Signs and Symptoms of a Bad Catalytic Converter

Catalytic converters have an important role in the vehicle’s exhaust system. It assists in neutralizing hazardous compounds such as carbon monoxide that a vehicle produces. The converters are made to last the lifetime of the vehicle, but they can break down or wear due to unattended repair issues in other parts of the vehicle. So, what are the symptoms of a bad catalytic converter?

A damaged catalytic converter causes air pollution, affects vehicle mileage, and damages other exhaust system components. It is advisable to have the converter repaired immediately if you notice a problem. Therefore, it is crucial to understand the signs of a bad catalytic converter, the causes, and how to fix the issue. 

What Are the Common Symptoms of Bad Catalytic Converter?

When the vehicle starts running, the vehicle produces toxic gases that go via the catalytic converter. The internal structure of the converter has a honeycomb design. It is made of precious metals such as rhodium, palladium, and platinum. The converter is designed to work under high temperatures of about 800°F.

Chemical reactions happen to neutralize the harmful gasses from the vehicle and change them to safer elements such as carbon dioxide, water, and oxygen. These clean elements then get out from the vehicle through the muffler. Listed below answers the question on how to tell if the catalytic converter is bad:

sign of bad catalytic converter

1. Engine Misfire

Engine misfires are among the first symptoms of a bad catalytic converter. It signifies that the catalytic converter is overheating. It is an implication that the combustion process in the cylinder is incomplete due to the ineffectiveness of the catalytic converter.

The converter overheats when it is clogged, leading to other damages if it is not looked at properly. Contact an expert mechanic as soon as you notice an engine misfire to prevent it from melting the catalytic components. This will eventually lead to other expensive repairs and replacements.

2. Rotten Egg Odor from the Exhaust

A weird odor coming from the vehicle is among the major car symptoms of bad catalytic converters. It is a sign that something could be wrong with its internal components. A rotten egg smell is a symptom of a catalytic converter failure. The fuel contains an amount of sulfate, which smells like rotten eggs. The converter converts the sulfate to other odorless gases.

Therefore, when the catalytic converter is not in order, the fuel will still have a stinky rotten egg odor. As soon as you experience this smell, let a mechanic fix it and replace the converter if need be.

3. Check Engine Light Is On

Vehicles have an internal computer that uses the check engine light to show any internal issues. A bad catalytic converter affects the fuel and air ratio, which results in the emission of toxic pollutants.

When the check engine light illuminates, it is a sign that the converter is not working properly. The engine light can also illuminate due to other issues in the engine. Therefore, the prudent thing is to get a mechanic to check the exact problem.

4. Problems Starting the Vehicle

A problem starting the car is one of the clogged exhaust symptoms. A clogged catalytic converter does not convert toxic gases from the vehicle to less toxic ones. The converter, therefore, keeps toxic gases in the engine, leading to increased exhaust pressure. This later causes the engine to stall or sputter, making it difficult to start.

5. Reduced Fuel Efficiency

A clogged catalytic converter reduced airflow, thus causing the engine to burn more fuel than normal. This is a result of poor acceleration. When you have minimal exhaust flow, you press the gas pedal more because acceleration is affected. This makes the engine inject more fuel which then results in a richer fuel mixture than required.

A damaged catalytic converter also prevents the engine from getting the required oxygen amount. Less oxygen supply in the engine makes the engine work harder, thus consuming more fuel. This means that the vehicle will go for fewer miles with a gallon of fuel than normal.

Therefore, if you discover an unexplained decrease in gas mileage, it signifies something wrong with the catalytic converter. However, this does not mean the converter needs replacement, but if it is combined with any of the other signs, it may be a requirement.

6. Poor Acceleration

Engine efficiency can reduce due to an array of factors. One of the major reasons an engine can lose its efficiency is due to a clogged catalytic converter. Sometimes, mechanics may misdiagnose when the converter is clogged partially, resulting in unnecessarily changing some parts. Carbon build-up in the honeycomb is what causes the converter to block.

The blockage can also happen when internal parts start melting due to excess heat caused by unburned fuel. The engine operates at high performance when there is a good flow of exhaust. A clogged converter reduces the flow, thus reducing engine power when accelerating, towing a heavy load, or going uphill.

Check the vehicle’s exhaust using your hand to know if the low power acceleration is due to a clogged catalytic converter. Get a helper to hold the vehicle at 1800 to 2000 rpm as you put your hand behind the tailpipe. If you feel the heat coming from the exhaust, it is a result of a clogged converter. It would also be advisable to learn how to unclog a catalytic converter.

7. Degraded Performance

Another common symptom of a bad catalytic converter is the degradation of engine performance. A bad converter creates backpressure that prevents the engine from running freely. This eventually makes the vehicle shake. The sudden pressure outbursts can also make the engine too when driving.

8. Increased Emissions

Another significant sign of a failing catalytic converter is increased carbon emission from the exhaust. The converter is a part of the emission system, and thus, when it fails, it reduces gaseous emissions.

