Leafy is correct, it deletes the egr system, throttle valve, cat, and dpf. Truck needs to be tuned to run correctly. I have my tuned by anarchy diesel with 5 different tunes hooked to a switch to shift on the fly. Stock, tow, street, hot street, race are the 5 different tunes I got. I also got the transmission tuning from anarchy which increases trans line pressure to create greater clamping power on the clutches to prevent slipping of the transmission. Warranty will be out the window. Avg 19mpg with delete was 14 before. I also run an edge insight cts2 monitor to keep an eye on everything going on with the truck.