How Do You Clear Out a Clogged Catalytic Converter?
A clogged catalytic converter has to be cleared in order for your car to perform correctly. This is no laughing matter, as catalysts can run up to 1,500 F to burn away harmful emissions. If they become fouled they still run hot, but it's your car that may be burning down. To clear a clogged cat, drive up and down a highway at speed for a distance, with several high speed stops and starts to blow out any debris. If it remains clogged, try a very high-speed run, slow a bit and then go back up to high speed. If this all fails, you will have to replace the catalyst.
For more info, see How to Repair a Catalytic Converter at Home