Sounds like maybe (and this is mostly just me guessing wildly) your coolant
temp sensor is either unplugged, loosely plugged, or defective, and that's
throwing off the ecu enough to skew the mixture and thus the catalytic
converter's having a hard time doing its thing... or maybe due to erroneous
temp readings, the ecu just thinks the converter isn't up to snuff.

The more qualified guys I'm sure will throw in their 2c worth.