How to Know Your Catalytic Converter Has Failed

Updated April 17, 2017

The catalytic converter is an integral part of the automobile exhaust system: it removes toxic emissions from engine exhaust. There are many indicators of potential catalytic converter failure. A vehicle displaying symptoms of failure should be taken to a professional mechanic for an emissions test to determine if the catalytic converter has failed, and for repair of any engine problems that may have contributed to the failure.

The number one symptom of a failed catalytic converter is a sulphurous or "rotten-egg" smell coming from the exhaust. Of all the possible signs of catalytic converter failure, this is the most obvious. However, it is also the last symptom to appear, as it is the result of damage to the converter caused by an improper fuel mixture.

Assess vehicle performance. A lack of power when accelerating or climbing a hill,and /or a pronounced drop in fuel efficiency are both indicators of possible catalytic converter failure. Other indications of catalytic converter failure are instruments showing the engine is running hot and a rattling noise emanating from the vehicle undercarriage when driving.

Use the car jack and jack stands to raise the car to a level that you can access and inspect the catalytic converter. The converter is part of the exhaust system and looks somewhat like the muffler, but it is closer to the engine.

Inspect the exterior of the converter for visible damage. Discolouration and/or warping of the converter housing are indicators of thermal damage to the converter substrate caused by overheating. Thermal failure is usually the result of a too rich fuel mixture entering the converter. This mixture then burns in the converter itself and melts the substrate, causing clogging and accompanying power issues. Thermal shock occurs when the hot converter comes into direct contact with ice or cold water, which can cause the interior substrate to crack, resulting in converter clogging, heat build-up and loss of power.

Check the exterior of the converter for other non-heat-related physical damage caused by hitting road hazards. If there are obvious large dents or dings in the converter housing, it is likely the substrate inside is cracked or broken. This damage will result in exhaust system back-pressure and reduced vehicle performance.

Determine if there is damage to the interior substrate by gently tapping it with a rubber mallet. If the substrate is damaged, there will be a rattling noise from within the converter. Be very careful not to tap too hard to avoid damaging the substrate yourself.

If the visual inspection indicates a problem with the catalytic converter, take the vehicle to a professional mechanic. If the failure has been caused by outside forces (thermal shock, road hazards) the mechanic can simply replace the converter. But if the issue is caused by overheating, it is essential the underlying engine problem (fouled spark-plugs, bad exhaust valves, bad oxygen sensor, etc.) be corrected to prevent the catalytic converter failure from recurring.


Make certain to use jack stands to hold the vehicle up. Do not get under a car which is supported by only a jack. Be careful when touching the catalytic converter. It can be extremely hot.

Things You'll Need

  • Car jack
  • Jack stands
  • Flashlight
  • Rubber mallet
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About the Author

Doug Donald has been writing online since 2004, covering business, relationships, health/exercise and food. His work has also been featured in the "Grosse Pointe News," magazines and corporate newsletters. Donald holds a Bachelor of Science in business and economics, as well as a Master of Business Administration from Central Michigan University.