To understand why a catalytic converter fails, you need to know how it works. The catalytic converter is part of the automobile exhaust system. It converts harmful compounds in exhaust into harmless compounds. In a typical passenger car, the catalytic converter, which resembles a muffler in shape, is between the engine and the muffler. It's on the underside of the car, usually underneath the passenger seat. Maybe you have felt its warmth through the floor on a long trip.

Catalytic converters have been standard on U.S. automobiles since the mid-1970s. The catalytic converter helped drive the push toward unleaded gasoline as well. Leaded gasoline contaminates the catalyst used inside a catalytic converter, destroying its usefulness and leading to a clogged converter.

After the engine exhaust gases pass through the catalytic converter, the gases go through the muffler or mufflers, depending on the make of the automobile. Some vehicles use a pre-converter as well, to perform a similar function. The catalytic converter generally lasts the life of the automobile and rarely has a problem with being clogged or plugged during its lifetime.

The inside of the catalytic converter is a honeycomb set of passageways or small ceramic beads coated with catalysts. A chemical reaction takes place to make the pollutants less harmful. There are many passages for the exhaust gases to flow, to allow for the maximum amount of surface area for the hot gases to pass.

The catalysts include:

  • Oxidation catalysts: Palladium (Pd) and platinum (Pt) metals in very small amounts (to keep the catalytic converter price down) convert the hydrocarbons of unburned gasoline and carbon monoxide to carbon dioxide and water.
  • Reduction catalysts: Palladium and rhodium (Rh) metals also in very small amounts convert the nitrogen oxide to nitrogen and oxygen. Nitrogen oxide is a big contributor to smog.
Many states and localities have legislated annual automobile emissions testing that checks the actual emissions content. The exhaust emissions test checks for the absence of a converter or a malfunctioning one during an inspection. It's illegal in some states and localities to remove a factory-installed catalytic converter. A mechanic can sometimes temporarily remove it and replace the catalytic converter with a test pipe, but the rules on this can vary from place to place.

There are two ways a converter can fail:

  • It can become clogged.
  • It can become poisoned.
There really is no "inspection port" for the consumer or mechanic to see an actual clog in a converter. Often, the only way to tell if a catalytic converter is malfunctioning (plugged) is to remove it and check the change in engine performance. When a clogged converter is suspected, some mechanics temporarily remove the O2 sensor from the exhaust pipe ahead of the catalytic converter and look for a change in performance.

A catalytic converter relies on receiving the proper mix of exhaust gases at the proper temperature. Any additives or malfunctions that cause the mixture or the temperature of the exhaust gases to change reduce the effectiveness and life of the catalytic converter. Leaded gasoline and the over-use of certain fuel additives can shorten the life of a catalytic converter.

A catalytic converter can also fail because of:

  • Bad exhaust valves on the engine
  • Fouled plugs causing unburned fuel to overheat the converter
Sometimes you can tell that a converter is clogged because you don't go any faster when you push the gas pedal. Also, there usually is a noticeable drop in gas mileage associated with a clogged catalytic converter. A partially clogged converter often acts like an engine governor, limiting the actual RPMs to a fast idle. A totally clogged converter causes the engine to quit after a few minutes because of all the increased exhaust back pressure.

The catalytic converter, like the rest of the emissions system, typically has a warranty length that exceeds the term of the warranty for the rest of a typical U.S. automobile.

Here is a safety reminder: Do not park your car over tall grass or piles of dry leaves. Your car's perfectly running catalytic converter gets very hot…enough to start fires! You can keep it running well by keeping the ignition system in top shape, to prevent any unburnt fuel from entering the catalytic converter.