What Happens When The Catalytic Converter Goes Bad (Symptoms)

What Happens When The Catalytic Converter Goes Bad

Have you ever asked yourself or your friend what happens when the catalytic converter goes bad? Well, you are at the perfect place to find the answer to such a question.

The catalytic converter in your vehicle is vital for ensuring that no harmful compounds escape into the environment.

In the vast majority of cases, you are never going to need to worry about issues with your catalytic converter. There isn’t much that can go wrong with them. This means that they will often last as long as your car lasts.

Still, it may be worth knowing what could potentially happen the catalytic converter goes bad. There are going to be a few noticeable signs and symptoms of a bad catalytic converter, which included:

1.         You Will Start To Leak Pollutants Into The Environment

2.         The Check Engine Light Will Switch On

3.         Dark Exhaust Smoke

4.         Rotten Egg Smell

5.         Extreme Heat Under The Vehicle

6.         Reduced Vehicle Acceleration

7.         Higher Fuel Costs

8.         Sluggish Engine

9.         You Struggle To Start Your Vehicle

You Will Start To Leak Pollutants Into The Environment 

Without going into too much scientific detail, the job of the catalytic converter is to ‘clean’ the emissions from your vehicle. This is to ensure that you are not pumping too much pollution into the environment.

Now, you are probably not going to notice that the pollutants are leaking yourself. Not unless you have a test kit, of course.

However, when you take that vehicle to be checked over, they will almost always carry out an emissions test on your exhaust. They will then be able to tell whether your catalytic converter is broken.

For most, this is the only time that they are ever going to get a hint that the catalytic converter is broken.

Read also: Can a Catalytic Converter Unclog Itself? (Clear a Clogged)

The Check Engine Light Will Switch On

The check engine light can get a little bit annoying. Often, it is because the check engine light doesn’t actually tell you what is wrong. It just tells you that something is wrong.

Inside of modern vehicles (and we are talking vehicles built within the last few decades here), there will be constant checks by the vehicle on the catalytic converter.

If the tests believe that the catalytic converter is broken, then that check engine light may switch on.

Of course, you shouldn’t ignore the problem if that check engine light switches on. You are going to need to make your way to the nearest garage.

If it isn’t the catalytic converter going bad, then it is probably going to be a serious issue elsewhere in the vehicle that needs to be rectified.

Read also: Can a Clogged Catalytic Converter Cause Rough Idle? (Misfire, Hard Shifting, Overheating, )

Dark Exhaust Smoke

Your exhaust should be letting out smoke that is white or gray. 

If you start to notice that the smoke coming out of the exhaust is black, or close to black, then you know that you have a problem with the catalytic converter. It means that there is nothing that is actually ‘cleaning’ that exhaust smoke.

Rotten Egg Smell

Started to spot a rotten egg smell coming from your exhaust?

Well, this is one of the issues caused by a bad catalytic converter. Again, this is all down to the fact that the catalytic converter is not cleaning the exhaust gases as effectively as it should be. 

If your vehicle has got to this point, then you know that you have a pretty serious problem. You are going to want to have an emissions test carried out as soon as possible.

Extreme Heat Under The Vehicle

If your catalytic converter is clogged, this can cause a build-up of heat underneath the vehicle. When you stop the vehicle and place your hand underneath (don’t touch the underside of the vehicle), it will be extremely hot.

Of course, if your vehicle has managed to get to this point, then that is a bad thing. It means that the area around the catalytic converter has become seriously clogged.

Of course, this extreme heat is going to be putting a ton of pressure on the various components in your vehicle. This could end up having a knock-on effect where, eventually, you end up having a massive repair bill.

This is why it is so important to deal with problems like this very quickly. The repair of a catalytic converter should be fairly affordable.

Repairing the rest of the vehicle due to that heat damage? Probably not quite so much.

