Bad Catalytic Converter Symptoms & How To Fix

The catalytic converter is a car component whose primary function is to stop the car from producing toxic gasses like carbon monoxide when operating.

A car owner needs to know the signs of a bad catalytic converter and how to solve them.

A bad Catalytic converter can be detected by several signs. A common sign is if the car doesn’t start immediately after the ignition key is turned on.

Another notable sign of a bad catalytic converter is that the car’s gas mileage decreases over time. You will notice this sign clearly when the tank of your car is filled with low-octane gasoline.

A quick fix to this is to use high-quality gasoline with a high-octane number. This practice will help flush out debris accumulated in the catalytic converter.

Catalytic converters are usually damaged due to low-cost and low gasoline to power the engine.

The most concrete way to prevent a bad catalytic converter is to choose high-quality and high-octane gasoline to run your vehicle.

If you fill your gas tank temporarily using high-octane fuel, you will be able to eliminate the catalytic converter’s waste. Explore more about the symptoms of a bad catalytic converter and how you can fix the issue by reading further.

Symptoms of a Bad Catalytic Converter

Symptoms of a Bad Catalytic Converter

Having established that changing a bad catalytic converter is expensive, it is essential you carefully maintain your vehicle’s catalytic converter to enable it to last for a long time.

Even so, you should be aware of the symptoms of a bad catalytic converter; here are some of them:

1. Difficulty Starting the Car

After turning on the ignition of a vehicle, if the engine does not start immediately, you should examine the vehicle to determine the cause of the problem.

One of the few causes of a hard start in a vehicle is a bad catalytic converter. Catalytic converters can become clogged with dirt, which makes it difficult for exhaust gases to be easily removed from their cavities.

If this happens, the vehicle’s engine will be forced to stop due to elevated exhaust pressure in the chamber. If you notice that your car, initially running smoothly, begins to stall and then whine, then there is a high chance you have a blocked catalytic converter.

2. Failed Emission Test

If your vehicle fails an emissions test, this might be because you have a bad catalytic converter.

The catalytic converter is meant to process bad exhaust gases before they are released into the environment. If your vehicle fails an emissions test, it shows that the catalytic converter isn’t doing its job.

The catalytic converter is one of the most critical components of your vehicle as it makes sure the car releases healthy emissions. If the catalytic converter doesn’t perform this function, it has a problem.

3. Drop in Fuel Efficiency

A constant drop in fuel efficiency is another symptom of a bad catalytic converter. If you notice that you are spending more on gas than usual, this implies a decrease in the mileage of your fuel tank.

If this drop in mileage continues, you are advised to quickly check if your catalytic converter is malfunctioning as it might be blocked with carbon deposits.

If there is a limited amount of oxygen entering the engine’s combustion chamber, it will be more difficult for the engine to work as there will be a low oxygen level. This will make the engine consume more fuel.

4. Illuminated Check Engine Light

Modern automobiles are equipped with electronic devices that monitor all the parts of the vehicle as you drive. One of them is the “Check Engine light.”

If the Check Engine light of your vehicle sets off when you are driving, do well to examine your catalytic converter to check for problems.

The Check Engine light sets off mostly when the air-to-fuel ratio in the combustion chamber in the vehicle is incorrect.

The vehicle’s tailpipes feature sensors that can dictate if the air-to-fuel ratio is accurate. If it is not accurate, that means your car is releasing unburned fuel, which pollutes the environment.

The triggered Check Engine Light signifies that the diagnostic system has examined the vehicle, and it has been discovered that the catalytic converter isn’t functioning as it should.

Also Read: Service Engine Soon Light: Meaning, Causes (How to Fix it)

5. Poor Acceleration

You might have a bad catalytic converter if your car does not accelerate correctly when you hit the gas pedal.

The emissions gases are trapped inside or in front of the catalytic converter due to carbon deposits blocking it.

While driving, if you observe that your vehicle stalls or completely fail when you speed up, then the fuel could not reach the engine. A common cause of this is a blocked catalytic converter. Removing waste or unblocking the converter is a simple way to resolve this problem.

It is advisable to check the catalytic converter frequently. In this way, you can easily maintain and protect the converter from more serious issues.

6. The Smell of Rotten Egg

Your vehicle’s exhaust system might smell like rotten eggs if you have a bad catalytic converter. Contaminated converters develop hydrogen sulfur (H2S) that smells like rotten eggs.

Additionally, a bad catalytic converter can also lead to ammonia (NH3) emission, which can be perceived within the exhaust system.

