if you use the check box and uncheck the o2 your car will try and run off the afr plugged into those spots in your map. with the o2 sensor checked your car will try and run at 14.7 for the boxes that have a higher value than 128. so basically the o2 tries to tune the afr to 14.7 with out it you'll run what ever the cell is setup for possibly. now that all cars run different unless you tune those cells the car might run richer or leaner than whats in the cell.