Read also: How to Clean a Catalytic Converter With Seafoam (Best Tips)

Reduced Vehicle Acceleration

In rare cases, a clogged catalytic converter can result in poor acceleration. You may struggle to push your vehicle beyond a certain speed.

This is because a clogged system will put a lot of pressure on the engine. Sometimes, the pressure on the engine is going to be so high that it just doesn’t have the power to make you go forward.

We cannot stress enough that this is going to be a rare problem. For most, the only time that this is really going to present an issue is if they have ignored every other sign that their catalytic converter is going bad.

Read also: How to Remove a Catalytic Converter Without Tripping the Check Engine Light

Higher Fuel Costs 

As we just said, a catalytic converter that has been clogged is going to put pressure on the engine. This means that it needs to work a lot faster.

Part of the reason why your engine has to work harder is that there isn’t enough oxygen flowing it for it to work efficiently.

Of course, the harder your engine needs to work, the more fuel it will require.

This means that if you have a bad catalytic converter, then you may notice a huge increase in your fuel costs. In fact, they could be an extreme increase if the problem is quite serious.

Now, do bear in mind that higher fuel costs over the life of your vehicle are completely normal. This is because as an engine ages, it tends to be more expensive to run.

However, if they have suddenly gone up and you are noticing poor fuel efficiency, then you know that you have a problem, likely with the catalytic converter. Gradual increases are just normal. 

Sluggish Engine

A bad catalytic converter can lead to a sluggish engine. It may chug about a little bit. You may notice small speed drops here and there. You may notice that the speed doesn’t really climb at a gradual rate.

If you have been driving your vehicle for a while, then you will know when the engine has become sluggish. it is something that feels incredibly noticeable when you are driving.

Again, this is not always going to be an issue with the catalytic converter. There are a lot of problems that can lead to a catalytic converter.

However, when your catalytic converter does go bad, then you can bet your bottom dollar that a sluggish engine is going to be one of the issues that crops up.

You Struggle To Start Your Vehicle 

Starting your vehicle? Well, you may find that it struggles a little bit to start up. You may have to give it several tries before you kick that engine into life.

In exceedingly rare cases, you may find that your vehicle doesn’t start at all. This is because the area around the catalytic converter is so clogged that there is no way that the engine is ever going to start.

However, once again, this is something that is only going to happen if you have ignored every other warning sign that there is a problem with your catalytic converter. 

Does the catalytic converter always need replacing when it is bad?

Sometimes, yes.

However, surprisingly, a lot of the issues that people discover with their catalytic converter will not actually be down to the catalytic converter itself. Instead, the problems are going to be caused by bad spark plugs. If you replace those, then the catalytic converter may start to work properly again.

Of course, it may still always be worth running your vehicle by a garage. This way they will be able to tell you for 100% certain whether there is a long-term issue that needs to be rectified.

Remember, the earlier that you deal with the problem, the cheaper it is going to be to fix. A bad catalytic converter is not a good thing, but it is not the end of the world.

It only becomes the end of the world when you have let the problem get out of hand and it has had a knock-on effect for the rest of your vehicle. 


In most cases, your vehicle is going to continue to function, even if the catalytic converter is broken. Although, this doesn’t mean that you should continue to drive the vehicle. The catalytic converter plays an incredibly important role in protecting the planet.

So, if you do notice one of the issues that we highlighted on this page, then you really should be making a beeline for your local garage. This is a problem that you really are going to need to have dealt with. 

Remember, not all of the issues that we have highlighted on this page will have been caused by a bad catalytic converter.

After all, a vehicle is an incredibly complicated piece of kit. This is why we wouldn’t really recommend that you diagnose the problem yourself. It is something that is going to be almost impossible to do.

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Jeff is an automotive technician, technical writer, and Managing Editor. He has held a lifelong passion for cars, with a particular interest in cars like the Buick Reatta. Jeff has been creating written and video content about transportation, automotive, electric cars, future vehicles as well as new, used for more than 18 years. Jeff is based in Boulder, Colorado.

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