Bad Catalytic Converter – What Are The Symptoms?

Bad Catalytic Converter – What Are The Symptoms?

August 1, 2021 0 By CarGuy

So, you are experiencing how your car is down on power. And it’s getting difficult to keep the engine running because every time the car is trying to stall? You shouldn’t worry, maybe you just have a bad catalytic converter.

The catalytic converter is one of the most important pieces in the combustion process. Its main role is to purify the exhaust fumes into something less toxic and damaging for the environment.

These catalytic converters are doing their magic and protecting us and the environment from these harmful particles. But unfortunately, these particles can clog the converter and cause it to go bad.

A broken catalytic converter has many symptoms. We are going to cover most of them, and these are the most important ones you need to know. The ones we will cover are a dead giveaway if you have a bad catalytic converter.

Catalytic converters can be pricey. And because of the precious metals that are installed inside of these converters they are stolen quite often. So, you need to be aware of this also.

In this article, we are going to cover everything when it comes to bad catalytic converters. We are going to learn what is a catalytic converter and from which materials are these converters built. Then we are going to cover the main symptoms of a broken catalytic converter. Then we are going to discuss the cost to fix. And lastly, we are going to answer some common questions when it comes to bad catalytic converter and how to prevent thefts.

What Is A Catalytic Converter?

A catalytic converter is a device that is installed in your exhaust system. It has a cylindrical shape and looks like a muffler from the outside. In front and after the converter there is an O2 sensor that is measuring the exhaust gasses.

It has has a honeycomb structure. This honeycomb structure is built from many precious metals like platinum, palladium, nickel, copper, and rhodium. These metals are very expensive, which makes the price of these parts extremely high. The high price of these components and the easy access from below makes the thieves go crazy for these converters. That’s why you need to pay attention to where you park your car.

This honeycomb structure is purifying the exhaust gasses. It works at an operating temperature of 600 degrees. So, if you are running your car on short distances it will not be is not getting up to temperature and may cause bad catalytic converter symptoms to develop. These symptoms we are going to cover in the next chapter.

Bad Catalytic Converter Symptoms

Bad catalytic converter symptoms can be many. But we are going to cover the most common ones in detail and also their possible outcomes.

Engine Misfires

Engine misfires are probably one of the most pronounced symptoms. If the catalytic converter is clogged or there is a blockage on the inside. Then you are probably experiencing misfires as well.

How this happens is really simple to explain. There is a blockage inside the honeycomb structure and your O2 sensor is sensing that there is something bad going on. The readings are not good. Then it tells the engine to add more fuel and these symptoms are getting even worse.

Then there is fuel that is not ignited in the exhaust. This fuel that is not burnt can foul your spark plugs, clog your catalytic converter even more and also damage your O2 sensor.

If you are experiencing misfires you are going to hear some pops and booms coming from the exhaust pipe. This is a clear sign that something is going on inside of the combustion process and has to be resolved.

You can connect your car to a computer and see if the sensors are ok. If they are good, that means that either your spark plugs are bad or your converter is broken.

Carbon Buildup

Carbon buildup is also very frequent if your catalytic converter is clogged up. This clogging will cause a buildup of carbon on the spark plugs, valves, exhaust ports of the head, exhaust manifold.

The carbon will start to build upon these components and will make your engine run poorly. If you notice one of these symptoms it is good to act quickly in order to prevent carbon buildup inside of the engine. Because if there is too much carbon you will need to do a lot of cleaning in order to bring the engine back to decent shape.

Rough Engine Idle

Rough idling along with misfires is also very noticeable when you have this problem. You will notice how at idling speed the needle of the tachometer will start to shake and act weird.

This is caused by the increased backpressure that is in the engine. The exhaust gasses can’t pass freely out of the exhaust and there is a backpressure buildup inside of the exhaust. This makes the engine run badly at idle and sometimes will even try to stall.

Poor Acceleration

Poor acceleration is also another symptom when there is too much backpressure inside of the exhaust system. When you apply the gas pedal and if there is a blockage inside of the exhaust the gasses will not be able to pass and your vehicle will accelerate very slowly.

This slow acceleration is caused by the huge backpressure. Basically, your engine is suffocating and trying to breathe. Since there are no exhaust gasses coming from the back that means less air is sucked from the intake. This results in poor acceleration and a possible engine stall.

