Most motorists view catalytic converters as a mixed blessing. They improve air quality by reducing tailpipe emissions, but they also add back-pressure that handicaps performance. This was the case with early pellet-based catalytic converters, but according to tests in "Import Tuner" magazine's October 2006 issue, the monolith "brick" ones we use today have a minimal effect on the engine. Since replacing a bad catalytic converter can cost thousands, here are some ways to buy time.
Clean it
If your converter has a carbon buildup or mild oil fouling, try cleaning it before replacing it. Catalytic converter cleaner/deodorizers are available for $13 to $25, and you can add them to your gas. As you drive, the cleaner removes deposits from the converter and the rest of the engine. If this doesn't solve the problem, your converter might be clogged with sulfur from your fuel and phosphorous from your oil. Scientists at Madrid's Institute of Catalysis and Petrochemistry have discovered that diluted citric acid can remove these deposits without affecting your converter's platinum. Just soak the catalytic converter in a citric acid solution for six hours; and if it doesn't have other problems, it'll be as good as new.
Stop the Rattles
When a catalytic converter begins to fail, its monolith substrate filling cracks, and the pieces rattle. Since the substrate absorbs the toxic exhaust gases, the longer you keep it from coming apart the better. The best way to do this is by using a mechanic's stethoscope to find out where the loose pieces are. Drill a small hole at each location and put a large drywall screw in it. The screws will hold the loose pieces in place and delay the inevitable replacement of your converter. It's also possible for your catalytic converter's heat shield to start rattling. You can fix the problem by screwing hose clamps together to make a giant hose clamp that fits around the heat shield. Just crank it down until it's tight enough to keep the shield from moving.
Gut it
When all else fails, you have one more option before replacing the catalytic converter. You should only choose this option when the monolith substrate filling of your converter is too cracked to repair or too fouled to clean out. At this stage you can only unclog the converter by removing its substrate filling. Gutting the converter's contents will relieve the back-pressure in the exhaust and make your car drivable until you get a new converter. Start by unbolting the intermediate pipe from the back of the converter to expose its interior. Insert a big crowbar into the converter and start banging it around. Hit the crowbar with a hammer from time to time to speed up the process. You can check your progress with a flashlight and use a shop vac to suck out substrate pieces. You can also idle the car a few times to blow out stubborn chunks. After a final pass with the shop vac, you can reconnect the intermediate pipe and get back on the road. Always use a mask and creeper for this job to avoid contact with the substrate.