Catalytic converters are used in exhaust systems to reduce toxic by-products of fuel into less hazardous substances. These less hazardous substances include carbon dioxide, water vapor, and nitrogen gas. Toxic by-products are chemicals such as nitrogen oxides, carbon monoxide, and hydrocarbons (in the form of unburned gasoline). The catalytic converter contains a honeycomb structure that is coated with a catalyst, often platinum or palladium. When the catalyst becomes hot, the exhaust gases passing over it will change so that gases that cause smog are removed. Excessive unburned fuel in the exhaust can cause the catalyst to reach extremely high temperatures, which can damage the catalyst materials. When these materials are damaged, the reduction of the hazardous substances decrease which in turn will increase the harmful gases exiting through the exhaust.
A defect as simple as a thermostat stuck open or a bad spark plug can cause the failure of a catalytic converter. Rapid temperature changes can also cause damage. The cat normally operates at temperatures above 400 degrees Fahrenheit. Rapid cooling can cause the internal ceramic structure to break apart and obstruct the exit into the exhaust pipe. Engine management defects that allow too much fuel into the cat can also cause the honeycomb structure to break apart. When the cat becomes plugged there is a noticeable drop in engine power. In addition, the check engine light may turn on.
A customer brought in their 2008 Toyota Sienna complaining of a rattling sound coming from the vehicle while idling. When the vehicle was at higher speeds, the noise would diminish but would become louder when slowing down. One of our technicians inspected the vehicle and discovered that the insides of the catalytic converter were broken and bouncing around inside causing the rattling sound. We recommended changing the catalytic converter on the Sienna. The customer agreed and in turn, the concern went away!