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Terms of Use wikiHow is where trusted research and expert knowledge come together. Learn why people trust wikiHow
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Cars & Other Vehicles
Cars
Car Maintenance and Repair
Exhaust and Fuel Parts
9 Signs That Your Catalytic Converter is Going Bad Explore this Article IN THIS ARTICLE 1 Lit check engine light 2 Rotten egg smell 3 Rattling noises 4 Reduced fuel efficiency 5 Reduced acceleration 6 Stalling or not starting 7 Blue exhaust smoke 8 Glowing converter after driving 9 Failed emissions test + Show 6 more... - Show less... Other Sections Questions & Answers Tips and Warnings Related Articles References Co-authored by Jason Shackelford and Hunter Rising
Last Updated: January 27, 2026 Fact Checked
X
This article was co-authored by Jason Shackelford and by wikiHow staff writer, Hunter Rising. Jason Shackelford is the Owner of Stingray Auto Repair, a family owned and operated auto repair shop with locations in Seattle and Redmond, Washington. He has over 24 years of experience in auto repair and services, and every single technician on Jason’s team has more than 10 years of experience. There are 9 references cited in this article, which can be found at the bottom of the page. This article has been fact-checked, ensuring the accuracy of any cited facts and confirming the authority of its sources. This article has been viewed 68,807 times.
Are you noticing problems with your vehicle's performance or exhaust system? Catalytic converters filter the exhaust fumes from your engine and make them harmless, but they can cause a few issues when they start to break down. The catalytic converter is an important part of your vehicle, but there are easy signs to tell if it's the part giving you trouble. Keep reading so you can recognize all of the tell-tale symptoms that it's time to check or replace your catalytic converter.
Steps
1
Lit check engine light
A bad converter triggers sensors that will turn the engine light on. Your vehicle has oxygen sensors that connect to the catalytic converter and monitor the emissions your vehicle produces. If the converter doesn’t filter enough of the emissions, it alerts the sensor and turns on the light so you know something is wrong.[1]XResearch source [2]XResearch source
Since the check engine light turns on for many different reasons, use an OBD code scanner to scan your vehicle for the problem. Catalytic converter issues usually have an error starting with P0420 or P0430.[3]XResearch source
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Rotten egg smell
A bad odor means sulfur is passing through the converter unfiltered. Exhaust passing through the honeycomb material inside the converter usually changes to an odorless gas called sulfur dioxide. When the converter is old, clogged, or going bad, then you’ll notice the strong sulfur odor any time you drive.[4]XResearch source [5]XResearch source
3
Rattling noises
The rattling could be broken material inside the converter. The honeycomb material in the filter starts to wear away as the filter gets older or damaged, so it could break apart inside. Listen for a rattling sound coming from underneath your vehicle while it’s running or when you rev the engine to determine if the converter is the issue.[6]XResearch source
Rattling could also be a sign that parts of your exhaust system are loose and need to be tightened.
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Reduced fuel efficiency
Your engine will overwork and use more fuel with a clogged converter. Broken material inside of a catalytic converter blocks the line to your engine, so fuel suddenly won’t burn as efficiently. Since your engine will try to keep up, it will work harder and eat up your fuel faster than it has in the past. You may notice that you’re filling up more frequently and or aren’t getting the mileage that you’re used to.[7]XResearch source
5
Reduced acceleration
A clogged converter can make your vehicle misfire and accelerate slower. When the material breaks up inside of the converter, it blocks the exhaust and creates backpressure on the engine, causing it to misfire. Since there’s more pressure on your engine, it will work less efficiently and won’t have as much power as usual when you try to speed up.[8]XResearch source
As your catalytic converter gets in worse shape, your engine will lose more and more power.\
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Stalling or not starting
Broken material in the converter clogs the exhaust and blocks the engine. If the honeycomb material breaks and causes a clog, the pressure builds up so much that your engine can’t get enough air to function. You may notice it right when you start your vehicle or while you’re driving. Even if stalling doesn’t happen every time you drive, it’s a good sign that you should get the converter checked out.[9]XResearch source
The broken material in your catalytic converter can cause a misfire when it blocks the exhaust, but it may fall out while driving. When it gets jostled back into your exhaust again, then it makes your engine stall.
