PRO Courses Hot Guides Tech Help Pro Expert Videos About wikiHow Pro Upgrade
QUIZZES All Quizzes Hot Love Quizzes Personality Quizzes Trivia Quizzes Taylor Swift Quizzes
EDIT Edit this Article
EXPLORE Tech Help ProAbout UsRandom ArticleQuizzes Request a New ArticleCommunity DashboardTrendingForums Arts and EntertainmentArtworkBooksMoviesComputers and ElectronicsComputersPhone SkillsTechnology HacksHealthMen's HealthMental HealthWomen's HealthRelationshipsDatingLoveRelationship Issues Hobbies and CraftsCraftsDrawingGamesEducation & CommunicationCommunication SkillsPersonal DevelopmentStudyingPersonal Care and StyleFashionHair CarePersonal Hygiene Quizzes Love Quizzes Personality Quizzes Fun Games ForumsArts and EntertainmentFinance and BusinessHome and Garden Relationship QuizzesCars & Other VehiclesFood and EntertainingPersonal Care and Style Sports and FitnessComputers and ElectronicsHealthPets and Animals TravelEducation & CommunicationHobbies and CraftsPhilosophy and Religion Work WorldFamily LifeHolidays and TraditionsRelationships Youth
LOG IN
Log in
Social login does not work in incognito and private browsers. Please log in with your username or email to continue. FacebookGooglewikiHow Account No account yet? Create an account
RANDOM
Home
Random
Browse Articles
TrendingNew
Quizzes & Games
All QuizzesHot
Love Quizzes
Personality Quizzes
Fun Games
Dating Simulator
Learn Something New
Forums
Courses
Happiness Hub
Explore More
Support wikiHow
About wikiHow
Log in / Sign up
Terms of Use wikiHow is where trusted research and expert knowledge come together. Learn why people trust wikiHow
Categories
Cars & Other Vehicles
Cars
Car Maintenance and Repair
Exhaust and Fuel Parts
9 Signs That Your Catalytic Converter is Going Bad Download ArticleExplore 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 Tips and Warnings Related Articles References Co-authored by Jason Shackelford and Hunter Rising
Last Updated: July 6, 2025 Fact Checked
Download Article 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 67,244 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
Download Article
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
Advertisement 2
Rotten egg smell
Download Article
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
Download Article
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.
Advertisement 4
Reduced fuel efficiency
Download Article
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
Download Article
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.\
Advertisement 6
Stalling or not starting
Download Article
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
Download Article
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.
Advertisement 8
Glowing converter after driving
Download Article
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
Download Article
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.
Advertisement
Expert Q&A
Search Add New Question Ask a Question 200 characters left Include your email address to get a message when this question is answered. Submit Advertisement
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 1
Submit a Tip All tip submissions are carefully reviewed before being published Name Please provide your name and last initial Submit Thanks for submitting a tip for review! Advertisement
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 1 Not Helpful 0
Advertisement
You Might Also Like
How to Diagnose and Fix the Source of a P0430 CodeHow to Fix a Clogged Catalytic Converter with or without Removing It10 Signs It’s Time to Replace Your Vehicle’s Oxygen SensorThe Top Causes & Fixes for Emission System IssuesHow to Identify the Cause of a Rotten Egg Smell in Your CarWhy Does My Car Stall When I Come to a Stop?Has Your Catalytic Converter Been Stolen? How to Tell and What to Do If It Has BeenHow toMake Your Car Run Faster While Going UphillMy Car Runs Fine: Why Is My Check Engine Light On?How toClean a Catalytic Converter7 Symptoms of a Bad Spark PlugWhy Your Check Engine Light Comes On and OffVehicle Jerking While Accelerating: Causes & Fixes ExplainedHow to Diagnose a Check Engine Light That Flashes Then Stops Advertisement
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 67,244 times. 30 votes - 80% Co-authors: 4Updated: July 6, 2025 Views: 67,244 Categories: Exhaust and Fuel Parts In other languages Portuguese Japanese
Print
Send fan mail to authors
Thanks to all authors for creating a page that has been read 67,244 times. If you buy through links on our site, we may earn a commission.
