What is the most important emissions control device on a vehicle today? The catalytic converter, because it cleans up any exhaust pollutants that exit the engine. It operates at temperatures of 600 to 1000 degrees F.

BTW: Parking over a pile of dry leaves or the like can start a fire..

Providing the converter does its job efficiently, the vehicle will meet emissions and pass both a tailpipe emissions check and/or an OBD II plug-in emissions test. OEM converters are engineered to last well over 150,000 miles, but a number of things can interfere with their ability to clean up the exhaust — and some may eventually cause the converter to prematuely fail.

The causes of failure that can actually damage an engine to the point of engine replacement

Ignition misfire (fouled spark plug and/or shorted plug wire)
Compression misfire (leaky valves or head gasket)
Internal coolant leaks (cracks in head or leaky head gasket)
Oil burning (worn valve guides, seals, rings, cylinders)
Fuel contamination (lead)
Rust or physical damage

TYPES OF CATS

Before we go any further, we need to take a quick look inside the converter to understand how it operates. Inside the stainless steel outer shell is a ceramic or metallic honeycomb coated with a very thin layer of precious metals. These include platinum, palladium and rhodium. These metals all have the unique ability to set off chemical reactions. They are not consumed or used up over time, but only serve to ignite reactions between the pollutants in the exhaust and oxygen.

The earliest converters dating back to 1975 were “two-way” or “oxidizing” converters because the catalyst only reacted with hydrocarbons (HC) and carbon monoxide (CO) in the exhaust. These older converters did nothing to reduce oxides of nitrogen (NOX) in the exhaust.

In the 1980s, “three-way” (TWC) converters made their appearance. These have two catalysts inside, one to oxidize HC and CO, and a second to reduce NOX.

The powertrain control module (PCM) flip-flops the air/fuel mixture when the engine is warm by monitoring the rich/lean signal from the oxygen sensor in the exhaust. When the O2 sensor reads lean, the PCM makes the fuel mixture go rich. When the O2 sensor sends back a rich signal, the PCM shortens the on-time of the fuel injectors and leans the fuel mixture. The O2 sensor then send back a lean signal, and the PCM increases the on-time of the injectors to make the fuel mixture rich again. By rapidly changing the air/fuel mixture back and forth, the overall mixture averages out and keeps emissions at a minimum.

On some newer vehicles, a new type of “wideband” oxygen sensor (also called an “air/fuel ratio” sensor) is used. Instead of producing a high or low voltage signal, the signal changes in direct proportion to the amount of oxygen in the exhaust. This provides a more precise measurement for better fuel control and tells the PCM the exact air/fuel ratio. On most applications, you can also read the air/fuel ratio or lambda value on your scan tool.

I realize this is a complicated article to understand. The most important thing to know is that a clogged cat on any car or truck will inhibit the power and performance dramatically. Possibly cause an engine failure due to heat build up in the engine. I have seen several vehicles, such as a Ford engine needing the be replaced with a used engine due to forcing the vehicle to go when it clearly had a problem and lacked power. 

I have also installed a used Toyota engine in Tundra pickup that had severe cat problems. Read our next post on GotEngines.com Blog, completeing your catalytic converter education.