I have used COS#5 on probably close to a 100 fbodys with no problems until now. I had one that just wouldnt run very well with the pcm using the O2 sensors. I fought and and fought this car, very simple car to tune, just basic bolt ons and a small cam. As soon as I would let the stft turn on the car would run like crap and drive horrible. I tied new O2 sensors, even a another wide band and got the exact same results. I then just took the same exact tune and converted it back to the stock OS (12212156) and the car ran and drove perfect. It will not run with a custom OS in. Has anyone every came across this?