Catalytic Converter: The 12 Signs of A Bad One

The purpose of a catalytic converter is to reduce emissions of harmful gases from vehicles. This is a crucial part of your vehicle’s exhaust system, and you need to learn to recognize the signs of a potential problem with your converter.

How Does A Catalytic Converter Work?

Vehicles have been equipped with converters since 1975 to reduce the number of pollutants generated by traffic. You need to have a vehicle with a converter that works properly to avoid exposure to harmful gases, reduce your environmental footprint, and pass an emission test if your state enforces emission standards.

converter reduces harmful gas emissions into the atmosphere by transforming these pollutants into gases that aren’t as dangerous.


Vehicles have been equipped with converters since 1975 to reduce the number of pollutants generated by traffic. You need to have a vehicle with a converter that works properly to avoid exposure to harmful gases, reduce your environmental footprint, and pass an emission test if your state enforces emission standards.

converter reduces harmful gas emissions into the atmosphere by transforming these pollutants into gases that aren’t as dangerous.

As exhaust gases move through your vehicle’s exhaust system, they go through a converter that contains a honeycomb structure or ceramic beads. The honeycomb structure or ceramic beads are coated with metals.

How Does A Catalytic Converter Work

The heat of the exhaust gases is transmitted to this metal coating, and the metal coating or catalyst becomes a filter that transforms harmful gases via chemical reactions. Converters don’t eliminate all harmful emissions, but they make a significant difference regarding the quantities of harmful pollutants released into the atmosphere. Converters are becoming more performing and are also combined with motors that are more efficient, which is why newer vehicles have a lower environmental footprint. Some vehicles are equipped with two-way converters. These converters work by oxidizing carbon monoxide and hydrocarbons. Oxidation transforms carbon monoxide into carbon dioxide, and hydrocarbons are transformed into carbon dioxide and water vapor. You will also find vehicles with three-way converters. These converters are more effective since they also use a chemical reaction known as reduction to transform nitrogen oxides into nitrogen and oxygen gases besides using oxidation to reduce carbon monoxide and hydrocarbons emissions.

What Happens Inside Of A Car Engine?

What Happens Inside Of A Car Engine

It’s important to understand the combustion process that takes place inside of an engine to get a better idea of the importance of the converter.

Gasoline contains thousands of chemicals. The primary ingredient is distilled petroleum, but manufacturers add ingredients such as ethanol to improve combustion. Some additives are also present to make gasoline more stable.

You will also find chemicals that make gasoline less corrosive, and some additives that keep the fuel system of your vehicle clean.

car engine

Most of the chemicals found in gasoline belong to the hydrocarbon category. These molecules are made from long chains of carbon atoms and hydrogen atoms. Gasoline is introduced into the engine of a vehicle to create a controlled explosion in a small space. This generates energy that is used to make the vehicle go forward. Most vehicles use motors with a four-stroke combustion cycle. Here are the stages of this combustion cycle:

  • Intake
  • Compression
  • Combustion
  • Exhaust

During the intake stage, the intake valve opens so that air and gas can enter the engine. The piston moves down to make room for the air and gas mix. During the compression stage, the piston moves up. This compresses the air and gas mix.The combustion stage begins when the spark plug emits a spark. This spark ignites the air and gas mix, and a small explosion is created. The energy of this explosion pushes the piston down. The last stage of the combustion cycle is the exhaust stage. The exhaust valve opens so that the byproducts of the combustion, the exhaust gases, can exit the engine.

What Happens To The Exhaust Gases?

Once the combustion cycle is over, exhaust gases are expelled from the vehicle after going through the exhaust system.

The exhaust manifold collects gases as they exit the combustion chambers of the engine. These gases go through a front pipe, converter, muffler, and exit the vehicle via the tailpipe.

exhaust system of a car

These parts are attached underneath your car by a system of hangers. The converter can be found just behind the motor, under the feet of the driver. It is connected to the rest of the exhaust system. The purpose of the converter is to turn harmful gases into gases that aren’t as dangerous. The purpose of the muffler is to reduce the noise the exhaust gases make as they are pushed out of the vehicle. The exhaust system is one of the simplest systems you will find in a vehicle, but it is crucial. The exhaust system helps improve the performance of a vehicle, reduce noises, and prevents exposure to harmful gases.

