually lists this stuff, but....

o2 sensor. the S4's have a temp sensor in the bottom of the radiator that turns on the closed loop system @65c. TPS being out might cause it, maybe vacuum or an exhaust leak.

and i think the definition of the code says something about the time in closed loop without seeing the o2 change.