citric acid bath if severe clogging - to the point of engine balking out. If the engine can start and rev up to atleast 50% engine rpm, then take the car out for a proper flogging - get the converters extremely hot - they will self clean. I helped a guy in houston with a mercedes W124 E420, he was not able to pass emmissions.
We both took the car for a good flogging, ran the engine at 4500 rpm for about 10 miles - when we arrived at the testing station, they the machine thought the car was switched off - Those cat converters were lava hot - they had started to glow a faint orange.