Hey! don't spend $1700 through your overpriced stealership!!!! If that cat is bad and you're ready to replace it grab a new one at
www.hyundaioemparts.com or follow the link below which shows your cat.
www.hyundaioemparts.com .
This is a great website run by a dealer down in Florida which also sells on the internet. I'm a VW guy and there's one for us called 1stvwparts.com out near Seattle for my VW parts, but since I'm lucky enough to now service my GF's 2001 Santa Fe I'm now applying my VW tricks in finding good quality parts here in the Hyundai world!
I just called them right quick and verified that a cat for your 4 cyl 2003 SantaFe is 700 bucks. Since it's OEM and a genuine Hyunidai part you're good to pass all your CARB tests out there in your wacky state of CA. So if you take it out, post some pics, let one of us geeks determine if it's a bad cat or not, and then purchase and replace accordingly.
I'm not sure about the Hyundai codes, but I would def try to determine if it's an O2 sensor, those guys are easy, usually don't require the special tool, as Jsinton said and are notorious for crapping out around 100k miles. If you do need a "lambda" wrench (O2 sensors are known as Lambda sensors in dork world) usually they're rentable at a local parts store.
Worst case scenario you're staring at $700 instead of $1700 for a new cat... Good luck and Godspeed. Hopefully that helps a little bit.
PEACE! :57:
~Lucky
Sidenote: Just do your plugs now, if you've got one out to check, you might as well change them all. no need to get dirty 2x.