2003 F150- Have to turn key to on but must wait a minute to crank and start

I have to turn the key to on and wait up to a minute before turning it the rest of the way to crank and start.  If I do not turn to on and wait before cranking, it will turn over but not start.  Seems to take less time on hot days and longer on cold days.  I do not think it is the fuel system.  I am getting a code about emissions that no one can figure out the exact cause.