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Understanding Fire Hazards with Catalyst-Equipped Cars

U.S. EPA publication on the history and precautions associated with automobile catalyst systems.

U.S. EPA Publication "Review of Catalyst Overheating Issue" March, 1983


Many 1975 and later model year automobiles are equipped with catalytic converters which serve as a primary means for reducing the emissions of air pollutants from these cars. The U.S. Environmental Protection Agency (EPA) has received reports concerning over-temperature problems or fire hazards from catalytic converters. This fact sheet has been prepared to respond efficiently to such inquiries.

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How Catalytic Converters Get Hot

Catalysts reduce emissions by accelerating the combustion of pollutants leaving the engine. In doing this job, they get hot. The outside metal temperatures of some types of converters may approach 800 to 1000 F under conditions of extremely high engine loading. However, measurements by the United States Forest Service have shown surface temperatures equally as high in the exhaust systems of pre-1975 cars at extreme engine load conditions. Therefore, with this discovery, catalytic converter surface temperatures do not represent a new type of problem for automobile manufacturers and users as long as the engine is running properly.

However, if there is a partial ignition system failure, such as one or more misfiring spark plug or defective ignition wires, the temperatures of the catalytic converter surfaces and the exhaust system downstream from the converter may reach 1200 to 1400 F. This is because of the abnormal amount of unburned fuel delivered by the nonfiring cylinders. Further, once hot, the converter will take longer to cool off than other parts of the exhaust system because of its greater mass. This points out the need for careful attention to vehicle maintenance and alertness by vehicle owners to any signs of abnormal engine operation.

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Precautions To Take

The EPA regulations require that any emission control system used by vehicle manufacturers "shall not in its operation, function, or malfunction result in unsafe conditions endangering the motor vehicle, its occupants, or persons or property in close proximity to the vehicle."

The vehicle manufacturers are aware of the need to provide protection from possible hazards or discomfort associated with high catalyst temperatures for both the vehicle occupants and vehicle components. In addition, protection is also necessary to avert possible fire hazards associated with driving vehicles through tall grass or other vegetation. The exact means taken by the different manufacturers to provide high temperature protection vary, and include such approaches as insulating the entire catalytic reactor so that the outside surfaces are not hotter than mufflers, installing protective metal shields between the converter shell and vegetation, and using thicker carpeting materials inside the car to protect the occupants from experiencing high floorboard temperatures. In addition, some cars have temperature-sensing devices to deactivate the catalytic reactor or alert the driver to abnormally high temperatures, which might be caused by misfiring spark plugs, etc.

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Fires May Still Happen

The EPA has received reports of vehicle and vegetation fires in which catalysts were involved from both vehicle owners and from the National Highway Traffic Safety Administration (NHTSA), an organization which has been monitoring the frequency of such incidents with individual manufacturers. In some cases, it appears that combustible undercoating material had been applied to the catalyst and other exhaust system hardware. In most cases, vehicles were also reported to have been running badly with evidence of nonfiring spark plugs or other ignition system defects. If an abnormal amount of unburned fuel is fed to any catalyst, which occurs when the engine is misfiring in one or more cylinders, the catalyst will attempt to "do its job" by burning this fuel instead of simply expelling it out the exhaust pipe as the case with older cars. When this happens, the surface temperature of the catalyst container and the exhaust pipe can become abnormally hot, possibly leading to charring or burning of undercoating inadvertently sprayed on the catalyst or exhaust system, charring of floor mats in the car, or ignition of dry vegetation if the vehicle is operated off-road. Vehicle service manuals caution against applying undercoating on the catalyst of exhaust systems.

It should be noted that vegetation fires caused by hot automobile exhaust systems occurred before the advent of catalyst-equipped cars, and will occur in the future. The Forest Service has periodically conducted tests of cars for fire hazards since 1967 because of the long-standing concern by that agency over vehicle-induced fires in national forest recreational areas.

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What Can Be Done in the Future

The EPA and NHTSA have been monitoring closely the frequency and type of such incidents. The NHTSA, on the basis of a review completed in December 1976, concluded that "the rate and nature of catalytic converter incidents do not present an unreasonable risk of health or injury to the public." The EPA will continue to require manufacturers to design their vehicles so that when properly operated and maintained they will pose no hazard to either life or property.

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What You Can Do

If you keep your car properly maintained as recommended in your owners manual, you should normally have no problems. If you notice the engine running rough, you may have a misfiring spark plug. Be sure to have that checked promptly, not only to avoid catalyst overheating but also to restore good performance and save fuel.

Never park a catalyst-equipped car, or any car, on a pile of dry leaves or other dry vegetation. Normal caution in how you use your car is all that is needed to avoid catalyst fires.

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