What Happens On A Chemical Level?

car engine

The chemistry behind combustion and exhaust gases is complex due to the many additives present in gasoline. However, a few gases make up the majority of exhaust gases.

The Combustion Process

Gasoline is primarily made out of hydrocarbon molecules. These molecules are complex chains of carbon and hydrogen atoms.

During the combustion process that takes place inside of the motor, these molecules are exposed to oxygen and heat. This causes a chemical process known as oxidation, and the hydrocarbon molecules are turned into carbon dioxide, carbon monoxide, and water vapor.


The nitrogen atoms present in gasoline bind with oxygen atoms during the oxidation process. This results in nitric oxide and nitric dioxide being created. The oxidation process also causes energy and heat to be released. This is what causes the piston to move. There are other byproducts of this reaction. Other chemicals can be present in exhaust gases due to the many additives present in gasoline. Because combustion engines need to harness the heat and energy produced during combustion, these controlled explosions happen inside the piston chambers. The quantity of oxygen is limited to achieve a very precise result and create just enough energy to push the piston. Because there is a limited quantity of oxygen available, the oxidation process of the combustion stage can’t be completed and some of the fuel is left unburnt. This means some intact hydrocarbon molecules end up in the exhaust gases. Carbon monoxide is another byproduct of this incomplete oxidation process. In a natural setting, there is an infinite quantity of oxygen available when oxidation occurs, and carbon monoxide molecules are eventually turned into carbon dioxide.

Here is a summary of the different molecules left at the end of the combustion process:

  • Carbon dioxide
  • Carbon monoxide
  • Water vapor
  • Hydrocarbon
  • Nitrogen oxides (Nitric oxide and nitric dioxide)
  • Other impurities

Exhaust Gases and the Role of the Catalytic Converter

The inside of a converter contains a honeycomb structure made out of ceramic or ceramic beads. The honeycomb structure or beads are coated with palladium, platinum, and rhodium.

experts discussing exhaust gases

The heat of the exhaust gases causes this metal coating to heat up once the gases reach the inside of the converter. This hot surface finishes the oxidation process that hasn’t been completed inside the engine.

If your vehicle is equipped with a three-way converter, it will also use a chemical process known as reduction to transform the nitrogen oxides present in exhaust gases.

the process of oxidation

The oxidation process that takes place inside of the converter causes carbon monoxide molecules to bond with oxygen atoms. These molecules are transformed into carbon dioxide. Hydrocarbon molecules are turned into carbon dioxide and water vapor during this process. The oxidation process is made possible by platinum and palladium coating.The reduction process breaks down nitrogen oxides into nitrogen and oxygen gases. Platinum and rhodium coating retain oxygen atoms when heated up. Nitrogen oxides are stripped of their oxygen atoms, and the remaining nitrogen atoms can bond and form nitrogen gas. Even though catalytic converters can’t transform every molecule that goes through, these parts can transform 90 percent of hydrocarbons, carbon monoxide, and nitrogen oxides.

Diesel Engines and Catalytic Converters

There are a few differences between gasoline and diesel engines. In a gasoline vehicle, fuel and air are mixed before entering the engine. The air and fuel mix is compressed and ignited so that the energy of the controlled explosion can be harnessed.

super charged engine

Diesel engines use a slightly different process. The air inside of the piston chamber is compressed before fuel is introduced. Fuel is injected inside of a chamber filled with compressed air, which means the air is already heating up. Diesel engines rely on the heat of the compressed air to ignite fuel inside of using spark plugs. The fuel used by diesel engines has different characteristics. It tends to be oilier, heavier and has a higher energy density. The emissions of a diesel engine are different because the fuel has a different chemical composition, and because the temperature of the engine is cooler compared to a gasoline engine. This means the oxidation process isn’t completed when exhaust gases leave the engine.


