poor gas mileage on 2002 Chevrolet Malibu

diagnostic code came up as bad oxygen sensor or catalytic converter, it happened today and I smelled strong odor of gas and the gas mileage just started to be very poor but the car runs well and idles good but just did not start up qiute as easy after I went to the auto parts store when after he put the compter on it and gave me the diagnosis

Asked by for the 2002 Chevrolet Malibu
that was not a diag, just a code pull. it never identifies a bad aprt just a system failure. a real diag from a real shop will give you your issues. fuel pressure regulators are a common failure for the symtoms you described.
get it checked before you kill you cat converter.