Have a Check Engine Light after your Header Install?

That's normal because of your catless header. To resolve this, record the code or codes that are being thrown (P0421 is the format). Then connect your car to the VT software again. We're going to re-flash the car but this time you'll click the EDIT button. You can then search for the code or scroll until you find it. Put a check box next to any code you'd like to disable and continue to flash the car. The code will no longer appear.