The whole efi system relies on the O2 sensor(s) to work and fine tune the system in closed loop mode. Modern efi is supposed to eliminate the need to "tune" vehicles occasionally to ensure peak performance and ensure the vehicle meets emissions requirements. Makes you wonder if we had California type smog tests if we would have to change those pesky O2 sensors sooner?
I've always believed O2 sensors should be considered a wearing item and need to be changed. We should consider ourselves lucky, I did a pair of O2 sensors on our VX and it cost about $120. Recently we did one O2 sensor on my boys Subaru, retail price was over NZ$500, trade was $395 but the parts guy was in a good mode and sold it to him for $295....