Symptoms of Catalytic Converter Failure

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Most people have heard of a catalytic converter, but it seems that only auto mechanics know what they do or how to tell when your converter is going bad. Because the catalytic converter is tied to many of the systems in your car, understanding the symptoms of failure can help you save money on converter repairs.


When a standard combustion engine burns gasoline, it emits several fumes that can potentially be harmful if breathed regularly. A catalytic converter is put in the exhaust line to help create several chemical reactions that lower the toxicity of the emissions that are given off by the engine as it burns gasoline. The converter helps to spur the oxidation of hydrocarbon and carbon monoxide from the more pure forms found in direct exhaust, and it also helps to break down toxic oxides in nitrogen back to basic nitrogen and carbon dioxide. The resulting fumes are more transparent and less harmful than the direct engine exhaust, but they are still dangerous to breathe.


The failure of a catalytic converter typically happens for one of four reasons.

The first is overheating. This results when fuel gets into the converter, usually from misfiring spark plugs, and burns out the mechanisms inside. Catalytic converters are not designed to handle the heat of burning fuel; when fuel starts to burn in the converter, it can cause a great deal of damage to the converter and the rest of the exhaust system. One of the noticeable symptoms of overheating in the converter is a significant loss of power from the engine.

Another converter failure is a contamination of the chemical mechanisms. In some cases, motor oil or antifreeze can find their way into a catalytic converter and prevent the converter from being able to perform its chemical reactions. A common symptom of this kind of failure is a raised level of emissions that could reach the point of being illegal.

A hot catalytic converter needs to cool down before it can be exposed to a cooling agent such as cold rain or snow. If a hot converter is hit by snow, ice or cold rain, this could crack the shell of the converter and begin to cause a rattling sound.

The final type of failure is simply running over a rock and having it impact the converter, or any other type of physical damage that could occur during the course of driving. Depending on where the converter is damaged, one of the symptoms of physical damage could be raised emissions or a drop in engine performance.


There are many signs that the catalytic converter may be in need of service, but two of the more common signs are engine performance and an odour associated with a failing converter.

A converter that is damaged can cause the engine to not function properly. For example, the engine struggles when going up inclines, it hesitates when you try to accelerate or you experience a noticeable drop in fuel economy. If a tank of gas is not taking you as far as it used to, then it may be time to have the catalytic converter looked at.

An imbalance in the air-to-fuel ratio can cause excessive amounts of sulphur to be burnt during the converter's operation. This can result in a distinctive "rotten egg" smell that is often associated with burning sulphur. Do not ignore that burning smell if you experience it while driving your vehicle--it could be your catalytic converter in need of repair.


A catalytic converter that shows any symptoms of failure will need to be replaced. In most cases, a catalytic converter cannot be repaired because it is an enclosed system that cannot be opened to be repaired, much like a battery cannot be opened and repaired because the battery is a closed system using chemical reactions to operate. Replacing the converter should not be the end of the repair job. To make sure the new converter is not damaged, you should find the cause of the defect and repair it. It could be a faulty computerised sensor causing the problem, a damaged cylinder in the motor or bad wiring to the spark plugs. If the source of the problem is not solved, then the new converter will meet the same fate as the old one.


Most catalytic converters installed on new vehicles are designed to last well beyond the 100,000-mile replacement threshold of most new car warranties. If you find that your catalytic converter is damaged before 100,000 miles, it never hurts to check and see if you qualify for a free replacement converter from the car manufacturer.