OWNER WAS DRIVING THE AUDI HOME WHEN HE SMELLED SOMETHING BURNING. HE PULLED OVER AND OBSERVED A FIRE IN THE ENGINE COMPARTMENT. FIRE BURNED WITH GREATEST INTENSITY IN LOWER LEFT REAR OF ENGINE COMPARTMENT, FROM LOCATION OF LEFT SIDE CATALYTIC CONVERTER. SPARK PLUGS REMOVED FROM ENGINE SHOWED OVERFUELING PROBLEM IN #6 AND #8 CYLINDERS ON LEFT BANK, CAUSING CATALYTIC CONVERTER TO OVERHEAT AND START THE FIRE. *NM