Symptoms Of A Bad Catalytic Converter

How do you tell if your car has a bad catalytic converter? Do you suspect that it is cracked? When should it be replaced? Here's a simple guide.

The catalytic converter is making the world a better place by converting toxic elements to non-toxic ones before being released into the environment through the exhaust.

A catalytic converter that does not work properly will almost always present symptoms that will give you hints that something is wrong.

Here, we will further elaborate on what these hints can be and if there is something you should be particularly careful about if these symptoms occur.

It should be noted that, in most cases, a catalytic converter that has gone bad is usually due to another reason than the catalytic converter itself.

But do not worry, we will cover everything you should know about bad catalytic converters.

Let’s jump into it!

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What Is A Catalytic Converter?

Catalytic Converter

The catalytic converter works passively to convert toxic gases into less toxic ones.

It does so by containing reduction- and oxidation catalysts, meaning that it converts the toxic gases without itself being affected.

This is done with the help of some precious metals, namely platinum, rhodium, and palladium.

Unfortunately, because of these precious metals, catalytic converters are a target for theft as they can easily be sold, and a small buck can be made by thieves.

The catalytic converter is located underneath your car, where all the exhaust gases from the engine go through before they are exposed to the environment.

Symptoms Of A Bad Catalytic Converter

Can You Drive Without A Catalytic Converter

As previously mentioned, a bad catalytic converter is due to another underlying problem that is not directly correlated with the catalytic converter.

For example, a blown head gasket, oil leak, or similar issue may get foreign substances into the catalytic converter, clogging and/or damaging it, which then calls for a replacement or a clean-up.

Below, are the most common symptoms that are associated with a bad catalytic converter.

Check Engine Light

How To Reset A Check Engine Light

The most common symptom of a bad catalytic converter is an illuminated check engine light on the dashboard.

Here, just like any other problem with a car, an OBD2 scanner will be very handy as it can narrow down the potential culprits for the check engine light.

If the catalytic converter is faulty, a code will likely be shown that specifically points out that something is not right with it.

There may also be some related codes that may show up, for example, a code related to the downstream O2 sensor.

In some cases, the catalytic converter may not be the problem at all. Instead, the O2 sensor may be faulty and give an incorrect reading.

Power Loss

A noticeable symptom almost always present when the catalytic converter has turned bad is power loss, and the reason is very simple.

As the catalytic converter does not work properly, either by damage, being clogged up, or both, it will hinder the exhaust from escaping, causing increased backpressure.

It can be seen as the faulty catalytic converter is strangling the engine, not letting it produce power as efficiently.

The power loss is most noticeable when accelerating, such as when trying to get into the highway. If it is noticeable, you will most probably also be notified by a warning light.

Failed Emission Test

car exhaust pipes

The catalytic converter plays a central role in the emissions of your car, and it is the main reason why they exist in the first place.

A faulty catalytic converter would therefore be ineffective in converting the harmful chemicals, leading to a failed emission test.

Failed emission test is not necessarily contained to a bad catalytic converter. Keep in mind that a bad catalytic converter is most likely due to another reason.

It is therefore important to fix any other potential problems and have in mind any other problems you have had and recently resolved, such as an oil leak or a bad O2 sensor, for example.

What Causes A Catalytic Converter To Go Bad?

Catalytic Converter

Knowing what can make a catalytic converter stop working is extremely important. If you do not know the reason why the catalytic converter does not work, a replacement will not do any good.

For example, a car that leaks excessive oil in the combustion chamber, which then gets into the catalytic converter, will lead to the new catalytic converter failing unless the leak is resolved.

You should, therefore, always make sure that the initial problem is resolved before replacing the catalytic converter.

Also note that replacing a catalytic converter is regulated to a great extent, meaning you should make sure that the catalytic converter you are replacing is appropriate.

Here are the main causes for why a catalytic converter goes bad:

Fuel In The Catalytic Converter

catalytic converter repair

The catalytic converter is designed to receive exhaust from the engine and dispose of it in a non-toxic way.