Did this article help you?
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: 4Updated: July 6, 2025 Views: 67,24480% of readers found this article helpful. 30 votes - 80% Click a star to add your vote 80% of people told us that this article helped them.
Quizzes & Games
Should We Break Up QuizTake QuizDepression TestTake QuizDo I Want a Divorce TestTake QuizWhat's Your Red Flag QuizTake QuizAm I Falling Out of Love QuizTake QuizCan You Spot The False Climate Claims QuizTake Quiz
You Might Also Like
How to Diagnose and Fix the Source of a P0430 CodeHow to Fix a Clogged Catalytic Converter with or without Removing It10 Signs It’s Time to Replace Your Vehicle’s Oxygen SensorThe Top Causes & Fixes for Emission System Issues
Featured Articles
How to Text Yourself: Fake Text Messages, Reminders, and MoreAccurate Omegaverse Quiz: 100% GuaranteeThe Hardest & Most Difficult Riddles for AdultsHow to Be More Spontaneous and Enjoy Life to the FullestHow toImprove Your PersonalitySigns You Were Meant For Greatness (Quiz)
Trending Articles
How Sexual Am I QuizHow Much Do I Mog QuizWho Is Secretly Crushing On Me Right Now QuizAccurate IQ Checker Quiz: How Smart Am I?Will I Ever Find Love QuizCan We Guess How Tall You Are Quiz
Featured Articles
210+ Shower Thoughts: Funny, Deep, Weird & Mind-BlowingHow to Meet Gay People Without a Dating AppCute, Thoughtful & Romantic Messages to Wish Her a Good Day at WorkWhat Movie Should I Stream QuizWhat Does the Term "Femboy" Mean?40+ Fun & Exciting Group Game Ideas to Play with 20+ People
Featured Articles
11 Things to Do If You’re Connected to Wi-Fi But Have No InternetThe Ultimate List of Funny Nicknames for Your CoworkersWhich Five Nights at Freddy's Character Am I QuizBest Haircuts for Men with Round Faces (& How to Style Them)What Is the Chubby Body Type? Plus, How It Compares to Other Body TypesDo I Have an Innocent Face Quiz
Watch Articles
How to Marinate Chicken: 4 Recipes and a Step-By-Step GuideHow toClean Shower TileHow toDo Oblique CrunchesSimple Ways to Riffle Shuffle and Bridge Playing CardsHow toMake Black CoffeeHow toPlant a Seed in a Pot
Trending Articles
Do I Attract the Male Gaze or the Female Gaze QuizHow Weird Am I QuizFunny Numbers to Call When You’re Bored or Want to Prank SomeoneThe 26 Different Types of Women in Psychology, Dating, & LiteratureMental Illness TestWhat Type of Person Do I Attract Quiz
Quizzes & Games
Is My Relationship Toxic QuizTake QuizDo I Need Glasses QuizTake QuizAre My Hormones Imbalanced QuizTake QuizWhat Chakra Do I Need to Balance QuizTake QuizTraffic Signs TestTake QuizAm I Being Gaslighted QuizTake Quiz
Categories
Cars & Other Vehicles
Cars
Car Maintenance and Repair
Exhaust and Fuel Parts
wikiHow Newsletter You're all set! Helpful how-tos delivered toyour inbox every week! Sign me up! By signing up you are agreeing to receive emails according to our privacy policy.
Download Article 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 Tips and Warnings Related Articles References Co-authored by Jason Shackelford and Hunter Rising 
Advertisement 
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 67,244 times. 30 votes - 80% Co-authors: 4 Updated: July 6, 2025 Views: 67,244 Categories: Exhaust and Fuel Parts In other languages Portuguese Japanese 
Take Quiz
Take Quiz
Take Quiz
Take Quiz
Take Quiz
Take Quiz 
Take Quiz
Take Quiz
Take Quiz
Take Quiz
Take Quiz
Take Quiz 
802 
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] 
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.