7. Rattling Noise

If you hear a rattling sound whenever you start your vehicle, this might mean you have a problem with your catalytic converter.

The catalytic converter is a significant component of your vehicle’s exhaust system. The catalytic converter can fail if other parts of the exhaust system are not functioning correctly. A good example is an exhaust leak which can cause the converter to fail.

For this reason, it is also advisable to check other parts of the exhaust system if you have a problem with the engine’s exhaust.

How to Repair a Bad Catalytic Converter

How to Repair a Bad Catalytic Converter

If you notice that your catalytic converter is malfunctioning, the first thing you should do is repair it. Instead of replacing it, you should first determine what caused it to malfunction.

Remember that replacing the catalytic converter can be very expensive. Here are some ideas for repairing a failed catalytic converter:

1. Clean the Catalytic Converter

The quick and simple way to fix a failed catalytic converter is to clean it. Here is a cost-free method. All you have to do is remove the catalytic converter from its location and remove all the debris stored up at both ends of the device.

You can also soak the device in water mixed with detergent and degreaser or a hot water mixture.

You must soak the catalytic converter if the contaminants are hard. This will help break down the debris, making it soft to be easily removed from the converter.

After soaking and cleaning the catalytic converter, you should let it dry thoroughly before putting it back into the vehicle.

2. Use Fuel Additives

One of the functions of fuel additives is to remove undesirable deposits that gather up around the catalytic converter.

If this additive is introduced into the car’s engine, it will help prevent the accumulation of debris in the catalytic converter, making it work efficiently.

3. Use High-Quality Fuel

The quality of fuel used in the car’s engine plays an essential role in the maximum functioning of the catalytic converter.

A fuel with high octane or a premium-quality fuel is an excellent solution for a bad catalytic converter.

You will notice the difference after a couple of drives if you put this method into practice.

4. Repair Exhaust Leaks

Exhaust leaks can also influence the performance of your car’s catalytic converter. Due to this fact, to repair a failing catalytic converter, you also have to repair corroded flex pipes, worn-out gaskets, and any other component of the exhaust system that might cause a problem.

5. Perform the “Italian Tune-Up.”

The “Italian tune-up” involves running the car hard enough to increase the temperature of the catalytic converter to a maximum of 1,832degF to 800degF.

The car can be pushed to its limit for a couple of miles by subjecting it to rapid and sudden accelerations.

When you do this, the catalytic converter gets heated up to the point it can melt the accumulated debris within its matrix and the cylinder heads, intake heads, oxygen sensors, and exhaust.

Check out this video for more tips on how to repair a bad catalytic converter

Also Read: Bad Oil Pump Symptoms & Replacement Cost

Related Questions – Bad Catalytic Converter

What Does a Bad Catalytic Converter Sound Like?

A bad catalytic converter makes Rattling noises. The catalytic converter comprises tiny, honeycomb-shaped parts that make a rattling sound when damaged. If you have a bad catalytic converter, it will produce rattling noises, and this can be noticed, especially when the vehicle is starting.

How Do You Diagnose a Bad Catalytic Converter?

The easiest way to check for a bad catalytic converter without special tools is to lose the connection between the exhaust pipe and at the top of the catalytic converter or remove the oxygen sensor. If the engine’s condition improves while you run it using the loose pipe or these connections(oxygen sensor), you might behave a catalytic converter issue.

How Long Can I Drive With a Bad Catalytic Converter?

It is possible to drive with a bad catalytic converter for a long time. However, if your catalytic converter fails completely, you will not be able to drive your vehicle. Mainly, the device might become damaged entirely over time and will need to be replaced immediately.

How Much Does It Cost to Replace a Catalytic Converter?

In most cars, replacing a bad catalytic converter will cost between $950 to $2500(parts and Labor inclusive). This price depends on your car’s make and model and your location.


It is easy to know if your catalytic converter is about to fail through the symptoms it manifests. A prevalent indicator is when the engine is had to start even as you turn on the ignition.

Another obvious sign is that the car’s engine consumes more fuel than usual. This indicates that your car fuel efficiency is very low, resulting from a bad catalytic converter.

There are other notable signs to look out for; however, you must repair or replace the bad catalytic converter immediately if you notice any of these signs.

The easiest solution to this problem would be to fill your vehicle’s tank several times with high octane gas. After that, examine the car to see if the issue has been resolved. If not, refer to this article to check for other methods to fix a bad catalytic converter.

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