Since the 1970s, catalytic converters have been standard on all American-made vehicles in order for car companies to meet emissions standards. The converter breaks up the harmful pollutants that are generated by the combustion process before they leave the exhaust pipe.
If a catalytic converter is in disrepair, a car's emissions will be higher, which will not only be harmful for the environment but will also make it difficult to pass an inspection. There are a few ways that catalytic converters can fail - they can become clogged or poisoned due to the use of any fuel additives or leaded gasoline. When a converter is clogged, you may notice a significant decline in gas mileage. When you push the gas pedal, a clogged converter may also limit the car's ability to accelerate.
The most obvious sign of a failing catalytic converter is a foul smell similar to rotten eggs, but it is also the last sign of improper fuel mixtures which can damage the engine. Other symptoms may include the internal engine temperature reading very high on the dash and a rattling sound coming from underneath the car when it is running.
If a problem is caught early enough, there is a chance that the converter can be repaired. However, if you notice the bad-egg smell, this is usually the point of no return and the converter will need to be replaced.