I am sorry you are having problems, I have to say that "Cats" are causing some problems to a lot of people sometimes quite unecessarily.

First, do you know the Engine Code for your car, as some Hyundai's do not require a Cat Test.

Next, presuming that it does, because the mileage is very low, it's probable that the Cat is quite "gunged" up inside, we have found that a good hard run to get the temperature of the Cat hot can allow Cats' with apparent problems to get through.

Many other things can also affect the readings, and with low mileage cars, heavy carbon deposits on Spark Plugs, old and brittle Plug Leads, dirty or blocked Air Filters, all or any of these should be looked at and changed if there is any doubt.

It will certainly save a huge amount of money if it saves a Cat being changed, and it will probably improve your fuel economy significantly as well. I hope this has helped, let me know the Engine Code if you can, I will check to see if a Cat test is required.

Regards - Bill