Symptoms of a Failing Catalytic Converter

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Your catalytic converter is often an overlooked part of your exhaust system. But this part of your car has an important role. 

If your catalytic converter goes bad, not only will your car begin to damage the environment it will also damage your car. 

Don’t worry. I’m here to help you avoid this. In this post, I will go over the 10 signs of a failing catalytic converter and what you can do about it. 

Let’s get started. 

1. Check Engine Light Turns On

One of the first signs that there is an issue with your catalytic converter is when you check engine light turns on. 

The check engine light can turn on for a variety of reasons. You can use the check engine light as a sign that you need to inspect your car further. 

The best way to determine the reason you check engine light turns on is using an ODB reader. An ODB will give you a code that tells you what is causing the issue with the car. 

You can use a traditional ODB scanner, or you can use a Bluetooth ODB scanner. A Bluetooth ODB scanner typically connects to your phone using an app. 


Code P0420 

When using an ODB scanner, the code P0420 tells you there is an issue with your catalytic converter. 

This code applies to all vehicles built before 1996.  

You should reach out to a mechanic to determine the reason for your failing catalytic converter. 

You should also reach out to a professional if you receive another ODB code so you can address that issue. 

2. Your Vehicle Won’t Start

Another common sign that your catalytic converter is bad is if your car is not starting.

This will typically happen because excess fumes will become trapped inside your exhaust system with a failing catalytic converter. The trapped exhaust will cause a buildup of pressure inside your engine.

This will make it difficult for your car to start. In some cases, your car might start, but it will cause your engine to stall or turn off after a short period.

If your catalytic converter is only slightly clogged, this issue might not be less severe at first. At first, you might experience stalling or difficulty starting your vechile.

As the issue progresses, these issues will get worse, and your car may stop starting completely.

3. Failed Emissions Test

The catalytic converter turns the harmful exhaust fumes of your engine into safe gasses that will have a less harmful impact on the environment.

When your catalytic converter is failing, it won’t be able to convert these fumes to safe gasses. This will cause you to fail your emissions test.

You don’t have to take your car to a professional to test your emission. Most standard ODB readers and Bluetooth readers can test your emissions.

You will need to read your ODB manual to determine how to find the emissions test or code.

This will typically require you to drive the car before it can give you a proper reading.

4. Rotten Egg Smell Coming From Your Car

If you smell a rotten egg smell or sulfur smell coming from your car, this is a sign that your catalytic converter is not working properly.

The rotten egg smell comes from the hydrogen sulfide that is produced during the combusting process. A functional catalytic converts the hydrogen sulfide into sulfur dioxide, which is orderless.

When the catalytic converter is not working properly, it won’t convert the hydrogen sulfide into sulfur dioxide. As a result, your car will emit a rotten egg smell when it’s on and running.

The smell will be strongest at the back of your car or the side of your car, depending on where your exhaust is pointing.

5. Dark Smoke Coming From Your Exhaust

Another common sign of a bad catalytic converter is black smoke coming from your exhaust. Again, the intensity can vary, but this is a major concern if there is dark smoke coming from your exhaust.

One of the main reasons a failing catalytic converter causes black smoke is because it’s clogged. When this happens, if your car is running on a rich fuel mixture, then your car will generate dark exhaust.

In some cases, the catalytic converter can eliminate small amounts of dark smoke. With a bad catalytic converter, any black smoke will come straight out of the exhaust.

6. Engine Misfires

Another sign of a bad catalytic converter is engine misfires. This happens because a bad catalytic converter can cause your engine to overheat.

This can cause your engine to combust when it’s not supposed to. Also, this will cause certain pistons to misfire.

If you are not familiar with a misfire, a misfire sounds like a popping or backfiring sound. These sounds are sudden and occur quickly.
Below is a video of what you might hear when your engine misfires.

7. Rattling Noise

Catalytic converters have a honeycomb structure inside. The material and structure inside the catalytic converter allow it to eliminate harmful chemicals inside your exhaust.

When the structure becomes damaged either due to heat or damage, this will cause your catalytic converter to go bad. Typically when this happens, you will be able to hear a rattling noise.

You will hear this when you are accelerating or going over a bump. When there is severe damage, you will hear this rattling noise even when you are idle.

8. Damaged or Discolored Housing

If there is damage on the outside of the catalytic converter, this is a sign that its bad.

One of the most common signs is if there is clear damage, such as a dent, hole, or crack in the housing. Typically, damage on the outside will cause damage on the inside and reduce its effectiveness.

Another common sign is if the housing of the catalytic converter is warping or discolored. This typically happens when a catalytic converter is overheating.

The excess heat will damage the interior and exterior of the catalytic converter. This damage will ruin your catalytic converter.

9. Reduced Fuel Economy

Catalytic converters can also reduce your fuel economy. This happens because a bad catalytic converter creates a back push, forcing your vehicle’s engine to work harder to push out exhaust fumes. This process rob’s your vehicle of horsepower and reduces fuel economy.