Rotten Egg Smell

Rotten egg smell is also one of the symptoms. This rotten egg smell appears because in every fuel there is a small amount of sulfur.

When the catalytic converter is bad. This sulfur doesn’t get converted into something less smelly and toxic. That means it comes out from the exhaust freely and pollutes the environment with toxins. These toxins also produce a rotten egg smell. If your car produces this type of odor from the back, it means that there is something wrong with the converter. And that needs to be fixed as soon as possible.

Check Engine Light

Since your catalytic converter doesn’t measure the exhaust gasses as it should when its clogged. That means that you are also going to get a check engine light on your dashboard.

This is not always the case, but there are some serious cases where check engine light is a common occurrence when you are dealing with this issue.

If this light appears, then it is a good idea to get an OBD2 scanner and scan your car for codes. If you get some codes from your O2 sensor then it is a good idea to check the values that you get. These readings from the O2 sensor unfortunately cannot be accessed with basic scanners and you will need something more advanced. Or you can take your car to a mechanics shop.

If the sensor is still alive then it’s a good idea to remove the exhaust and inspect the converter and see the honeycomb structure’s overall health. If its clogged up. Then it will require a thorough clean.

Black Smoke

Black smoke is also one of the symptoms. It appears because of the rich fuel to air mixture caused by the bad readings of the O2 sensor.

It can also appear if the honeycomb structure inside of the catalytic converter is damaged. If the honeycomb structure is damaged it means that there is no catalytic converter and the gasses go into the atmosphere. Since the gasses are not purified, they are released as they come from the engine.

Rattling Noise

Rattling noise can also happen if your honeycomb structure starts to decompose and breaks apart in pieces. When you apply the throttle, if some weird sound comes from the underside of the car, then probably either the catalytic converter has shattered in pieces and these pieces are hitting the inside of the catalytic converter housing, or there is some exhaust leak.

Exhaust Leaks

Exhaust leaks can also happen if there is a damaged catalytic converter. If the converter is completely clogged. The exhaust gasses will try to find a way to escape. And if they cannot escape from the exhaust, they are going to create a lot of heat and backpressure. This pressure will start to burn the metal and create holes inside the exhaust. These holes will start to make some weird hissing noises. Exhaust leaks are fairly simple to spot.

Increased Emissions

If the honeycomb structure is damaged or destroyed. The car will also going to have bad emissions. You will most likely be going to fail to pass the emission test and you won’t be able to register the vehicle.

In order to register the car, you will have to return the vehicle in factory condition. And that means that you need to find a replacement for your catalytic converter and you are probably going to pay some money to replace it.

Although if the catalytic converter is still good, there are a couple of ways how you can bring it back in good shape. And we are going to cover some of these ways later in the article.

How To Check If Your Catalytic Converter Is Bad?

The only good way to be 100% sure that your catalytic converter is bad is to do the job manually. Sensors and other symptoms that appear will only tell you that you have a problem. Your job is to further investigate and see what is the cause of these symptoms.

By removing the catalytic converter, you are going to see the overall condition of the converter if it’s repairable or not. This is important because if the catalytic converter is only clogged there are ways to clean it. If the honeycomb structure is broken or gone, it means that you are going to have to buy a new catalytic converter.

If the converter is clogged, you will be able to clean it. And in the next chapter, we are going to explain how to clean a clogged catalytic converter.

How To Unclog A Catalytic Converter?

You can unclog a catalytic converter by following these simple directions. If the catalytic converter is not clogged too much, then you will be able to clean it without removing it. If the catalytic converter is extremely clogged, you will have to remove it.

1. Drive Your Car On The Highway

This is the simplest way to clean your catalytic converter. Take your car to the highway and give your car a good run for its money. Run the car at higher rpm 3000-3500 for 10-15 minutes. This will take your catalytic converter up to temperature and will ignite everything that is on the inside and clean the catalytic converter. Your catalytic converter basically gets clogged because it never gets up to temperature. And this is the way to solve this problem.

2. Try Cata Clean

CaClean is a product that reportedly will help cleaning the buildup in the honeycomb structure. You pour it into the gas tank and you give your car a good run for its money. This cleaner, along with the first method is a good combo to try if you don’t want to remove your catalytic converter from your car. Will save you a lot of money and effort and is a good idea to try. If none of these things work then it’s time to get your hands dirty.

