1997 Honda lx showing code 420 why would it read that

okay, evap system~ be sure the gas cap is on, and that when you filled up, you only took it to the "first click"...if just fillin' it up until the first click next time and NOT tryin' to top it off, this problem will go away~ if not use the obd-II scan port to read then RESET the trouble codes~

and that's P0420 not 420....which university again?~