Bad Catalytic Converter – Symptoms and How to Fix

The catalytic converter prevents the formation of toxic gases like carbon monoxide inside your car while you are driving. But what if it goes bad? You need to know about bad catalytic converter – symptoms and how to fix them.

Several symptoms will show you if your catalytic converter is going bad. One of the most common symptoms is if the engine does not start right after turning the ignition key on. Another symptom is if your car’s gas mileage keeps on going down. 

These symptoms will show up if you usually fill up your gas tank with cheap low-octane gas. As such, the first bad catalytic converter fix that you can perform is to run your car on a few tanks of good high-octane gas. Hopefully, this will flush out the accumulated debris in the converter.

Read on to learn more about the symptoms of a bad catalytic converter and how you can correct the problem.

Bad Catalytic Converter

Bad Catalytic Converter

You don’t have to be surprised when your catalytic converter goes bad. Several symptoms will alert you about it. One of the most common signs is if your engine does not readily start when you turn on the ignition key.

Another typical symptom is if your car’s fuel efficiency continuously goes down. This means you are forced to spend more fuel for the same mileage you are driving than before.

Catalytic converters usually go bad because of cheap and low octane fuel to run the engine. So, a straightforward fix to this problem is to use good and high octane fuel for your engine. If you fill up your gas tank a few times with high octane fuel, you can flush out the catalytic converter’s deposits.

Catalytic Converter Price Lookup by Serial Number

Functions of a Catalytic Converter

Processes All Exhaust Gases That Leave the Engine

As part of the exhaust system, the catalytic converter performs an important role in the efficient running of the engine. It processes all exhaust gases that leave the engine before they are allowed to escape into the environment.

A Bad Catalytic Converter Can Lead to Engine Failure

If the catalytic converter can go fully bad, it can lead to total engine failure. However, it does not mean that you need to replace the device when it shows signs of a bad catalytic converter. It is possible to correct the problem by fixing the converter.

Uses Heat and Metals to Oxidize and Lower Exhaust Emissions’ Toxicity

The catalytic converter takes care of the unburned gases from the engine’s combustion chambers. This device uses high heat and precious metals to oxidize and lower the toxicity of exhaust emissions.

Components of a Catalytic Converter

These unburned gases are changed into safer compounds such as nitrogen (N2), carbon dioxide (CO2), and water (H20). But catalytic converters are quite expensive due to their special construction. They are made using rare metals like rhodium, palladium, and platinum.


On average, they cost more than $1,000 apiece. So, replacing it with a good vs bad catalytic converter can be an expensive proposition.

Symptoms of a Catalytic Converter Going Bad

bad catalytic converter signs
Hard to Start Engine.

Now you know that catalytic converters are not that cheap. So, you need to take care of your car’s catalytic converter so you can use it for a long time. You need to know how to tell if the catalytic converter is bad. Watch out for the following signs:

1. Hard to Start the Engine

If you find that you are constantly having difficulty getting up in the morning, you have a problem with your system. It’s the same with your car’s engine. If you can’t start it right after you’ve turned on the ignition, you need to check what’s causing the hard starting problem.

One of the common causes of this issue is a bad catalytic converter. It could be clogged with deposits. If it is clogged, exhaust gases will remain in its cavities.

This situation causes the engine to stall because of the increased exhaust pressure inside its chamber. So, if you notice that your car seems fine at the start but then begins to stall or sputter right after, your catalytic converter may be clogged.

2. Failing Car Emission Test

If you brought in your car for an emission test and it failed, it could be the fault of the catalytic converter. It is supposed to clean the exhaust gases before emitting them through the exhaust pipe. The result shows that it did not.

It is the main component of your car that enables your car to give off healthy emissions. The failed result shows that it is not working as it should. Its causes could be any of the following:

  • Contaminated coolant or motor oil,
  • Increased carbon deposits, or
  • Other relevant reasons.

3. Drop in Fuel Efficiency

Another typical symptom of a failing catalytic converter is a continuous drop in fuel efficiency. If you notice that you have been spending more money to buy gas, this means your fuel mileage is beginning to drop.

Before it drops any further, you need to check the catalytic converter. It might be clogged with carbon deposits. There will be less oxygen coming into the engine’s combustion chambers if it is. The less oxygen, the harder your engine works. In this condition, the engine will consume more fuel.

4. Triggered Check Engine Light

If you have a modern car, it will be fitted with electronic instruments that monitor all its parts while you are driving. One of the instruments you need to be aware of is the Check Engine light.

When the Check Engine light is triggered while you’re driving, you’ll have to check the catalytic converter. Why? If the air to fuel ratio entering the engine’s combustion chambers is not right, the check engine light will turn on.

There are sensors in your tailpipe that determine if this ratio is correct. If it is not, it means your car is emitting unburned fuel that pollutes the atmosphere. Your car’s diagnostic system has tested the catalytic converter, and it says that it is not doing its work.

5. Poor Acceleration

When you step on the gas and your car fails to accelerate correctly, it could be a sign of a bad catalytic converter. It could be clogged with carbon deposits. Perhaps the exhaust gases are trapped in front or inside the converter.

If you accelerate and your car hesitates or stalls completely, the fuel may not reach the engine. This is also caused by a converter that is clogged. You can remedy this situation by just unclogging or removing the debris inside the converter.