This is especially easy to measure if you follow the same commute every day.

Let’s say you refill your tank every Monday. If all of a sudden you need to refill on Saturday, then this might be a sign of reduced gas mileage.

In newer vehicles, the gas mileage is displayed on your dashboard or the central display.

Keep track of this value over several weeks.

You can compare your MPG to the MPG recommended for your car’s specific year and model.

An easy way to determine your vehicle’s average gas mileage is using the U.S Department of Energy’s fuel economy website.

If your car does not display the MPG on the dashboard, you can use an app to track your MPG.

If your car has a lower MPG than the recommended MPG for your vehicle, I suggest you inspect your catalytic converter or take it to a professional to determine the reason.

10. Poor Engine Performance

A bad catalytic converter is not healthy for your engine. A clog or lack of filtration can result in many issues while you drive.

You might experience a loss in horsepower, reduced acceleration, and engine stalling or jerking with a bad catalytic converter.

This will also place additional wear on your engine because you’re making your engine work harder than it needs to.

If your engine begins to operate differently, then you should inspect your catalytic converter.

What Causes A Catalytic Converter To Fail?

1. Antifreeze or Oil leaking into the Exhaust System

One of the most common reasons that cause catalytic converters to fail is when oil, fuel, or antifreeze leaks into your exhaust system.

When these liquids make their way into your exhaust system, they will clog your catalytic converter. Also, if there is oil or fuel in your catalytic converter, it can cause it to overheat and damage the interior structure.

2. Misfiring Engine

The engine misfire will increase exhaust temperature that could melt or damage the catalytic converters. A misfire can sound violent depending on your make and model vehicle. The misfire sounded like a gunshot. It’s common to see a small flame coming from your exhaust during a misfire.

If you are not familiar with a misfire, a misfire sounds like a popping or backfiring sound. These sounds are sudden and occur quickly.
Below is a video of what you might hear when your engine misfires.

3. Excessive idle time

If you leave your car idling for an extended period, this can ruin your catalytic converter. When idling, this will cause your catalytic converter to run hotter than normal.

This will damage your catalytic converter destroying the interior and causing them to fail much earlier than it normally would.

4. Bad Spark Plugs

Bad spark plugs can cause your engine to misfire. This can cause fuel to leak into your catalytic converter. This fuel will damage your catalytic converter, either causing it to clog or causing it to overheat and damage.

How To Extend The Life Of Your Catalytic Converter?

The catalytic converter is connected to your exhaust right underneath your vehicle. This design makes your catalytic converter susceptible to impact underneath your vehicle.

Low sports cars are more susceptible to impact underneath the vehicle compared to an SUV with higher ground clearance.

As an automobile owner, you have to take this information into account when picking out the right vehicle for transportation. Here are some helpful tips to keep your catalytic converter working for an extended amount of time  

  1. Drive slowly over speed bumps. 
  2. Avoid potholes 
  3. Keep up with your regular maintenance on your vehicle. 
  4. Pay attention to the warning lights on your desk if they display. 
  5. Avoid overheating your vehicle. 
  6. Turn your vehicle off during a long idling period of time.
  7. Avoid debris in the road. 
  8. Don’t run over curbs

How Often Should I Replace My Catalytic Converter?

Fix or replace your catalytic converter if your vehicle starts to show signs or symptoms of a bad catalytic converter. Typically catalytic converters will last up to 100,000 miles or 7-10 years. This estimate is based on the standard USA person driving up to 13500 per year, translating into nearly 7.5 years to drive 100,000 miles.

If you suspect your catalytic converter is going out. Contact your local mechanic shop and set an appointment to get your vehicle diagnosed. This will give you a sure answer if your catalytic converter is causing the issues you are experiencing with your vehicle.

Will Removing The Catalytic Converter Damage Your Engine?

No, removing your catalytic converter will not damage your engine, but it will damage the environment.

Deleting your catalytic converters will make your vehicle fail an emission test, which is needed in some states in order for your vehicle to be street legal to drive. Your vehicle check engine will not turn off if your catalytic converter is deleted.

Many performance enthusiasts will run catalytic delete pipes to get rid of the catalytic converter completely. Enthusiasts advance the timing, increase airflow and increase the fuel delivery to their engines to increase horsepower.

This modification is called a tune to increase horsepower by playing with the three variables. A tune is a catalytic converter killer due to the increased fuel getting dumped into the exhaust and increased exhaust temperatures.

Note most performance modifications referring to CAT delete pipes are only legal for off-road use.

How Long Do Catalytic Converters Last?

Catalytic converters typically will last up to 100,000 miles depending on make and model vehicle.

The Department of Transportation’s Federal Highway Administration claims the average USA person drives 13,500 miles per year.

This means catalytic converters can last up to 7-10 years depending on how many miles you drive per year.  

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