Clogged Catalytic Converter

A car’s great performance can be seriously interrupted by a clogged catalytic converter. It can also cause a whole series of other types of problems with the exhaust system. This little catalyst is an integral part of the vehicle’s exhaust system.


There are two ways a converter can fail:


  • It can become clogged.
  • It can become poisoned.

There really is no “inspection port” for the consumer or mechanic to see an actual clog in a converter. Often, the only way to tell if a catalytic converter is malfunctioning (plugged) is to remove it and check the change in engine performance. When a clogged converter is suspected, some mechanics temporarily remove the O2 sensor from the exhaust pipe ahead of the catalytic converter and look for a change in performance.




The engine expels harmful gases out into the atmosphere, but the catalyst converts these harmful toxins into gases that are not dangerous for either the environment or people. If it does not work properly or it becomes clogged then the harmful toxins can then be released back out into the atmosphere.


In the catalytic converter, there are two different types of catalyst at work, a reduction catalyst and anoxidation catalyst. Both types consist of a ceramic structure coated with a metal catalyst, usually platinum, rhodium and/or palladium. The idea is to create a structure that exposes the maximum surface area of catalyst to the exhaust stream, while also minimizing the amount of catalyst required, as the materials are extremely expensive. Some of the newest converters have even started to use gold mixed with the more traditional catalysts. Gold is cheaper than the other materials and could increase oxidation­, the chemical reaction that reduces pollutants, by up to 40 percent.



The first symptom that your car has a clogged catalytic converter is that it will start to put off more exhaust. This is because when the unit is blocked and cannot perform properly the unburned engine gases will increase.  As the exhaust emissions are increased it can end up causing less exhaust. This is because the way it normally travels to get out and away from the car is through the exhaust and tail pipe, but the route is now blocked.


A partially plugged converter will create a restriction in the exhaust system. This, in turn, will cause an increase in exhaust backpressure that can strangle engine performance, particularly at highway speeds. The engine may start and idle normally, but lack high speed power. Fuel economy may also be down.


At first, you may suspect low fuel pressure or a plugged fuel filter (which can cause similar symptoms). A simple fuel pressure check with a gauge will tell you if idle pressure is within specifications, and a flow test will tell you if the fuel pump is capable of delivering enough fuel volume to meet the engine’s demands. You may also replace the fuel filter in an effort to cure your customer’s problem. But when the problem persists, it’s obviously not fuel-related. More likely, it’s an exhaust restriction.



The other noticeable symptom that your car has a clogged catalytic converter is a sudden drop in overall gas mileage. Since the engine cannot get rid of the excess gases, it will cause engine back pressure. This in turn will make the engine have to work harder to do its job. As it has to work so much harder, it will use more gas. This is a noticeable drop in gas mileage.


Does gas mileage always drop from a bad catalytic converter?


Yes it will always drop if the converter is clogged. Mileage will gradually degrade, and power will drop off until eventually the car won’t have enough power to drive safely.



If it is seriously blocked, the catalyst can cause all sorts of extra exhaust noise. This is usually in the form of popping or sputtering. The reason it will be sporadic type noises is that the exhaust flow is trying to get through the catalyst. But since it is blocked off the exhaust will try to exit around the plugged up unit. This will make a loud noise. It will basically sound like the exhaust is all of a sudden very loud. But it will be a different sound than when the muffler needs to be replaced.


If you start to hear very loud noises coming from the exhaust system of your car, it’s best that you get your car to a reputable mechanic. There could be a hole in your muffler or exhaust system. If you are hearing a rattling noise, it could be a more expensive fix, such as the catalytic converter.



These are some of the common symptoms that are very apparent and easy to spot when the catalyst starts becoming blocked and starts to not work properly. If you have any of these signs you might take the car to a mechanic to make sure it is a clogged catalytic converter so you can get it placed as soon as possible.


Is it safe to drive my car with a bad catalytic converter?

You’re polluting us.
Please fix it.



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