7
Blue exhaust smoke
Darker exhaust fumes are a sign your catalytic converter overheats. Normal exhaust usually looks clear or like light smoke. Since the converter usually limits the emissions coming out of your vehicle, thicker or darker smoke alerts you immediately that there’s an issue.
Exhaust can also be a sign that there’s a problem with the engine heating up too much, which damages the catalytic converter over time. Changing your oil can help keep the engine running smoothly.
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Glowing converter after driving
A bad catalytic converter will overheat after the engine runs. Try running your vehicle for 20 minutes and checking underneath your vehicle. If you can get a view underneath your vehicle, a bad catalytic converter will glow red-hot if it’s overheating. It’s normally a sign that the exhaust is getting caught in the converter without running through it properly.[10]XResearch source
9
Failed emissions test
An emissions test lets you know if the converter is ineffective. An emissions test checks how many pollutants your vehicle releases in the air. If your catalytic converter isn’t functioning properly, then the exhaust will pass through and cause you to fail the test.[11]XResearch source
Take your vehicle to a mechanic so they can tell you what needs to be fixed so you can pass the emissions test.
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Question What is the lifespan of a catalytic converter? wikiHow Staff Editor Staff Answer This answer was written by one of our trained team of researchers who validated it for accuracy and comprehensiveness. wikiHow Staff Editor Staff Answer A catalytic converter usually lasts about 10 years or more, or for around 100,000 to 150,000 miles. However, this can vary depending on engine problems, contamination, or poor driving habits. Thanks! We're glad this was helpful.Thank you for your feedback. If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow Yes No Not Helpful 0 Helpful 0
Question How much does it cost to replace a catalytic converter? wikiHow Staff Editor Staff Answer This answer was written by one of our trained team of researchers who validated it for accuracy and comprehensiveness. wikiHow Staff Editor Staff Answer The cost of replacing a catalytic converter can vary widely depending on what kind of car you have, but in general, you can expect to spend anywhere from $500 to $4,000. Thanks! We're glad this was helpful.Thank you for your feedback. If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow Yes No Not Helpful 0 Helpful 0
Question Can you fix a catalytic converter without replacing it? wikiHow Staff Editor Staff Answer This answer was written by one of our trained team of researchers who validated it for accuracy and comprehensiveness. wikiHow Staff Editor Staff Answer You can try to fix a catalytic converter if it’s clogged by using a catalytic converter cleaner additive like Cataclean, or by removing the converter and physically cleaning it. However, these methods will likely only buy time, and you’ll still eventually need to replace the catalytic converter if there are major issues with it. Thanks! We're glad this was helpful.Thank you for your feedback. If wikiHow has helped you, please consider a small contribution to support us in helping more readers like you. We’re committed to providing the world with free how-to resources, and even $1 helps us in our mission. Support wikiHow Yes No Not Helpful 0 Helpful 0
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Tips
If your vehicle makes a loud roaring sound when you turn it on, it may mean that your catalytic converter is missing. Catalytic converters are made of precious metals, so theft is a possibility.[12]XResearch sourceThanks Helpful 0 Not Helpful 0
Catalytic converters usually last around 10–15 years. Thanks Helpful 0 Not Helpful 0
You can try cleaning your catalytic converter with a cleaner fluid. Thanks Helpful 0 Not Helpful 0
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Warnings
Replace or fix a catalytic converter as soon as you notice an issue. Driving with a bad catalytic converter reduces your vehicle's performance and may be illegal in your area since it’s releasing harmful pollutants into the air.[13]XResearch sourceThanks Helpful 0 Not Helpful 0
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Co-authored by: Jason Shackelford Auto Technician This article was co-authored by Jason Shackelford and by wikiHow staff writer, Hunter Rising. Jason Shackelford is the Owner of Stingray Auto Repair, a family owned and operated auto repair shop with locations in Seattle and Redmond, Washington. He has over 24 years of experience in auto repair and services, and every single technician on Jason’s team has more than 10 years of experience. This article has been viewed 68,807 times. 31 votes - 81% Co-authors: 5Updated: January 27, 2026 Views: 68,807 Categories: Exhaust and Fuel Parts In other languages Portuguese Japanese
