Not sure if I need a new Cat converter or is it just an O2 sensor? on 2003 Kia Sedona

Engine light started to go on after I hosed off the engine. Light blinks on and off. Not having any signs of a bad converter as discribed on the web. Dealer wants to jump right in and change converter, shouldnt we troubleshoot with new sensors first. Autozone ck'd w/ diagnostic, most likely Faulty HO2S, rich air/fuel ratio, lastly was faulty converter

Asked by for the 2003 Kia Sedona
have live data read and watch 02 wave forms to determine if o2 is good b4 repl any parts. if unable seek help from mech as not to waste money