Hey guys, just had a cel code pop up on my wrx. Was happening for awhile, ages ago - only ever happens when i take the car on the freeway and sit on 100 or above in 5th. Kept popping as a rear 02 sensor, so replaced it and this is the first time it's come up again. Once more, sitting on 100~ on the freeway in 5th.

Would just like to get it scanned/removed and a possible explenation as to why this is occuring? There's no rapid accel/decel, just casual granny free way driving.