The catalytic converter is a part of the car's exhaust system usually located underneath the passenger seat, near the muffler. In terms of air quality, it's essential: Before noxious car fumes exit the car's exhaust, they pass through the catalytic converter, which chemically transforms some of the polluting gases into cleaner compounds. Catalytic converters can fail by becoming clogged or contaminated. There is no way to actually see a clog; the only indication of a malfunctioning converter is poor engine performance. A totally blocked converter will cause the engine to shut down from back pressure in the exhaust.
Diagnosing a failed converter may begin with the device, but car owners should have an open mind or seek an experienced mechanic. The failure may have little to do with the converter, but instead be the result of something else going on with the engine that's causing the converter's failure. If you assume it's the converter and replace it, but don't correct the underlying problem, you'll just go through your new converter in record time and be back at square one. For example, it may be rare for a converter to get clogged these days because of the widespread use of cleaner unleaded gasoline, but it can become contaminated. This happens when unsuitable materials enter the converter via fuel additives, causing changes in its temperature and shortening its lifespan. If you keep using the wrong additives, it could eventually happen again with the replacement converter. Sometimes even poorly tuned engines can cause damage to converters.
Naturally, a converter can be damaged by cracks and punctures, which could cause it to fail. Other physical damage can come from mounts, bolts or poor manufacturing and materials. Mechanical and electrical failures in the car, such as piston rings or the injection tube, can cause the catalytic converter to work poorly.
Another way your catalytic converter might "fail" is to disappear. It's not likely to fall off of your car, but thieves with saws can cut one from your car in a matter of seconds. Theft of the devices has risen all over the country because of increasing prices for scrap metal; converters usually are made from platinum, rhodium and palladium [source: McCarty].
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