Although the primary O2 sensor is the main sensor used by the ECM for fueling, the secondary O2 sensor can have an affect on the fueling. We learned this with Miller's car at the Maryland Mod Meet one year when the O2 extender on the sencondary O2 sensor failed. Basically it allowed air to be sucked in past the secondary O2 sensor while at idle, which read as very lean, basically air. What the ECM then did was after about 3 minutes if I remember, it ran the engine very rich for about 10 seconds, then ran normally. Then since it saw no response from the secondary O2 sensor, it did it again and ran the engine really rich for about 10 seconds. It did this a total of 3 times, after which it ran in kinda a fueling limp mode. I could be wrong on the times it took, etc, as it was about 4-5 years ago now, but it was pretty interesting. That being said, if the secondary O2 sensor is working normally the primary O2 sensor is used for fueling.