Car Catalytic Converter Problems? 9 Common Issues & Fixes

The catalytic converter in your car helps to burn any leftover fuel or oxygen after the combustion process and keeps your vehicle emissions low. This is an integral part of your car running at optimum performance.

Knowing the signs of a failing or degraded catalytic converter can mean the difference between an easy-to-fix issue or a very expensive bill.

We’re sharing the most common issues with catalytic converters, how to spot them, and their fixes to help keep you on the road.

#1 – You Have A Check Engine Light On

Your catalytic converter is monitored for effectiveness by the oxygen sensors in the exhaust system. The oxygen sensors are placed before and after the catalytic converter, and they detect a change in the amount of oxygen in the exhaust gas after it leaves the catalytic converter.

If there is a significant difference between the sensor readings, the Check Engine Light (CEL) will illuminate to signify there is a detected problem.

How to Fix it

If you have a Check Engine Light on, you can use a diagnostic scanner to check the stored trouble codes and determine what the CEL is indicating.

The most common trouble code associated with a catalytic converter is P0420.

• You can determine if your converter is partially or fully blocked by checking how much exhaust gas is coming from the tailpipe.
• Cover the end with your hand and have another person rev the engine to 2,000 RPM.
• If you don’t feel a significant change in the amount of exhaust gas coming out of the tailpipe, it means your catalytic converter is partially or fully blocked.
• A pour-in fuel cleaner can help remove contaminants from the converter if it is partially blocked.

A fully blocked converter may signify that it has internal damage, and it needs to be replaced.

#2 – Your Fuel Consumption Changes

An efficient engine will burn all of the air-fuel mixture in the combustion chamber, and the oxygen sensors in the exhaust system measure the oxygen change through the catalytic converter.

If there isn’t a significant change between the sensor readings, your car’s Engine Control Unit (ECU) may change the amount of fuel going into the engine. When that happens your fuel consumption will change, and that may be a bad thing.

How To Fix It

When the fuel consumption changes, most mechanics will verify the oxygen sensors are working correctly before investigating the catalytic converter as the source of the problem.

The sensors are easier to replace and quick to check for a problem. If they find the sensors are good, the next step is to look at the catalytic converter as a source of the issue.

A mechanic can use an OBD diagnostic tool to verify the readings from the oxygen sensors. These sensor readings will show how the exhaust gas is changing through the catalytic converter and if the converter can be cleaned or should be replaced completely.

#3 – You Notice A Change In Acceleration

The catalytic converters are part of a team that keeps your car running efficiently.

Spark plugs ignite the air-fuel mixture, the pistons use the energy to move the crankshaft in the engine, and that power is converted to motion for the car to accelerate.

If your converters are partially blocked or damaged, they will cause a backup and loss of acceleration of the car.

How to Fix It

Improper combustion will make your car less efficient, and it will need to make more power to have the same acceleration when you step on the gas pedal.

You may notice a change in fuel mileage indicating an issue with the catalytic converter.

Once you detect a problem, you or a mechanic can determine if the catalytic converter is partially blocked or has damage preventing efficient gas flow out of the engine. You may need to clean the converter if it has reduced flow or replace it if is damaged.

#4 – Your Exhaust Smells Like Rotten Eggs

A clogged catalytic converter will restrict the flow of the exhaust gases. Over time the air-fuel mixture will become rich (meaning more fuel than needed is required), and this leads to unburned fuel in the combustion chamber.

When the rich air-fuel mixture is burned it will smell like rotten eggs or burned sulfur.

How to Fix It

There are pour-in fuel cleaners that can help unclog a catalytic converter. This is the best option to try first as it’s a relatively low-cost fix.

It may take more than one bottle of cleaner depending on the severity of the clogged converter.

If the cleaner doesn’t fix the problem, the catalytic converter may need to be removed and replaced.

#5 – The Converter Housing is Discolored

A clogged catalytic converter will generate excess heat as it blocks the flow of exhaust gas leaving the engine. This heat can turn the housing blue, gold, or even dark brown.

Converters are normally a basic steel that is silver in color, so any dramatic change from that color indicates a problem.

How to Fix It

This is another situation that calls for the pour-in fuel cleaner that you add to the fuel tank. It should reduce the clog in the catalytic converter and restore the flow of the exhaust gas.

You’ll want to monitor how your engine is running before the cleaner and after to verify the change in performance of your car. The engine should run better after the converter is cleaned.

The color of the housing will not go back to a silver color, so once it’s changed color it’s a permanent change.

#6 – Your Car Develops Starting Issues

Starting issues, and running poorly in general, are a signal that your catalytic converter may be partially clogged or blocked.

The engine must remove the exhaust gas after combustion, and it will run poorly or have trouble starting if it can’t do that.

If your car starts and run for a few seconds, but then dies, it may be signaling the converter is partially blocked. The exhaust gas is building up around the converter and blocking the engine from releasing more gas.

How to Fix It

This is another situation to try the pour-in fuel cleaner that is formulated to clean the catalytic converter.

Try one or two bottles of the cleaner following the manufacturer’s directions on the bottle. If it doesn’t fix the issue, it may be time to replace the converter.

Related: Car Won’t Always Start The First Time (Check These Things)

#7 – Your Exhaust Emissions Significantly Increase

The job of the catalytic converter is to change the Nitrogen oxide, carbon monoxide, and hydrocarbons in the exhaust gas into safer elements like carbon dioxide.

If you have an annual or frequent inspection for exhaust emissions, you may fail the emissions test with a catalytic converter that isn’t doing its job correctly.

You may also notice that your exhaust smells like rotten eggs or burned sulfur if the converter has started to degrade due to age.

How to Fix It

Catalytic converters typically last 10 years, and they lose performance over time due to degradation.

When they have reached the end of their life, they will need to be replaced with new converters. This is best left to a professional mechanic because the converters contain restricted materials like rhodium and platinum.

The old converters need to be recycled properly.

#8 – Your Engine Develops a Misfire

The combustion process in the engine requires an air-fuel mixture to ignite and burn.

A clogged or degraded catalytic converter may block the exhaust from leaving the combustion chamber in the engine. That blockage can also prevent fresh air-fuel mixture from entering the engine. When you don’t have the air-fuel mixture, a misfire develops.

How to Fix It

You may not notice a misfire in the beginning, but your engine’s control module probably will. You may find the Check Engine Light (CEL) illuminated indicating there is a problem.

Check the diagnostic trouble codes to understand what the computer is detecting.  You may just need to clean the catalytic converter, or in the worst case, it may need to be replaced.

#9 – You Hear A Rattle In The Exhaust System

The exhaust system should not have rattles, but if you hear one, it indicates a problem.

The honeycomb structure inside the catalytic converter can break apart over time due to degradation or something hitting the outside of the converter housing.

These small pieces can rattle inside the converter housing or be carried further into the exhaust system. You may hear them rattling inside the muffler or eventually see them coming out of the exhaust pipe.

How to Fix It

The broken honeycomb inside the catalytic converter can’t be fixed. Some converters can be removed, and the honeycomb replaced, but most likely you’ll need to replace the complete converter as a whole unit.

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