When catalytic converters fail they normally clog up with debris and block the flow of exhaust gas from getting out of the system. this causes extreme performance problems. In the extreme cases it will even prevent the car from starting at all. Most of the time it acts like a governor and limits the engine RPM to a fast idle. So how do you check a catalytic converter without removing it from the car??

You can do a vacuum check at idle and then at 1500 RPM. If the vacuum is say 21 inches at idle and like 15 inches at 1500 then the cat is likely clogged. In a carbureted car you can remove the air cleaner and have someone hold a strong light near the intake of the carb. Then look across the top of the carb into the light and rev it up. If you see a cloud of fuel vapor pumping up and down sort of hovering over the carb then the cat is likely plugged.

Next, remove the oxygen sensor and start it up. The hole where the oxygen sensor was will relieve the back pressure and it should run a tad better. If so, then disconnect the exhaust pipe at the exhaust manifold and try it again. If it is a clogged cat it will scream like a banchee and rev up to its normal 4000 rpm.

For a good description of a catalytic converter go to this site.

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