From the Magnaflow/Car-Sound website:
"If the ceramic inside your converter becomes clogged or coated with carbon, lead or oil, then the converter’s efficiency is greatly reduced"
From Random Technology:
Direct Fit Catalytic converters - Note that it is illegal to remove a properly functioning catalytic converter and that aftermarket catalytic converters are not legal for use on OBD2 vehicles in California (some 1994 and 1995 and all 1996 and later vehicles).
"Under Federal EPA regulation, replacement of original catalytic converters is allowed only if the original converter(s) is missing, or the vehicle has more than 50,000 miles or is at least five years old and the need for a replacement has been established and documented, or a local inspection program has determined the existing converter is in need of replacement."