Testing a catalytic converter at home

The catalytic converter can be an expensive emissions device that you could be forced into replacing to pass state emissions or smog testing. The question is what are some of the ways to test a catalytic converter to verify replacement is necessary.

Picture of catalytic converter

Catalytic Converter

I can not say that it is real common to have a problem with the catalytic converter and probably many vehicles go to the junkyard with the original one installed on the automobile.

The key word would be many and certainly not all. Certain vehicles seem to require replacement more often than others. One symptom that would be common when a catalytic converter suffers a catastrophic internal failure would be a lack of power that is sometimes coupled with a stalling condition after the engine is started.


You can get a popping or backfiring from the throttle body when the converter is severely plugged. I have never seen it but I have heard of a converter being completely blocked and causing a no start condition.

How to test the catalytic converter

My favorite way of testing a catalytic converter would be to use a vacuum gauge to monitor the engine vacuum while the RPMs are raised. Testing engine vacuum is an efficient way of finding several different kinds of engine performance problems. When a catalytic converter is restricted or plugged engine vacuum will be lower than normal and fall off fast with higher RPMs.

Since a rubber mallet and a vacuum gauge are pretty cheap these are two tests that can be accomplished at home for those who have some DIY auto repair experience. There are several other ways of testing catalytic converters that are available to your auto repair shop.

Picture of 4 gas emission analyzer

4 gas emission analyzer

Proper diagnosis is so important especially in the case where the converter is recommended to pass emission testing and the car runs fine.

One of these tests is called an O2 storage test that checks the ability of the converter to store oxygen. There are also converter efficiency tests that can be performed with a four gas emission analyzer that many auto repair shops will have on-site.

Just because a vehicle fails an emissions test does not mean it needs a catalytic converter replaced. The fuel control system needs to work properly and should adjust the air fuel mixture so it goes back and forth from a lean to rich. If it stays lean or rich for too long emission levels will rise no matter the condition of the catalytic converter.

If your auto repairs shop states you need a new catalytic converter it may very well be true. But why not ask them a few questions about the tests they used to diagnose the problem. If you’re going to spend a lot of money on replacing this part you have the right to know why it failed and some sort of guarantee that it will help your vehicle pass the smog emissions testing.

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