Diesel exhaust gases contain smaller quantities of carbon monoxide, carbon dioxide, and hydrocarbons. However, there are a lot more nitrogen compounds. Soot is a byproduct that isn’t present in gasoline engines. There have been some significant improvements in diesel vehicles. The parts that control the direct injection of the fuel are more performing than before, and the refinement of diesel fuel has become more precise.Diesel vehicles rely on a converter to reduce harmful emissions and also use a filter to catch soot and other fine particles.

Why Is The Catalytic Converter So Important?

Exhaust gases have a negative effect on human health and the environment. Carbon monoxide is poisonous, and increased levels of carbon monoxide in the atmosphere can impact health.

Carbon monoxide can also react with hydroxyl radical once it is released into the atmosphere. This reaction increases levels of greenhouse gases carbon dioxide and methane.

Nitrogen oxides include different gases made from nitrogen and oxygen molecules. Once these gases are released into the atmosphere, they can react to sunlight and form smog. They can also react with other chemicals and create acid rain. Increased levels of nitrogen oxides in the air can cause health problems such as irritation of the airways.

Hydrocarbon molecules, which are the results of gasoline that hasn’t been properly combusted, are toxic to humans. These molecules are believed to increase the risks of developing cancer. They can also react with other pollutants present in the atmosphere and create smog. Carbon dioxide is one of the most well-known greenhouse gases. Increased levels of CO2 in the atmosphere are believed to be linked to heat waves and global warming and to increase the acidity level of oceans. This greenhouse gas can also increase the levels of other pollutants such as ozone. Legislation has been passed to create vehicles that are more efficient and to have manufacturers refine fuel that will result in fewer harmful emissions.

Some states require your vehicle to pass an emission inspection before you can register it. There are currently 34 states that enforce some form of testing. However, some vehicles are exempt from these testing requirements, and emission testing is only enforced in some cities or townships instead of being a state-wide measure in some areas. Many owners find out that there is an issue with their converter during one of these inspections. However, there are other signs to watch out for.

What Can Cause A Catalytic Converter To Go Bad?

exhaust system

Structural damage is a common cause behind a bad converter. You might be able to see a dent, tear, or other sign of damage on the outside of the converter.

If road debris hit the converter and damaged hit, the damage will probably allow some of the exhaust gas to go through the exhaust system without being properly filtered by the converter.

The hangers that suspend the exhaust system under your vehicle are also susceptible to damage. Road debris or driving over a bump can cause a hanger to get loose. Your converter can sustain structural damage if it’s no longer supported by the hangers.


A clogged converter is another common issue. The converter can become clogged if the honeycomb structure is broken or damaged. Unburnt fuel can also cause a clog to form inside of the converter.

A clogged converter means that exhaust gas can’t get through. It might also prevent the engine from getting enough air.

If you don’t address the issue quickly, pressure can build up in the exhaust system and the engine can overheat. A clogged converter will fail and could cause damage to the engine if it overheats.

For instance, an issue with your car engine can change the composition of the exhaust gases and affect the converter. The air-fuel mix not being properly dosed is another problem that will affect the converter.

Timing issues during the combustion process, a bad spark plug or damaged spark plug wires, or a faulty oxygen sensor might be to blame for a bad converter.

Your converter will also go bad if there is too much unburnt fuel that ends up in the exhaust system. This is an issue that can be caused by the engine not burning fuel properly, or by the air-fuel mix containing too much fuel.

Oil or antifreeze can also leak and end up inside of the exhaust system. This issue should be easy to pinpoint since your vehicle would be low on oil or antifreeze.

You should look for signs of structural damages to the exhaust system if you suspect there is an issue with your converter since this problem is easy to diagnose or rule out by simply looking under your vehicle.

You might be able to determine where the issue is coming from by going through a process of elimination or by looking for other issues such as using more fuel than usual, but it’s best to take your vehicle to a licensed mechanic so they can find the problem and fix it.

