SO I've had the P0158 code 3 times, first two times were a few days apart between re-setting, then again this morning which is 2 months later.

If I have no intention of spending sunny weekends under my A6 replacing sensors and tracking wiring to look for faults, what is the effect of not addressing the code and just re-setting it?
Worse MPG?
Loss in performance?

Unless I get the emmisions check letter in the post, I don't think I'm going to address this until the fall when garage time is a goos use of bad weather weekends.

 

OH NO!!! You have high voltage from the second O2 sensor on bank 2.

Stop being lazy and fix it!

 

I guess I could if I knew it was definately the O2 sensor, read lots of cases of people changing them out and the code comeing straight back (issue in the wiring?)
Lot's of parts places list them, but thought this was a dealer only sensor?

Also, is it necessary to replace both rear sensors at the saem time or even all 4?

 

the second set of 02 sensors is just for emissions. And no they aren't dealer only sensors.