code P0240 on 03 beetle

I just bought a used 03 volkswagen beetle a couple of weeks ago. Yesterday the check catalytic converter light came on on the dash. I took it and had it put on a computer (not at a volkswagen dealership). The code give to me is p0420. Does anyone know what this means? I called the dealership and they said many things could give this code, anything from not closing the gas cap correctly to driving the car with the low fuel light on. The local car repair shop said the catalytic converter needed replacing. So I am getting a little confussed... Any tips would help.

Before spending any $$$, I'd disconnect the negative terminal from the battery and reconnect after a few minutes - this should reset the codes. If the same code reappears then start investigating. Cat convertors only need replacing when blocked (high mileage) or they fail a pollution check. (at least in Australia). But first reset the codes as sometimes they can just be caused by a 'glitch'....and make sure you know the security code for your radio which should be in the owners manual in the glovebox. Good Luck.

Could be anything, gross evap leak or such...
wich would mean incorrect or open gas cap...
It also could be your cat. thers a recall/warranty extension on NB 99+ up to 120,000 miles. So if you hear rattling from under car, or crappy gas mileage.
you cat could be it.
~mike