Gas gage problem & code po420

gas gage say full when empty and empty when full and so on, and i got he code p 420 what does it mean how do i fix? thank

Welcome to AF. 0420 can be a bad cat or sensor after the cat, but the proper diagnosis of this can be complex. If you do a site or google search you will find lots of info. Sounds like a bad connection or tank sending unit. BTW, in the future post the year engine and mileage of your car.

Thank For The Help