Most Common Signs Of A Bad Catalytic Converter

worn out inverter

If there is an issue with your converter, your vehicle will not run properly. The severity of the issue will depend on how badly the converter is damaged or clogged.These aren’t signs that you should ignore since the converter can fail and cause damage to other parts. There is a risk for the engine to overheat if the converter is clogged. You should avoid driving and take your vehicle to a mechanic’s if you suspect there is an issue with your converter.These signs are easy to identify, but keep in mind that they can be caused by other faulty parts and systems.

Failed Emissions Test

The purpose of an emissions test is to make sure the exhaust gases of your vehicle are below certain levels. Depending on where you live, you might have to pass different types of emissions tests.

Some states require drivers to pass a dynamometer test. This test captures and analyzes exhaust gases to measure levels of harmful gases.

Your vehicle might also have to pass a gas cap test. A mechanic will check the gas cap to make sure it doesn’t leak. A damaged gas cap can cause harmful emissions from fuel that evaporates.

Some states use onboard diagnostic checks instead of dynamometer tests. A mechanic will use an OBD tool to collect data from different sensors and systems. This data will help them diagnose any problems with the engine or exhaust system and should also give them an idea of the composition of your exhaust gases.

A failed OBD or dynamometer test can indicate that your converter isn’t working properly. However, issues with other parts and system can cause you to fail these tests.

For instance, a faulty evaporative emission control system could cause gasoline to evaporate from your tank. The dynamometer test would pick up high levels of hydrocarbons even though your exhaust system is working properly.

Your vehicle could fail an emissions test if the air-fuel mixture is off-balance. A bad injector or faulty oxygen sensor could be to blame.

A damaged spark plug that doesn’t ignite fuel at the right moment could result in high levels of hydrocarbons in your vehicle’s emissions.

Hydrocarbon levels can also go up if the air filter of your vehicle needs to be cleaned or replaced. This is something you can easily check and remediate to.

If your vehicle fails an OBD test, the problem could come from the sensors that collect data.

If you fail an emissions test, you should have a mechanic pinpoint the source of the problem. You might need a new converter or might to get another system fixed. This should be a priority since a faulty fuel injection system or other mechanical issues with your car can eventually damage the converter and cause more expensive problems.

Changes In Exhaust Gas Color

Most states that require vehicles to be tested for harmful emissions only require you to pass a test every couple of years. You should keep an eye on your exhaust gases in the meantime and look for any changes.

A change in the color of exhaust gases can indicate a wide range of issues with your vehicle. A bad converter can cause some changes in exhaust gases, but these changes can also come from another issue that could damage the converter.

White smoke coming out of the tailpipe of a vehicle typically indicates that the engine is overheating. There are many possible causes, but your converter could be clogged and cause exhaust gases to increase the temperature of the engine.

If you notice black smoke coming out of the tailpipe of your car, your air filter might be clogged. The unusual color of the exhaust gases could also be caused by issues with the fuel injector or fuel pressure regulator.

If the fuel injector or fuel pressure regulator isn’t working properly, your converter could fail due to high levels of hydrocarbons ending up in the exhaust gases.

Other unusual colors such as grey or blue exhaust smoke don’t indicate that there is an issue with your converter, but you should still take your vehicle to a mechanic’s since there might be an issue with an oil leak or your transmission.

Unusual Smell

High levels of hydrocarbons in your exhaust gases can cause an unusual smell. Some people have compared this smell to sulfur. If you notice that the exhaust of your vehicle smells different, your exhaust gases might contain high levels of unburned gasoline. This could be the result of a faulty converter, but keep in mind that high levels of hydrocarbons in your exhaust gases can also result from issues with the air-fuel mix that goes into the engine. The spark plug might not be working properly, which causes unburned gasoline to end up in the exhaust system. Even though the converter might not be at the source of the unusual smell, high levels of unburned gasoline going through your converter can cause this part to fail.

Discolored Converter Housing

If overheating occurs inside of the converter, the outer housing of the part can become discolored. This is something you can look at by getting under your vehicle with a flashlight. Wait a few hours after you have turned the engine off to give the converter enough time to cool down.