A vehicle should have an emissions test annually. This test will help you to know if the converter is working properly. Some of the issues that may make the car fail emissions tests are contaminated motor oil or antifreeze or high levels of carbon emissions.

9. Rattling Noises

Weird sounds coming from the vehicle are a symptom of an issue with the internal parts. Therefore, what does a bad catalytic converter sound like? A bad catalytic converter produces a rattling noise from under the vehicle when driving or idling. The noise tends to be louder when starting the vehicle.

Excess heat or damage in the converter breaks the honeycomb materials, thus resulting in the rattling noise. If you hear any abnormal sounds, take the car to a mechanic and have it inspected. Replace the catalytic converter in case the issue is diagnosed to avoid further damage.

10. Black Smoke from the Exhaust Pipe

When the catalytic converter fails, gases start building up in the exhaust. As they build up, the gasses become thicker and have a darker color. Therefore, thick and dark-colored gases coming from the tailpipe are a symptom of a damaged catalytic converter.

driving with a bad catalytic converter

Bad Catalytic Converter Causes

Now that we have learned what happens when the catalytic converter goes bad, it is crucial to understand the causes. Most of us expect catalytic converters to last a lifetime since they have no moving parts.

However, as long as they last and are durable, they can still fail. Most of the problems in the catalytic converter are caused by engine problems. Let us now discuss the causes of bad catalytic converters:

Poor Engine Maintenance

As we have said above, most of the issues of catalytic converters result from a damaged vehicle engine. Engines need to be serviced from time to time. Old and poorly maintained engine burns oil due to worn-out valve guides, struck rings, and worn cylinder walls.

This burning oil produces products that can damage the converter. Incorrect timing and incorrect fuel and air mixture also lead to a catalytic converter failure. These factors damage not only the converter but also other engine parts.

Unburned Fuel

Excess heat is a major cause of issues in the internal vehicle parts. The engine exhaust is always hot. When contaminants like unburned fuel leak to the exhaust system, they burn up, thus causing the honeycomb structure to melt.

The honeycomb helps the catalytic converter to perform. Damaged or melted honeycomb blocks and restricts exhaust flow. This is what causes the engine to start misfiring and making a rattling noise.

Coolant Leaks

Coolant or antifreeze leaks in the combustion chamber can lead to serious problems. A blown head gasket mainly causes these leaks. When the leaks get to the exhaust system, they clog the catalytic converter, preventing it from working effectively.

Coolant in the exhaust system creates soot and thick carbon, which blocks air passages in the honeycomb structure of the converter. This prevents the catalytic converter from getting rid of harmful emissions in the exhaust flow. It also clogs the pores of ceramic catalysts, thus restricting exhaust flow, which causes back pressure.

Bad Spark Plug Wires or Spark Plugs

Another cause of a bad catalytic converter is failed spark plugs or spark plug wires. When the plugs misfire or fail to fire, they force unburned duel to get in the exhaust system. The unburned fuel ignites, causing a complete or partial meltdown of the honeycomb structure.

Short Trips

If you only use the car for short trips, the catalytic converter does not get hot enough to burn hydrocarbons completely. This would cause clogging of the catalytic converter after some time. You can prevent this by driving your vehicle for longer to allow the converter to heat up sufficiently and burn the deposit build-up completely.

Malfunctioning Oxygen Sensor

A malfunctioning oxygen sensor sends the wrong readings of the exhaust gases to the car’s computer. These readings lead to improper fuel and air mixture. When the mixture is too rich, unburned fuel gets in the converter. Too lean a mixture prevents the converter from converting hydrocarbons to safe elements, making the vehicle fail an emissions test.

How Do You Fix the Bad Catalytic Converter?

Each car owner should know how to check if your catalytic converter is bad. Once you diagnose that the vehicle has issues with the catalytic converter, you should fix it right away. You can learn more about how to fix a catalytic converter from this video. Here are some ways you can fix a bad catalytic converter without replacement:

how to check if your catalytic converter is bad

Fuel Additives

One easy way of fixing a clogged catalytic converter is using fuel additives. The fuel additives help the engine run more cleanly, and they will help clean catalytic converter deposits. For instance, if you run a vehicle with low-octane fuel, you can fix the converter by changing it to high-octane fuel.

Italian Tune-Up

The Italian tune-up method is used to fix quite a number of issues in a vehicle, and a bad catalytic converter is one of them. All you are required to do is to push your vehicle harder to hear the catalytic converter enough. The converter should be heated to a high temperature of between 426 °C (426°F) and 1,000 °C (1832°F).

Run the vehicle a little longer than usual, and this will help burn off any carbon deposits in the catalytic converter, oxygen sensors, cylinder head, and intake valve. This, however, may not work if the converter is extremely clogged.

Fix Exhaust Problems

 you could easily fix the problem with your converter if you deal with the cause. If your vehicle has fuel, oil, or coolant leaks, fix them immediately, and this could restore the converter’s performance. Also, replace corroded flex pipes and worn-out gaskets for they affect the functioning of a converter.

Cleaning the Converter

At times you may have to go the old-fashioned way of cleaning the catalytic converter. Remove the catalytic converter from the vehicle and wash it using a pressure washer. This will help to blast out any contaminants and deposits in the system. Ensure that you flush the converter from both ends.

