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Clogged Catalytic Converter Symptoms

Clogged Catalytic Converter

Catalytic converters are found on exhaust systems and depending upon the size of the vehicle, there may be multiple. They take harmful gases that are a result of the combustion process and turns them into gases that are less damaging to the environment.

If you have ever looked inside of a catalytic converter, you will notice a distinctive honeycomb mesh material. Also inside are other precious metals, which all work together to perform the chemical reaction.

Although many manufacturers claim their catalytic converters will last the life of the vehicle, this is not always the case. You can attempt to prolong the lifespan by using a catalytic converter cleaner but sometimes they can be clogged up beyond repair.

Below is a list of common symptoms of a bad or clogged catalytic converter.

Table of Contents

1. Check Engine Light

If the vehicles check engine light is on, this could mean anything is wrong with the engine. However, a bad catalytic convert will trigger the light but it’s best to connect a car scanner to the OBD2 port of the car to confirm the issue.

2. Loss of Power

By far one of the most common symptoms of a clogged catalytic converter is a lack of performance. If you attempt to accelerate aggressively or uphill and notice a lack of power, the catalytic converter may be the issue.

Clogged catalytic converters can be diagnosed by analyzing the exhaust whilst under load. Whilst parked, have someone stand behind the car and put their hands near the exhaust. Once setup, raise the car’s RPM to around 2,000 and ask the other person if there is a hot flow of air coming from the exhaust. This is a clear sign of a clogged catalytic convert.

3. Odd or Rotten Egg Smells

Due to the fact that fuel contains sulfate, as a result, if it’s clogged within the catalytic converter, it can produce odd smells. The most distinctive foul smell is rotten eggs, which clearly indicates that the catalytic converter is clogged.

4. Dark Exhaust Smoke

You can tell a lot about a car by examining the exhaust smoke coming from your car. When the exhaust outputs black smoke, this is a red flag that can be a result of a bad catalytic converter. Due to the fact the gases can’t escape, it builds up and becomes thicker, which when the gases escape, it produces black smoke.

5. Reduced Fuel Economy

As a result of reduced airflow through a clogged catalytic converter, the gas mileage will drop quite significantly. It causes your engine to burn extra fuel and waste additional gas, which can even make the converter clog even more.

6. Excessive Misfires

A misfiring engine is a clear sign that there is an issue and it should be addressed sooner rather than later. The clogged catalytic converter cause gas within the engine to heat up more than it should, which causes excessive misfires to occur. Failing to solve the misfiring can result in other components becoming damaged.

7. Rattling Sound Underneath Car

Once you begin to hear rattling underneath the car and coming from the catalytic converter, you should expect the worst. This usually signifies that parts of the honeycomb mesh or other internal component have broken down. These tiny bits of debris will vibrate and rattle inside of the converter and will need to be changed.


Catalytic converters are an expensive component to change once they go wrong. Once they have failed, there is virtually no way of repairing them and they must be replaced in order to return optimum performance to your vehicle. Maintaining your car appropriately and monitoring the emissions output is the best way to prolong its life.

procarreviews author

Written By Dan

About the Author

Dan is an automotive journalist and owner of over 10 cars ranging from supercars, tuned cars, classics and your good old beater cars. He always likes to get his hands dirty with nut and bolt restorations or detailing sessions using the best products on the market.

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