Large LTFT imbalance B1-B2
This is something you definitely need to correct. I had this issue a while back when I had a leaking band clamp. It was about the same difference as yours was off. The passenger side would read +12 and the other 0 or +2. Once I replaced the band clamp with one that would actually fit around the y-pipe tight it completely got them back in sync. As far as what it could be you sound like you know some of the typical culprits but if you had a bad heater code then I would switch your O2 Sensor's around and if it the problems follows the sensor you know just to replace the bad one and be done.