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Yes No Advertisement If you buy through links on our site, we may earn a commission.Cookies make wikiHow better. By continuing to use our site, you agree to our cookie policy. Co-authored by: Jason Shackelford Auto Technician Co-authors: 5Updated: January 27, 2026 Views: 68,80781% of readers found this article helpful. 31 votes - 81% Click a star to add your vote
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Co-authored by: Jason Shackelford Auto Technician This article was co-authored by Jason Shackelford and by wikiHow staff writer, Hunter Rising. Jason Shackelford is the Owner of Stingray Auto Repair, a family owned and operated auto repair shop with locations in Seattle and Redmond, Washington. He has over 24 years of experience in auto repair and services, and every single technician on Jason’s team has more than 10 years of experience. This article has been viewed 68,807 times. 31 votes - 81% Co-authors: 5 Updated: January 27, 2026 Views: 68,807 Categories: Exhaust and Fuel Parts In other languages Portuguese Japanese 
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A bad converter triggers sensors that will turn the engine light on. Your vehicle has oxygen sensors that connect to the catalytic converter and monitor the emissions your vehicle produces. If the converter doesn’t filter enough of the emissions, it alerts the sensor and turns on the light so you know something is wrong.[1] [2] 
A bad odor means sulfur is passing through the converter unfiltered. Exhaust passing through the honeycomb material inside the converter usually changes to an odorless gas called sulfur dioxide. When the converter is old, clogged, or going bad, then you’ll notice the strong sulfur odor any time you drive.[4] [5] 
The rattling could be broken material inside the converter. The honeycomb material in the filter starts to wear away as the filter gets older or damaged, so it could break apart inside. Listen for a rattling sound coming from underneath your vehicle while it’s running or when you rev the engine to determine if the converter is the issue.[6] 
Your engine will overwork and use more fuel with a clogged converter. Broken material inside of a catalytic converter blocks the line to your engine, so fuel suddenly won’t burn as efficiently. Since your engine will try to keep up, it will work harder and eat up your fuel faster than it has in the past. You may notice that you’re filling up more frequently and or aren’t getting the mileage that you’re used to.[7] 
A clogged converter can make your vehicle misfire and accelerate slower. When the material breaks up inside of the converter, it blocks the exhaust and creates backpressure on the engine, causing it to misfire. Since there’s more pressure on your engine, it will work less efficiently and won’t have as much power as usual when you try to speed up.[8] 
Broken material in the converter clogs the exhaust and blocks the engine. If the honeycomb material breaks and causes a clog, the pressure builds up so much that your engine can’t get enough air to function. You may notice it right when you start your vehicle or while you’re driving. Even if stalling doesn’t happen every time you drive, it’s a good sign that you should get the converter checked out.[9] 
Darker exhaust fumes are a sign your catalytic converter overheats. Normal exhaust usually looks clear or like light smoke. Since the converter usually limits the emissions coming out of your vehicle, thicker or darker smoke alerts you immediately that there’s an issue. 
A bad catalytic converter will overheat after the engine runs. Try running your vehicle for 20 minutes and checking underneath your vehicle. If you can get a view underneath your vehicle, a bad catalytic converter will glow red-hot if it’s overheating. It’s normally a sign that the exhaust is getting caught in the converter without running through it properly.[10] 
An emissions test lets you know if the converter is ineffective. An emissions test checks how many pollutants your vehicle releases in the air. If your catalytic converter isn’t functioning properly, then the exhaust will pass through and cause you to fail the test.[11] 
wikiHow Staff Editor Staff Answer This answer was written by one of our trained team of researchers who validated it for accuracy and comprehensiveness.