The catalytic converter, standard equipment in all newer vehicles, has the job of altering pollutants in the car's exhaust gas. The catalytic converters do this by chemically altering the harmful chemicals present in exhaust gases, such as oxides of nitrogen (NO x ), unburned hydrocarbons (HC), carbon monoxide (CO) and oxides of sulfur (SO x ) into harmless variants of these chemicals, such as carbon dioxide (CO 2 ) and water (H 2 O). For a more in-depth description of the chemical reactions in exhaust gases, refer to the beginning of this section.
Two things can to destroy the catalyst: the use of leaded gas and excessive heat. The use of leaded fuel was the most common cause of catalyst destruction. The lead coats the thin layer of platinum and palladium (that actually promotes final combustion of the almost completely burned hydrocarbons that enter the catalytic converter). The coating keeps the mixture from actually contacting the noble metals. The lead may also cause the passages of the "substrate''-the material that carries the noble metal coating-to clog. This would cause the car to run at reduced power or even stop altogether.
Excessive heat results in the converter when raw fuel and air with a high oxygen content enter the catalytic converter, which greatly accelerates the combustion process. This most often occurs when a spark plug wire is disconnected. Excessive heat during misfiring due to poor vehicle maintenance and prolonged testing with the ignition system wires disconnected are two common ways a catalyst may be rendered ineffective. Test procedures should be accomplished as-quickly-as possible. The car should be shut off whenever misfiring is noted. Misfiring due to extremely lean or extremely rich mixtures will also damage the catalyst.
While the catalyst itself is a maintenance-free item, it should be understood that long life depends completely on proper fueling and good maintenance. The car should be tuned as required, and the fuel and air filters, as well as the oxygen sensor, should be changed as specified in the maintenance intervals chart in Routine Maintenance . Ignition wires and the distributor cap and rotor should be inspected/tested and replaced if necessary to prevent misfire.
Two catalysts are used on each car on model years through 1985. On dual catalyst systems, a small one located just after the exhaust manifold ignites early in the engine warm-up cycle, and a larger one located under the car body completes the clean-up process during warmed-up operation of the car. If the unit should be damaged, it must be replaced-no service is possible.
1986 and later models employ a 3-way catalyst. Two converters, working under different fuel/air mixture conditions are used. The first catalyst is fed exhaust from the engine that is at the chemically correct mixture ratio of 14.7:1. At this point, the exhaust gases contain oxygen that has combined with the nitrogen in the air to form nitrogen oxides (a pollutant); and unburned (or oxygen-short) hydrocarbons and carbon monoxide (another pollutant). This converter creates conditions which cause the oxygen in the nitrogen oxides to combine with the unburned material. This is called "reduction'' of the nitrogen oxides. Once the nitrogen oxides have been reduced, the air pump adds extra air (and oxygen). The second converter uses this extra oxygen to complete the job of oxidizing the unburned material.
REMOVAL & INSTALLATION
For the removal and installation of the catalytic converter, refer to the end of Engine & Engine Overhaul .