3. Clean The Catalytic Converter Manually

This is the best way to clean your catalytic converter if you want to do this job permanently and not to worry if your converter will soon again develop some buildup.

If you want to perform this work, you will need to remove the converter from the vehicle. Give it a thorough inspection and if the honeycomb is good then you will be able to clean it.

Get a bottle of soap and water and fill a big bowl with it. Place your catalytic converter in the bowl and leave it overnight. In the morning the converter should be soaking wet and then it’s time to clean the catalytic converter.

In order to perform this work, you will need a pressure washer. A good pressure washer will do the job right. Place the pressure washed on one side and the dirt should start coming off from the other side. Then you should repeat this process again from the other side.

Repeat this process until you get the result that you wanted. This is a much safer method than those online methods with lacquer thinners and other products.

If this doesn’t work, then your catalytic converter is too far gone and will require to be replaced with a new one.

Cost To Replace A Bad Catalytic Converter?

Cost to replace a broken catalytic converter is high, and how high? Well, these converters can cost from $1300 up to $2000 depending much on the make and model. Some makes have cheaper catalytic converters and some have more expensive ones.

The labor will also going to cost you around $500. Because you need to get under the car in order to remove the bad part and this includes unbolting all the bolts. Most of these are rusty and will require a lot of time. That adds up pretty quickly. So, we can sum up that cost to replace a catalytic converter doesn’t come cheap.

Bad Catalytic Converter F.A.Q

We have discussed the symptoms of a broken catalytic converter and now let’s answer some common questions connected to this problem. These questions are one of the most frequent when it comes to this issue and for sure they deserve an answer.

What Happens When The Catalytic Converter Goes Bad?

A lot of things happen when the catalytic converter goes bad. And these problems include check engine lights, misfires, low engine power. Basically, your car will no longer be drivable and will feel ill. And what are you doing when you feel ill? You visit a doctor.

The best way to solve these problems is by visiting some skilled mechanic who will investigate the issue and will quickly make the right decision and what are going to be the next steps in fixing the car. If you have a good recommendation that will be great because good mechanics are not that easy to find. There are many who promise a lot and do not deliver anything from their promises except taking your money away from you.

Can A Bad Catalytic Converter Cause A Car To Stall?

Yes, a broken catalytic converter may make your car stall. This can be the result of a clogged catalytic converter. Your car’s exhaust gasses will not be able to escape from the exhaust and this can create a lot of backpressure inside of the exhaust manifold and act as an airbrake. When there is a lot of backpressure your car will slow down and when there is a complete seal inside of the catalytic converter the engine will shut down and stall.

You may be able to start your car again. But that will not going to last for long because not long after this, your car will going to come to a stop again and stall again. The only fix for this issue is replacing the catalytic converter or give it a clean if possible.

Driving Long Distance With Bad Catalytic Converter

Driving on long distances with a broken catalytic converter is not a recommended practice. If your car is not in perfect working order means that you are getting yourself in danger.

For example, you can get stranded somewhere in the middle of nowhere where is no roadside assistance available to help you out. You should keep this in mind.

You can never know where your converter is going to fail and cause some major headache for you. So, it’s the best idea to avoid traveling long distances.

Driving With A Bad Catalytic Converter

Also driving with a bad catalytic converter is not recommended at all. Your car is basically not roadworthy, especially if your catalytic converter is clogged more than 90%.

Your car can stall in the middle of the traffic and leave you to push your car to the side of the road and frustrate other drivers in the meantime.

Also, if the honeycomb structure is torn to pieces your car will pollute a lot and cause clouds of black smoke. This can be a signal for the police and if they notice your car, they are going to stop you and possibly give you a ticket.

They can also send you with the car to inspection and see how much your car pollutes and order you to get your problem resolved before you bring your car back on the road. So, the best thing is to deal with these issues on time and avoid some headaches.


In this article, we have covered a lot when it comes to the problem of a bad catalytic converter. This problem can make your car produce many different symptoms. Some of these symptoms as we said were: misfires, check engine lights, low power, engine stalling, black smoke, rattling noises.

All these symptoms indicate that you have a problem and need to further investigate the issue.

We also covered the cost to repair this problem and how to fix it These converters are expensive and can cost around $1,300 or even more in some cases.

We finally answered some common questions that many people want to know the answers to when they are having a bad catalytic converter. Once again, thanks for reading and if you like our articles, please subscribe to our social media in order to be in touch with our future article releases.