is it the tune itself? It's my understanding I can use the tuner to remove any codes that would cause the CEL to come on. But my question is can the tuner turn off the o2s? Basically, I'd like to get one E85 tune for the car. There may be times on a road trip where I need to run regular fuel, so I'd return the tune to stock where E85 isn't regularly available. At that point, the CEL would come back on due to an offroad exhaust. Do I need a tune to turn off the rear o2, or will the tuner do that? I'd rather not have to get a 2nd tune just to keep the CEL off on pump gas.