While there is a good safety margin for other substances to get into the catalytic converter, an excessive amount of unburnt fuel will damage the catalytic converter.

To reach amounts that will damage the catalytic converter, a greater amount of fuel must reach the converter, which is usually due to faulty spark plugs or ignition coils.

Problematic spark plugs and/or ignition coils will make the combustion ineffective or, in severe cases, absent, causing the fuel to run directly to the catalytic converter.

A catalytic converter has an operating temperature of more than 1000 Fahrenheit, which risk igniting the gasoline in the catalytic converter itself, causing damage.

If there is a problem with the spark plugs or ignition coils, there will most definitely be more symptoms that will occur, as well as an OBD2 code warning that something is wrong.

Oil & Antifreeze Leak

Engine oil stains of car Leak under the car when the car is park

Similar to having fuel in the catalytic converter, having oil or antifreeze is not any better.

While it may be logical that some small amount of fuel does not get ignited and reaches the catalytic converter, oil and antifreeze should not reach it in any amount.

The cause of oil and antifreeze reaching the catalytic converter is almost always due to a leak and should be dealt with immediately.

In many cases, a blown head gasket is the cause, which can lead to detrimental damage to the engine if it is not resolved quickly.

It is, therefore, very important to exclude a blown head gasket to limit any potential damage.

This does not mean that the only reason why oil or antifreeze reaches the catalytic converter is a blown head gasket, something less serious may be the reason, but it should never be ignored.

Oil and antifreeze will clog the catalytic converter, not only leading to poor emissions but also potential damage to the engine due to backpressure.

To put it simply, a clogged catalytic converter will block the only way the exhaust can escape the combustion chamber.

This, in turn, will cause resistance in the form of pressure building from the catalytic converter to the combustion chamber.

The pressure can damage multiple parts of the engine, including the pistons, head gasket, valves, and more.

Faulty O2 Sensor

Bank 1 vs Bank 2 Oxygen Sensor Location

A faulty O2 sensor may trick the ECU, causing it to make the engine run too rich or lean, triggering problems for the catalytic converter.

For example, if there is too much fuel in the combustion chamber, there is a significant risk that a lot of the extra fuel will get into the catalytic converter.

An engine that runs lean due to the O2 sensor is not as serious but can make your catalytic converter ineffective, not doing its work properly, and can cause you to fail an emission test.

External Damage

Car damaged in an accident

Even if you have maintained your car properly, you may be unlucky to hit something on the road, damaging the catalytic converter.

If this is the case, you are most definitely looking for a catalytic converter replacement, as cleaning it will not help with any structural damage.

Unfortunately, this means that the repair bill will be close to $1000.


The catalytic converter makes toxic gases non-toxic before they get released into the environment.

A bad catalytic converter can be bad not only for the catalytic converter itself but also for the whole engine due to the backpressure that forms.

Replacing a catalytic converter can be quite expensive, therefore, unless the catalytic converter has not been physically deformed, cleaning it can be a viable and more economical choice.

Unfortunately, sometimes that may not be enough, and a replacement is needed.

The most important thing to remember is that a catalytic converter that turns bad does so because of external factors, meaning that the catalytic converter is not the cause of the problem.

Before replacing or cleaning the catalytic converter, it is very important to diagnose and solve the problem that caused the catalytic converter to go bad.

In most cases, this is due to excessive fuel reaching the catalytic converter, an oil leak, an antifreeze leak, or a faulty O2 sensor.

If the catalytic converter has been physically deformed, it is most probably due to hitting something while driving the car, which does not require any further intervention than replacing it.

Symptoms of a bad catalytic converter overlap with many other problems, and, more often than not, a bad catalytic converter is a byproduct of the main problem.

Illuminated check engine light, power loss, poor acceleration, and a failed emission test are the most common symptoms seen.

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Bajro Muric

Bajro is a big car enthusiast with a love for writing and teaching. He writes about anything regarding cars - from common problems and fixes to racing.

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