Another method of cleaning a catalytic converter is by soaking it overnight in a combination of degreaser and hot water. This will help the deposits clogging the converter to dissolve. Once the converter is clean, dry it completely and reinstall it.

Benefits of Using a Catalytic Converter

A catalytic converter acts as an exhaust emission control unit for reducing toxic gases. Engineer Eugene Houdry invented the first converter in 1950. Catalytic converters work with two types of catalysts, namely, an oxidation catalyst and a reduction catalyst.

Catalytic converters have two major benefits in vehicles. One, it reduces the amount of dangerous pollution produced by the vehicle during the combustion of fuel. Secondly, it reduces emissions of nitrous oxide by 62%, carbon monoxide by 85%, and hydrocarbon by 87% in the life of a vehicle.

How to identify a bad clogged catalytic converter YouTube

Frequently Asked Questions (FAQs):

Q: What Happens When the Catalytic Converter Goes Bad?

The main work of the catalytic converter is to clean emissions from the vehicle and prevent air pollution. A catalytic converter is among the most durable part of a vehicle, but after some time, it can get physically damaged, clogged, or overheated.

A bad catalytic converter causes the check engine light to illuminate, the engine to misfire, and the vehicle may fail to start. Other signs are dark exhaust smoke, the smell of sulfur from the exhaust, and reduced acceleration.

Q: What Does a Bad Catalytic Converter Sound Like?

Disturbing noises when the vehicle is running or idling is a sign that something is wrong with its internal components. A bad catalytic converter produces a rattling noise from below the vehicle.

Excess heat in the catalytic converter can cause the honeycomb structure in the converter to break. The broken parts are what cause the rattling noise. This rattling noise will be more when you start the vehicle, and it becomes worse over time.

Q: How Bad Is It to Drive With a Bad Catalytic Converter?

Most people wonder how dangerous it is driving with a bad catalytic converter. Partially clogged catalytic converters do not affect the vehicle’s driveability, but they will affect your car’s performance.

However, if the converter fails, you may not be able to drive the vehicle. In this case, you will require to have it inspected and replaced immediately. It is prudent to fix issues with the catalytic converter as soon as you find out to prevent further damage.

Q: Will Check Engine Light Come On for Bad Catalytic Converter?

When the check engine light illuminates, it is a sign that something is wrong with the engine parts. A bad catalytic converter triggers the check engine light on. Some causes of a bad converter that turn the engine light on are broken oxygen sensors or worn-out spark plugs. Therefore if the light turns on, get the vehicle checked by a mechanic to distinguish what the cause is.  

Q: How To Know If You Need to Replace the Catalytic Converter?

A failed catalytic converter does not always mean that it needs replacement. At times you can fix the issue by just cleaning the converter. However, when the issue escalates, you may need to replace the unit. Some of the signs and symptoms that the catalytic converter should be replaced are check engine light on, decreased engine performance, and rattling noises from under the vehicle.

Q: How Long Should a Catalytic Converter Last?

Catalytic converters are designed to last for the lifetime of the vehicle. It is one of the longest-lasting and durable parts of a vehicle. However, due to the excess heat, clogging, and other factors in the system, it may fail.

For how long does a catalytic converter last? This unit can last for at least ten years without issues or around 100,000 miles. However, the time varies greatly depending on the environment you are driving in and your driving style.

Q: Can a Catalytic Converter Cause Shake?

A catalytic converter can become broken due to various factors. When you press the gas pedal, the broken pieces of the converter will produce rattling and vibration noises. Check for the broken parts of the catalytic converter by shaking it by hand and listening to the noise.

Q: Can a Bad Catalytic Converter Cause Transmission Problems?

When the catalytic converter is clogged, it makes the engine work harder, making it challenging to shift transmissions. The gears can also have issues if the fuel injection system or fuel filter is dirty or if the engine is too low or too full. The vehicle will have issues accelerating due to the clogged fuel and air filters.

Final Words:

A catalytic converter performs a vital role in the vehicle. Its primary function is to convert dangerous emissions produced by the vehicle to less harmful gases. A vehicle with a good catalytic converter enjoys good acceleration and high fuel economy and passes the emission test.

After learning the symptoms, causes, and how to fix a bad catalytic converter, you should understand how do you check for a bad catalytic converter. The signs above tell you when it is about to fail. If you notice any of the above signs, have a professional check and fix the vehicle. It is prudent to take the converter for an emissions test every year.


Hi there, I am R. Hasan Tito, a mechanic, and owner of this website. My friend and I created this website to share our knowledge, expertise, and experience with our fellow mechanics' community and car users. I am a specialist and certified automotive mechanic (Both Heavy Commercial and Private Cars). I worked as a Mechanic and Mechanic Supervisor for over fifteen years at Global Rebound Automotive companies - Toyota, TATA, BMW, Nissan, TVs, and Others. Now, I enjoy my new role of leading a team of automotive experts (in their respective fields) and publish new content on a regular basis on my website and social media.

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