Check the thermostat. The car could be continuously running in open loop, and the computer is seeing that and not taking the cat's input.
I work for Honda...go figure. But I'm still a Hyundai fanatic, don't worry!
Past: 2005 Accent GL, 1.6L DOHC -- my baby. R.I.P.
Current: 2002 Buick Regal LSX, L32 3800 Series III Supercharged 3.8L V-6 (swapped from stock L36 N/A 3800 Series II) , Midnight Pearl Blue - SLP 1:8.5 Rockers w/ LS1 springs, ZZP XP-Cam, HP Tuners for custom PCM, 3.69 Gears, built trans., Exhaust U-bend and resonator removed, straight-pipe cat-back with no muffler, some suspension stuff.
PB 1/4-mi: 13.12 @ 108.