Signs That a Catalytic Converter is Clogged or Broken

In this article we are going to take a closer look at this part, what it does and some of the signs that your unit may be developing an issue. Sometimes a simple repair can avoid a full replacement and save you a lot of money.

What Is a Catalytic Converter?

If you grew up during the 1970s and '80s you may recall occasionally driving around in cars with the windows down and smelling a sulfur rotten egg smell from time to time. After exclaiming “What is that smell?” someone in the car likely enlightened you to it being a catalytic converter.

This simple answer doesn’t mean much so let's explore just what a catalytic converter actually is. Basically catalytic converters are devices that capture the emissions from the burning of petroleum. Once captured these fumes are stripped of carbon monoxide, nitrogen oxides and hydrocarbons.

The remaining emissions are then released from the catalytic converter in the form of carbon dioxide (CO2) and Water (H2O). These emissions of course are far less harmful to the environment meaning that the fuel burning process is cleaner.

How Do Catalytic Converters Work?

There are many different types of catalytic converters but they all work along the same principals. Essentially inside these devices there are chemical elements that are used as catalysts. There are reduction catalysts and oxidation catalysts.

These catalysts are metals such as platinum, rhodium or palladium which by the way are not inexpensive. This often means that replacing the catalytic converter is not cheap. The metals are often coating ceramic structures and will trap and react with the carbon monoxide, nitrogen oxides and hydrocarbons as they pass through the device.

Firstly reduction catalysts such as platinum or rhodium act upon the nitrogen oxides ripping away the nitrogen atoms from the compound. As an example when nitrogen dioxide (N02) passes over these catalysts the nitrogen (N) is ripped away leaving only the two O atoms which for those who might not know is simple oxygen.

The next phase is the oxidation catalysts which might be platinum or palladium. These catalysts with the help of the extra oxygen from the reduction phase take care of carbon monoxide CO and hydrocarbons. Instead of removing atoms they actually force a bond between the O2 and CO molecules turning oxygen and carbon monoxide into carbon dioxide (CO2).

Although excess CO2 is still not great for the environment it is much more preferable to carbon monoxide which can be lethal. Poorly maintained gas burning heating systems for example might produce excess carbon monoxide in your home. The accumulation of this is poisonous and can kill.

Symptoms That Indicate a Clogged or Damaged Catalytic Converter (CAT)

We have already mentioned the sulfur smell that is given off by catalytic converters. Well they actually only do this when there is a problem with the part. If you are smelling that rotten egg sulfur smell when your engine has been running for a while and you get a check engine light you may have some issues with the CAT.

The signs of a bad catalytic converter include the following:

Check Engine Light

This warning light may illuminate any number of potential issues related to your engine, not just the catalytic converter. If you want to determine if this has come on because of the CAT then you might want to use an OBD2 scanner tool. Compare the error codes you get against the list of codes associated with your make and model and you may find out exactly what the issue is.

You will usually find a list of these error codes in your owner's manual. A common code you may see if the issue relates to the CAT is P0420.

A Drop in Acceleration

When a CAT gets clogged up it causes back pressure in the engine because the exhaust can not vent correctly. This will hamper engine performance and decrease acceleration power. When people fit a straight pipe exhaust system they will actually remove the catalytic converter in order to gain acceleration speed and a few extra horsepower.

A drop in acceleration is not a clear cut indication of a CAT issue but there is a way to test the theory somewhat. Have someone rev the engine up to 2,000 RPM while you hold your hand over the end of your exhaust pipe. If the exhaust seems meager then you may have a blockage. If possible, compare the flow to a similar car.

A Change in Fuel Economy

You may know based on years of using your vehicle that a certain journey takes half a tank of gas. One day you may note that it has taken more gas than normal or perhaps even less gas. A change in your general fuel economy in the plus or negative could both indicate CAT issues.

A clogged CAT makes the engine work harder and uses up more fuel as a result. If you have an exhaust leak in or around the CAT this might decrease the normal engine back pressure and actually improve your fuel economy.

It should be noted that either eventuality is not a good thing as high back pressure can make your engine struggle but low back pressure may come at the cost of creating bad emissions. In certain states if you are found to be emitting unfiltered exhaust you may face fines.

Sulfur Smell

The clogged CAT causes that excessive back pressure which in turn causes the air/fuel mixture in the engine to become too rich. When we say too rich we mean there is more fuel than should be in the cylinders so the combustion is not perfect.

The result of this hampered combustion is a smell similar to sulfur or rotten eggs. The unburnt fuel is the cause of this smell and it is a clear sign that the exhaust is not venting correctly through the catalytic converter.

Discolored Housing

This is a very visual indicator of an issue but you will have to look under the car with a torch to catch this clue. A clogged catalytic converter can become very hot, more so than normal and the effect can be a blue tinge to the metal of the unit itself.

Issues With Starting

If you start your engine and it runs for a few seconds before shutting off you may have a CAT clog issue. The increased back pressure can take a few seconds to build and when it does it may trigger the engine to shut down.

Dark Exhaust Fumes

Most of the time you can’t really see exhaust smoke if all things are well with the catalytic converter. You may see white steam on a cold morning but otherwise you should not see much. If however you see dark smoke then clearly your emissions are not as clean as they should be.

This can be due to damage to the CAT or burning of oil in the engine's cylinders. If you were to fail a state mandated emissions test such as happens in California this would also be an indication that the catalytic converter is not working well.

A Misfiring Engine

A rough idle when at a standstill or a vibration upon acceleration can be an indication of misfiring cylinders in your engine. This goes back to the engine back pressure messing with the fuel/air mixture. The combustion is not complete so it is considered a misfire.

A bad catalytic converter is not the only cause of misfires but if this tallies with other signs it can be a good indicator. You should fix a misfire as quickly as possible as these failed cylinder ignitions can cause damage to your engine.

Catalytic Converters are Targets for Thieves

The precious metals in catalytic converters like platinum and rhodium are a big reason that the theft of these automotive components is not uncommon. The aim may be to either extract the precious metals or sell the part on to someone else.

Thieves will crawl beneath a car and using a grinder or saw of some description literally cut the catalytic converter out of the exhaust system. This will leave a huge gap and subsequently the exhaust fumes will be released further under the vehicle.

How Much Does It Cost to Replace a Catalytic Converter?

If your catalytic converter is completely broken and needs replacing it is going to be a costly repair. As mentioned, thieves steal these parts due to the value of them and the precious metals that can be extracted.

Generally speaking you could be looking at between $400 - $2500 for a new replacement catalytic converter depending on your vehicle. Some high performance vehicles require very expensive CATs due to their more powerful engines.

How Long Should a Catalytic Converter Last?

On average a catalytic converter should last about 10 years or 100,000 miles. Obviously there are factors that can change this including the quality of the part and any issues with the engine that may cause undue stress on the CAT.

If you keep your car well maintained and make use of fuels that help produce lower emissions then you may extend the life of your CAT and your vehicle at the same time.

Conclusion

There are plenty of signs that your catalytic converter may be experiencing issues although some might indicate other issues as well. If you look for a collection of symptoms however it may become clear that the CAT is clogged or broken and may be in need of repair if possible but very likely replacement.

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