DIY

Bad Catalytic Converter Symptoms: Signs to Watch Out For

Reading Time: 9 minutes

When I was 20 years old, I owned a Pontiac Trans Am. Of course, it was bright red because, at that age, I wanted something flashy. The car also had a loud dual exhaust system to announce my arrival (an important feature when you’re young).

But there was a problem—the Trans Am suffered from a slight misfire at idle. When I owned the car, I was an apprentice automotive technician who didn’t know much. So, in an attempt to fix the issue, I replaced a few parts, such as the spark plugs, but nothing I tried worked. I was too young and too green to have the diagnostic skills needed to remedy the concern.

Also, because the misfire was so minor, it didn’t trigger the check engine light. Consequently, I opted to ignore the problem—despite the fact that my car’s exhaust smelled like rotten eggs.

a catalytic converter seen from the underside of a car
Catalytic converters often experience overheating and premature failure due to engine performance problems, such as a misfire.

Ah, to be young and naive. I let the misfire go for several months and, eventually, it took out my catalytic converters.

You see, the thing with catalytic converters is that they often experience overheating and premature failure due to engine performance problems, such as a misfire.

The moral of the story is: Don’t ignore engine performance problems the way I did when I was younger. If you do, you may end up destroying your catalytic converters.

What are the Symptoms of a Bad Catalytic Converter?

Check engine light turns on

Usually, an illuminated check engine light is the first and only sign of a bad catalytic converter. When the light pops on, your car’s engine control module (ECM) will also store a diagnostic trouble code (often a P0420 code) in its memory.

Poor engine performance

In some cases, the catalytic converter may become clogged or restricted, creating excessive exhaust backpressure. That chokes the engine, leading to performance problems such as lack of power and stalling.

Car won’t start

If the catalytic converter is severely restricted, it can create enough exhaust backpressure to prevent the engine from starting.

Failed emissions test

A catalytic converter’s job is to clean up the harmful emissions your car’s engine produces. As such, a bad converter can easily cause your vehicle to fail a state emissions test.

What Does a Catalytic Converter Do?

a typical car catalytic converter
A catalytic converter transforms hydrocarbons, carbon monoxide and oxides of nitrogen into harmless CO2 and water.

The catalytic converter—also referred to as a “cat”—is an emissions control device that transforms hydrocarbons, carbon monoxide and oxides of nitrogen into harmless CO2 and water. All modern cars have one or more catalytic converters located in the exhaust system.

How does the catalytic converter work its magic? The device contains precious metals, such as platinum, that create a type of chemical reaction, called a catalyst. It’s this reaction that cleans up the toxic gases leaving your car’s engine.

It’s also worth noting that, typically, there are oxygen sensors located near both ends of the catalytic converter. Your car’s ECM monitors the sensor located near the converter’s outlet (referred to as the “downstream” oxygen sensor) to gauge the efficiency of the catalytic converter. If from the signal, the module determines that the cat isn’t doing its job, it will turn on the check engine light.

What to Do If You Think You Have a Faulty Catalytic Converter

A faulty catalytic converter is bad for the environment and, in some cases, it can cause engine performance problems. That means you should replace a faulty catalytic converter as soon as possible.

How to Replace a Catalytic Converter

jack stands used when replacing a catalytic converter
You will need a jack and jack stands to replace your car’s catalytic converter.

Now we get to the fun part: catalytic converter replacement. Are you ready to grab some tools and get dirty? Good.

Although some catalytic converters are integrated with the exhaust manifold, we’re not going to cover that type of converter here. Nor are we going to discuss converters that are welded into place. Instead, we’re going to focus on bolt-in converters that are located between the exhaust manifold and exhaust pipe (or muffler).

Before we start, keep in mind that all vehicles are different. Be sure to follow the repair information for your specific application. Repair manuals, such as those from Chilton, are useful, but an ALLDATA subscription is even better.

ALLDATA has single-vehicle subscriptions for DIYers that provide detailed factory repair information.

The tools needed to replace a catalytic converter can vary, depending on what type of vehicle you have. But in general, you’ll need:

Note: The following are general guidelines for educational and entertainment purposes only. Consult your vehicle’s factory information for specific repair instructions and recommended safety procedures.

How to Remove a Catalytic Converter

protective goggles
Make sure your eye protection wraps around the side of your face, as rust and debris from the exhaust can easily get in your eyes.

Exhaust components can get extremely rusty, so it’s a good idea to apply penetrating oil to all of the fasteners holding the converter and heat shield in place. The oxygen sensor threads should receive some penetrating oil as well. Let the oil soak in overnight before you start working on your car.

1. Put on your safety glasses or protective goggles. Make sure your eye protection wraps around the side of your face, as rust and debris from the exhaust can easily get in your eyes.

2. Safely raise and support the vehicle using a jack and jack stands. Chock the rear wheels and set the parking brake.

Warning: Make sure the engine and exhaust system are cool before proceeding. Do not service the exhaust system while it is still hot. Touching a hot exhaust can cause severe personal injury.

3. Disconnect the electrical connector(s) going to the oxygen sensor(s).

uninstalling the oxygen sensor before removing the bad catalytic converter
You can choose to remove the oxygen sensor once the converter is out of the vehicle, if that’s easier.

4. Remove the oxygen sensor(s) using the correct size oxygen sensor wrench. You can also choose to remove the sensor(s) once the converter is out of the vehicle, if that’s easier.

5. Remove the fasteners securing the heat shield. Then remove the heat shield.

6. Remove the fasteners securing the catalytic converter to the exhaust pipe.

Note: If the catalytic converter is large, you should support it and/or have an assistant help you steady it before removing the retaining fasteners.