The temperature of your converter can vary in function of different factors, such as the fuel intake, air-fuel mix, and ignition system. If your converter is overheating, there is probably an issue with the engine that is causing extremely hot exhaust gases to go through the converter.

The high temperature of the exhaust gases will damage the converter and make the part useless. If you notice discoloration on the outer housing of the part, the inside of the converter is more than likely damaged as well.

Overheating can also be caused by a clogged converter. If exhaust gases can’t exit the converter, the pressure will build up inside of the part and cause the temperature to go up. Note that the hot exhaust gases can backfire and be recirculated into the engine, which could cause it to overheat as well.

Reduced Fuel Efficiency

Keep track of how often you put gas in your vehicle. If you have owned the same car for a few years, you should have a good idea of how far you can drive on a tank of gas. Keep in mind that driving in traffic or letting your car idle can impact fuel efficiency. However, if you notice a significant change in fuel efficiency, this might be a sign that a lot of gasoline is going through the engine without getting burnt. Unburned gasoline increases hydrocarbon levels in your exhaust gases and can cause your converter to fail. There might be an issue with the air-fuel mix, fuel injection system, spark plug, or another part that is causing the reduced fuel efficiency.

No Acceleration

You need proper tuning and a fuel injection system that is in working order for your vehicle to accelerate when you use the gas pedal. If you notice any changes in how your vehicle responds, there probably is an issue with how air or gasoline is getting into the engine.

The fuel injector might be clogged, or you might need to replace the fuel pump. You might be able to fix the problem by cleaning the fuel filter or replacing the airflow sensor.

However, a clogged converter might be the cause behind the change in performance. A clogged converter can prevent your engine from getting enough air. This could impact acceleration and cause other issues with the fuel intake system of your vehicle.

You might notice that the vehicle is not accelerating like it usually does, or doesn’t accelerate at all.

Your Car Won’t Start

There are many potential causes for a vehicle that won’t start. The battery might be dead or the started might no longer be working. However, your vehicle might be unable to start because of a clogged converter. If the converter is clogged, exhaust gases will create pressure inside of the exhaust system and end up recirculating inside of the engine. This can cause the engine to overheat and make it so your car won’t start. Note that exhaust gases recirculation inside of the engine can also cause your car to start after a few tries, or to cause it to respond in an unusual manner when you start it even though you can eventually get the car running.

Rattling Noise

You might hear a rattling noise coming from under your vehicle if there is structural damage to the converter. The honeycomb structure that can be found inside of the converter can break and the parts will rattle inside of the converter.

The rattling might also be caused by the hangers that keep the converter securely attached to your vehicles. If these hangers are damaged or come loose, your entire exhaust system could be damaged.

Keep in mind that rattling noises can be caused by other parts. You should take a look under your vehicle to see if you can pinpoint the source of the unusual noise.

If you don’t see any signs of damages, the honeycomb structure inside of the converter might be to blame for the noise.

Changes In Performance

Any changes in the performance of your vehicle could be tied to a bad converter. You might notice a difference when you accelerate or might notice that it is difficult to maintain a certain speed.

This probably indicates that there is an issue with fuel intake. Your vehicle might have a faulty spark plug or your engine might not be getting the correct air-fuel mix.

There are a lot of potential issues that can affect how your vehicle performances. You might see an improvement after you get an oil change if you are due for one. You might need to replace a faulty oxygen sensor.

You shouldn’t ignore these signs since they can indicate that there is an issue with the fuel intake system of your vehicle, and your converter could fail if the issue isn’t addressed.

MIL Or Check Engine Light Comes On

The Malfunction Indicator Lamp or Check Engine Light is a small indicator located on your dashboard. This light will come on to indicate that an issue requires your attention.

The Check Engine Light can light up for a wide range of reasons. A sensor might be picking unusual temperatures or humidity and the light will go away after a while.

