2003 Huyndai Accent won't pass smog?

I have a 2003 Hyundai Accent that recently did not pass smog (CA). mechanic suggested new spark plugs, repaired, came back, no luck. High NOX?  Anyhow, he stated that is was the catalytic converter, but that there is no aftermarket part, only dealer part. When I call the dealer to inquire about part purchase, they ask if the check engine light was on to indicate catalytic converter, otherwise, they say I should get a 2nd opinion since the part is non-refundable and installing it won't guarantee that the car will pass smog. Any other ideas as to what is causing the high NOX (?) value, I was told that an oxygen sensor alone won't help and we would probably need to replace that along with the catalytic converter.. Any suggestions? One small detail: my daughter accidentally put 1/2 gallon of diesel into her car 8 months ago (ran out of gas, went into dads garage and filled from gas container that she didn't realize was not gas) , it bogged and smoked like a dog until it was out, but no other apparent mechanical issues. we did change the spark plugs and fuel filter after that..Any suggestions?