7. Remove the fasteners securing the catalytic converter to the exhaust manifold.

8. Remove the catalytic converter from its rubber hanger(s).

using a wrench to remove an old catalytic converter from other exhaust parts
If the old catalytic converter is stubborn, you may need to gently pry it away from the other exhaust components or tap on it with a dead-blow hammer.

9. Remove the catalytic converter and corresponding gaskets. Have an assistant help you if the converter is large. Also, if the converter is stubborn, you may need to gently pry it away from the other exhaust components or tap on it with a dead-blow hammer.

Note: If the catalytic converter fasteners don’t come loose or they break off during removal, you may need to bring your car to a professional for repair.

How to Install a Catalytic Converter

car diagnostic scan tool
After installing the new catalytic converter, use a scan tool or code reader to clear any diagnostic trouble codes set by the original catalytic converter.

1. Compare the new catalytic converter to the old catalytic converter to ensure that they are the same design. Also, be sure that you know whether your vehicle is designed for California or Federal emissions (typically, this information is listed on a decal on the underside of the hood). The new converter must match your car’s emissions standards.

2. Apply a light coat of anti-seize to the threads of the oxygen sensor(s). Then, install the oxygen sensor(s) into the converter by hand. If possible, use a torque wrench to tighten the sensor to the manufacturer’s specification. It’s easy to damage the oxygen sensor threads, so be careful.

cleaning the mating surface on the exhaust pipe before installing a new gasket for the catalytic converter
The gasket mating surface on the exhaust manifold and exhaust pipe will likely be dirty. Clean it before installing new gaskets.

3. Clean the gasket mating surface on both the exhaust manifold and exhaust pipe.

4. Install a new gasket between the catalytic converter and exhaust manifold. Then, loosely install the bolts that secure the catalytic converter to the exhaust manifold. Do not tighten the bolts down completely at this time.

Note: If the catalytic converter is large, you should support it and/or have an assistant help you steady it during the installation process.

installing a new gasket on the exhaust pipe
Install a new gasket between the catalytic converter and exhaust pipe.

5. Install a new gasket between the catalytic converter and exhaust pipe. Use a torque wrench to tighten the converter-to-exhaust pipe fasteners to the manufacturer’s specification.

6. Use a torque wrench to tighten the converter-to-manifold fasteners to the manufacturer’s specification.

7. Push the catalytic converter back into the rubber hanger.

8. Reinstall the heat shield. Use a torque wrench to tighten the retaining fasteners to the manufacturer’s specification.

9. Reconnect the electrical connector(s) going to the oxygen sensor(s).

10. Safely remove the jack stands and lower the vehicle.

11. Use a scan tool or code reader to clear any diagnostic trouble codes set by the original catalytic converter. If you don’t clear the codes, you will have to wait for the control module to run its system self-tests before the check engine light will extinguish.

Many experts recommend that new catalytic converters undergo a “break-in” procedure. According to the publication, Auto Service Professional, to break-in a converter you should:

Catalytic Converter Replacement Cost

new catalytic converter
If you decide to have a professional replace your catalytic converter, expect to pay anywhere from around $500 to upwards of $2,000.

Catalytic converters are expensive. If you decide to have a professional replace your catalytic converter, expect to pay anywhere from around $500 to upwards of $2,000. Exactly how much the job costs will depend on many factors, including the type of vehicle you have.

Should you decide to tackle the job yourself, expect to pay anywhere from approximately $100 to over $1,000 for a replacement catalytic converter. Once again, exactly how much the part may cost will depend on many factors, including the type of vehicle you have.

Sometimes, you’ll find universal catalytic converters that are relatively inexpensive. The problem is, typically, those components have to be welded or clamped into place, and that makes replacement a lot more difficult.

Stick with a direct-fit converter—you’ll be happy you did.

Frequently Asked Questions

How long can you drive with a bad catalytic converter?

The answer depends on how the converter failed. If the converter is clogged, it can create enough backpressure to cause your engine to run rough, stall, and lack power (obviously, you can’t continue to drive like that). In some cases, the problem can even be severe enough to prevent your car from starting.

On the other hand, if the catalytic converter is worn out but not clogged, technically, you could keep driving for a long time.

How do you unclog a catalytic converter?

There is an array of products and methods that claim to unclog a plugged-up catalytic converter. And while some of these approaches may work, many of them will not. The only foolproof way to address the problem of a clogged catalytic converter is to replace it with a new one.

How long does a catalytic converter last?

In many cases, a catalytic converter will last the life of the vehicle, as long as the engine is properly maintained. Typically, you can expect to get at least 10 years out of nearly all catalytic converters.

How much does a catalytic converter cost?

The cost of replacing a catalytic converter will vary, depending on the year, make, and model of the vehicle. On average, however, you can expect to pay between $500 to upwards of $2,000 to have a professional replace your car’s catalytic converter.

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Author

Mia Bevacqua

Chief Mechanic at CarParts.com

Mia Bevacqua is an automotive expert with over 15 years of industry experience. She holds ASE Master, L1, L2, and L3 Advanced Level Specialist certification, as well as a bachelor's degree in Advanced Automotive Systems.

Throughout her career, Mia has applied her skills toward automotive failure analysis inspections, consulting, diagnostic software development, and of course, freelance writing. Today, she writes for companies around the world, with many well-known clients showcasing her work.

Mia has a passion for math, science, and technology that motivates her to stay on top of the latest industry trends, such as electric vehicles and autonomous systems. At the same time, she has a weakness for fixer-upper oddballs, such as her 1987 Chevy Cavalier Z-24 and 1998 Chevy Astro Van AWD.

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