It may not even start... And if it does it will run like s___. It will likely start out in third gear going forward and the shift into reverse will jar your teeth. I found out the hard way when I forgot to put the 2BAR back after I flashed to stock. I didn't look at codes, but it lit the CEL. It's only 1 screw and a connector... Really recommend not leaving the 3BAR in after you flash to stock.
I was accidentally sent a 2 bar tune once, and I had a 3 bar installed. Car would barely idle, and the dashboard lit like a Christmas tree. Spooked the hell out of me.