2001 Hyundai Accent Emissions test

HI, my 2201 accent has failed the emissions test. It passed on all tests except for the NOX levels were too high. My dealership says they have tried everything and now want to replace the catalytic converter just to see if that will make it pass. I have not seen the paperwork stating that the tests have failed so I need some help to diagnose if this is the problem or if there is something else it could be. The NOX level was 995 on the first test and should have been down around 600 to pass. There is often a smell of burning oil but they said that there is no oil leak and they also said the O2 sensor was fine. I have alrady replaced the mass air flow sensor. Any hep would be appreciated! I need info fast. Thanks.