If the light doesn’t disappear after a day or so, you should take your vehicle to a mechanic’s to figure out where the issue is coming from. Here are the most common issues that cause the Check Engine Light to come on:

  • Oxygen sensor
  • Loose gas cap
  • Spark plug wires
  • Airflow sensor
  • Converter

Your mechanic will obtain a diagnostic code and get a better idea of where the issue is coming from. Your vehicle doesn’t necessarily need repairs if the Check Engine Light comes on since the light might be triggered by a faulty sensor.

However, it’s best to have your vehicle checked by a mechanic to make sure there are no serious issues with the exhaust system or another part.

Fuel Vapor Test

If your vehicle has a carburetor instead of a fuel injection system, your mechanic can perform a simple test to determine if there is an issue with the converter.

Ask them to remove the air cleaner and rev the engine. You might see some fuel vapor come out of the carburetor. If this is the case, your converter might be clogged.

This is a test you can do if you have noticed other symptoms that could indicate there is a problem with the converter. You might be able to eliminate the converter as a potential cause if you don’t see any fuel vapor.

Temperature Difference

You can check the temperature at the intake and outtake valve of the converter to determine if this part is working properly.

It’s best to have a mechanic perform this test. They will be able to safely get under your vehicle by using a pit or by lifting the vehicle.

A converter will start working when exhaust gases reach a temperature of 400 to 600°F. However, it’s normal to measure temperatures between 1,200 and 1,600°F inside of the converter.

If the outtake valve temperature is higher than the intake valve temperature by 500°F or more, there is a lot of unburnt fuel that is going through the converter. The converter has either failed or is about to fail due to exhaust gases that contain too many hydrocarbons.

If the temperature reaches 2,000°C, the converter is overheating and is likely too damaged to function properly. Remember that you shouldn’t measure the temperature of your converter yourself since you could seriously injure yourself, especially if the part is overheating.

What To Do If You Have A Bad Catalytic Converter

If you notice any of the signs mentioned above, you should look into getting your converter replaced as soon as possible. The issue can worsen and cause the converter to become completely clogged, which means your vehicle might no longer start.

You can drive with a bad converter. You will notice a difference in performance and fuel efficiency. Keep in mind that driving with a bad converter means your car will emit more harmful exhaust gases.

The issue that is causing your converter to fail might also be affecting the fuel efficiency of your vehicle. You should get the issue taken care of as soon as possible to avoid overspending on gas, especially if you drive a lot.

You should take your vehicle to a mechanic’s, a muffler shop, or a dealership if it’s still under warranty. A certified mechanic should inspect your converter and determine if the part is failing.

A visual inspection can reveal problems with the converter. In some cases, your mechanic might need to remove the part to see if there are any structural damages inside. A pressure test might also be needed to see if there are issues with your exhaust system.

It’s important to take your vehicle to a repair shop as soon as possible because a failing converter usually indicates that there is a problem with the engine.

How Much Does A New Catalytic Converter Cost?

The cost of a new converter depends on the year, make, and model of your car. Older vehicles made before the 1980s have a two-way converter that is more affordable than newer parts.

If you have an older vehicle, a new converter might only cost you between $100 and $600. If you have a vehicle manufactured after the 80s, you will need to purchase a three-way converter. This part can cost anywhere between $200 and $2,500 depending on the make and model of your car.

The installation process can be complex on some newer vehicles. Labor can cost between $100 and $500 depending on how long the installation process takes.

An OEM part will cost more than an aftermarket part. You might be able to find a used converter for your vehicle, but keep in mind that used parts need to pass an inspection.

Converters typically come with a warranty that lasts for 80,000 miles or eight years. The manufacturer of your vehicle will cover the cost of the new part and repairs if the warranty is still valid.

Because a faulty converter is often the cause of an issue with the engine or fuel injection system, you might have to pay for additional repairs to the engine. The cost of these additional repairs depends on where the issue is coming from and on the make and model of your vehicle.

The catalytic converter is an important part because it considerably reduces the harmful gases that your vehicle emits when you drive. Learn to recognize the signs of a faulty converter and get your car checked as soon as possible if you suspect that this part is failing.