What Causes a Catalytic Converter to Go Bad?

Find out what causes a catalytic converter to go bad, the symptoms of a bad catalytic converter, and more with these tips from our expert mechanics.

What Does the Catalytic Converter Do?

Exhaust gases from the engine go into the catalytic converter to burn up unburned fuel. Overall, they’re reliable and last long. They can break prematurely, like from excessive fuel, oil, coolant or external damage.

Where Is the Catalytic Converter Located?

You’ll find the catalytic converter after the exhaust manifold and before the exhaust pipes connected to the muffler. It’s normally placed closer to the engine, and on some vehicles it may be part of the exhaust manifold.

What Causes a Catalytic Converter to Go Bad?

There are a few different causes that can make a catalytic converter go bad, such as fuel and coolant leaks.

Excessive Fuel

If the engine is not running efficiently, excessive fuel can dump in the converter and heat and plug it up. With the engine running harder, you’ll be wasting gas, and you’ll notice a lack of acceleration and horsepower.

Spark Plugs and Ignition Coils

Spark plugs and ignition coils are the most common reason for fuel getting into the exhaust. Not changing the plugs or coils regularly can cause a misfire and flashing check engine light.

More on how to diagnose a misfire

Fuel Injectors

The fuel injector can also remain stuck open, dump fuel into the engine cylinder, and cause a misfire. To test this, you can swap the fuel injectors to see which injector is failing if you know it’s not the ignition coil or spark plug.

Signs of bad fuel injectors

Oxygen (O2) Sensors

Oxygen (O2) sensors not sending the right signal to the computer can cause the catalytic converter to fail. The computer will take a false reading and dump more fuel.

Air Intake System Parts

Intake system parts like the air filter, mass air flow (MAF) sensor, throttle body, or intake hoses can cause problems with the catalytic converter. Leaks can cause excessive fuel and contamination.

How to check air intake system parts

PCV System

If there is an issue with the engine burning oil, there could be a problem with the PCV system. If it’s not working properly, excessive oil can get into the combustion chamber and exhaust. It could be plugged up or not restricting enough and causing oil to leak into the intake.

Valve Seals

Valve seal
Valve seal

Failed valve seals could be contributing to the engine burning oil. If they’re not sealing properly, oil will get past the valves and into the combustion chamber. If you have an excessive oil consumption problem, it could be from the valves.

Pistons and Piston Rings

Piston and piston rings
Piston and piston rings

The engine piston and piston rings can have carbon build-up that sticks the rings in the grooves. This can cause improper sealing. Oil can leak past the piston and into the combustion chamber.

Head Gasket or Intake Gasket

A worn head gasket is the most common reason for coolant to leak into the exhaust. You’ll find it between the cylinder head and engine block. Coolant will move through the ports and close to the combustion chamber. If the gasket is cracked, coolant will seep past and into the exhaust.

Along with the head gasket, on some vehicles the coolant passes through the intake gasket. If that’s leaking, coolant can leak into the exhaust system.

How to diagnose a blown head gasket

Symptoms of a Bad Catalytic Convertor

Shiny Convertor

If the catalytic converter is glowing, that’s a sign there is too much air and fuel dumping into it.

Physical Dents

Physical dents in the convertor can damage it. This can happen if driving off road for example and the converter take a hit from a a rock that leaves a big dent. The converter may leak if ripped open, and damage like this can break its internals.

Rattling Catalytic Convertor

Catalytic converter honeycomb

The catalytic converter has a honeycomb inside. Unburnt fuel will travel in, disperse, and burn up. Converters that are plugged up will have a lot of spots on the filter or a solid piece. Pieces could also be rattling around, meaning it’s damaged inside and is broken.

Check Engine Light for Code P0420 or P0430

Catalytic converter with two O2 sensors and cut open to show honeycomb
Catalytic converter with two O2 sensors and cut open to show honeycomb

The converter has one O2 sensor in front reading exhaust gases and one at the back. The computer determines the difference and sees if the cat is working efficiently. If it isn’t, a check engine light could appear for codes P0420 or P0430, which are inefficiency codes of the catalytic converter.

More on codes P0420 and P0430

How to Diagnose a Catalytic Converter

Test the Converter with a Back Pressure Gauge

You can remove the O2 sensor and put in a back pressure gauge. Then start the engine. If the engine at idle has a reading above 1 pounds per square inch (psi), then the convertor is plugged up. Or, if you rev the engine and it has a reading above 1.5 psi at 2500 RPM, then there’s a problem with the converter. If it’s plugged up, it’s because your vehicle is wasting fuel, causing the engine to overwork.

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What Causes a Catalytic Converter to Go Bad? - Expert Tips - 1A Auto
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What Causes a Catalytic Converter to Go Bad? - Expert Tips - 1A Auto
If you're wondering what causes a catalytic converter to go bad or what the symptoms of a failing one are, check out this video and article and learn what the experts at 1A Auto have to say
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