How do I know if my catalytic converter has failed?

by | Apr 8, 2010 08:00 PM ET


Welcome to Brain Stuff from, where smart happens.

Marshall Brain

Hi, I'm Marshall Brain with today's question, how do I know if my catalytic converter has failed. To understand why a catalytic converter fails, you need to know how it works. The catalytic converter is the part of the automobile exhaust system that converts harmful compounds in the exhaust into harmless compounds. It does this by using a catalyst, which speeds up chemical reactions. The catalyst can take unburned hydrocarbons in the exhaust stream and burn them. It can turn carbon monoxide into carbon dioxide and so on.

The thing that does this is a honeycomb piece of ceramic with the catalyst embedded in it. And if you were to ever look at this, the holes going through that catalyst are very small. So one of the easiest ways for a catalytic converter to fail is for it to become clogged. Often, the only way to tell if your catalytic converter is plugged up is to remove it and check for the change in engine performance. Some mechanics temporarily remove the oxygen sensor from the exhaust pipe ahead of the catalytic converter and see if that helps. It opens up a big enough hole to let exhaust through if the converter is plugged up.

Sometimes you can tell that a converter is clogged because you don't go any faster when you push on the gas pedal at a certain point. Also, there's usually a noticeable drop in gas mileage associated with a clogged converter. A partially clogged converter often acts like an engine governor. A totally clogged converter causes the engine to quit after a few minutes because of all the increased exhaust backpressure.

A catalytic converter can also fail because of bad exhaust valves on the engine or fouled plugs, which cause unburned fuel to overheat the converter. A catalytic converter relies on receiving the proper mix of exhaust gases at the proper temperature. Any additives or malfunctions that caused the mixture or the temperature of the exhaust gases to change reduced the effectiveness and the life of a catalytic converter. Leaded gasoline and overuse of certain fuel additives can also shorten the life of a catalytic converter by poisoning the catalyst.

Here's a safety reminder. Do not park your car over tall grass or piles of dry leaves. Your car's perfectly running catalytic converter gets incredibly hot, hot enough to start fires.

Do you have any ideas or suggestions for this podcast? If so, please send me an email at


For more on this and thousands of other topics, go to And be sure to check out the Brain Stuff blog on the How Stuff Works homepage.

More To Explore