This is why you need to check your catalytic converter’s condition regularly. You can prevent the more severe problem from happening to your converter and the engine by checking it regularly.

6. Smell of Rotten Egg

When you smell rotten eggs in your exhaust system, it could be a sign that your catalytic converter is contaminated. Hydrogen sulfide (H2S) forms inside a contaminated catalytic converter. In addition, you will also smell Ammonia (NH3) if the converter is not functioning properly.

7. Rattling Noise

If you hear rattling noise when you start the engine, it could also signify that the catalytic converter is bad.

The catalytic converter is a component of your car’s exhaust system. So, it could also fail if another part of this system fails. For instance, bad spark plugs will cause the converter to fail. That’s why you must consider other parts of the exhaust system if you are encountering exhaust engine problems.

Again, what are the symptoms of a bad catalytic converter? Some of the symptoms of a bad catalytic converter include a slow engine, the presence of dark exhaust smoke and the smell of rotten eggs, decreased acceleration, and too much heat from under your car.

Driving With A Bad Catalytic Converter – What Happens and How Long Can You Drive

How to Fix a Catalytic Converter

bad catalytic converter smell

If a catalytic converter does not function properly, your first move is to fix it. It is not wise to replace it right away before analyzing why it’s not working well. Remember, it is quite expensive to replace this device.

Here are some common fixes to a failing catalytic converter:

1. Clean the Converter

Cleaning the converter is the cheapest way you can solve this problem. Remove the converter from its place. With a pressure washer, blast out anything that contaminates its matrix. Flush out all the debris from both ends of the device.

You can also soak the converter in a laundry detergent, degreaser, and hot water mixture. Soaking the converter is necessary if the contaminants are hard. The soaking mixture will dissolve them into softer materials that you can easily remove. Dry the converter entirely before reinstalling it to its place.

2. Use High Quality Fuel

Feeding your engine with a higher quality fuel or higher octane level is an effective way to restore the catalytic converter to its full efficiency. Run your car with this upgraded fuel for a few full tanks, and you will see the difference.

3. Use Fuel Additives

Adding a fuel additive to your gas intake can also help remove the unwanted deposits accumulated on the catalytic converter, enabling it to function efficiently.

4. Perform the “Italian Tune-Up”

To do the “Italian Tune-Up,” you have to drive your car by pushing it hard enough so that you can bring up the converter to its optimum working temperature, which is between 800°F and 1,832°F.

Drive your car harder for a few miles with several hard and sudden accelerations. It will heat the converter enough to burn off the deposits in its matrix, as well as those that have accumulated in the intake, exhaust, cylinder heads, and oxygen sensors.

5. Repair Exhaust Leaks

Exhaust leaks can also affect the performance of the catalytic converter. So, to restore a failing converter, you may have to repair corroded flex pipes, replace worn-out gaskets, and fix anything in the exhaust system that may contribute to the problem.

Why Do Catalytic Converters Go Bad

You also need to know why a catalytic converter goes bad. This will help you avoid practices that may compromise its full functioning. What are the usual reasons why converters go bad?

1. Engine Issues

Most catalytic converters go bad because of engine issues. For instance, unburned fuel will settle in the catalytic converter if excess fuel enters the engine. If the condition is not corrected, the converter will be clogged with soot.

You must ensure that only the correct air to fuel ratio is entering the combustion chambers so that there will be no unburned fuel that will go to the exhaust system. Other engine issues include:

  • Bad spark plugs,
  • Failing oxygen sensors, and
  • Incorrect engine timing.

2. Failed Gaskets

Sealing is critical in the operation of the engine. The proper seal ensures that oil and fuel will not mix with the coolant as the engine burns fuel in the combustion chambers. Failed gaskets, worn piston rings, and bad valve seals are the common culprits of failing catalytic converters and bad exhaust systems.

How to Take Care of a Catalytic Converter

Since replacing a bad catalytic converter is very expensive, taking care of your current converter is the best move you can take.

Here are some tips for making your converter last longer:

  1. Observe regular preventive maintenance of your car’s exhaust system.
  2. Fix the Check Engine Light problem right away.
  3. Drive your car regularly and often.
  4. Use high octane fuel.

Catalytic Converter Theft Prevention – Full Guide

Conclusion: Bad Catalytic Converter

You will know if your car’s catalytic converter is going bad by the symptoms it is showing. One of the most common signs is a hard-starting engine. The engine does not readily start when you turn on the ignition key.

Another symptom is you are gassing up more often than before. This means the fuel efficiency of your car has gone down. These are not all the symptoms, but if you experience them, you need to fix or replace the faulty catalytic converter.

A straightforward fix you can perform is to try filling up your gas tank with high octane fuel a few times. And then check if the catalytic converter problem goes away. If not, you have to try the other fixes discussed in this article.

Read next:

What Causes a Catalytic Converter to Go Bad?

P0420 Code

How to Fix Malfunctioning O2 Sensors

What Are the Symptoms of Bad Spark Plugs?

How Much Does It Cost to Fix an Exhaust Leak?

Can You Drive a Car Without a Catalytic Converter?

Muffler Vs. Catalytic Converter