The O2 sims are just there to lie to the computer that the cats are supposely still there. The back 2 O2 sensors are really of no importance. There job is to let the computer know when the cats are missing!!! or not working correctly. When that happens the check engine will turn on. The sims correct this from happening. I believe they will have nothing to do with the mixture control. That is what the front O2 senors are for.