Suppose that a car has 120,000 miles on it and the catalytic converter fails. From what I read (see here as an example), age doesn't appear to be a general cause for failure. Is this a possibility if a car is old?

Note the site lists the following reasons:

  • Thermal Failures
  • Contamination
  • Coated or Fouled
  • Other in pictures: Melted Brick: Engine running too hot, Burned Converter: Exhaust temperatures too high, Oil-Fouled Brick: Engine oil entering exhaust, Plugged Brick: Incomplete combustion by-products, Road Damage: Impact under vehicle crushing converter.

I found this; it indicates they last about 100,000 miles.

  • 1
    Using medical terminology, the can fail at any time from an "acute" condition like fuel contamination or road damage, but "chronic" failure is more related to the number of hot/cold cycles than the total mileage. Eventually, the thermal cycling starts to cause cracks the internal structure starts to disintegrate, and the cat gets blocked up by the debris to the point where the back pressure impacts the engine performance. The final stage of this can be quite rapid - i.e. going from a slight loss of power to an engine that won't even idle within 20 or 30 miles of driving.
    – alephzero
    Commented Nov 7, 2016 at 22:36

3 Answers 3


It looks like usually, catalytic converters, if totally undamaged and treated correctly, shouldn't die with age. The article you referenced says,

The truth is, on modern vehicles, the catalytic converter should last the life of the car or truck, given an "average" life of about 100,000 miles (160,934 kilometers).

However, this does not indicate how much longer after that you should expect your cat to last. And I question an average life of 100,000 miles... :)

According to agcoauto.com, cats are usually guaranteed for 80k miles:

The EPA mandates a warranty on most new vehicle converters. This warranty covers the converter and installation. Catalytic converters, found defective, may receive a no-charge replacement, for the term of the warranty. The term of the warranty is normally eight years or 80,000 miles. A few vehicles may also have a warranty extension beyond this.

One could assume that if they are guaranteed to 80k, they are expected to last a good bit longer than that.

That being said, treating your cat in a way which creates damage is quite easy to do: from agcoauto.com:

Too many short trips at low speed can also cause the catalytic converter to fail. Converters need highway driving to reach operating temperature. This is how they clean and regenerate the catalyst. If we do not drive our vehicle often or far enough, the converter may fail. Vehicles that get only intermittent use commonly have this problem.

Bottom line:

Essentially, a catalytic converter is a special structure of ceramic which is designed to renew itself automatically. Under ideal use, the car should die before the catalytic converter. However, mistreating it can be so simple that it may well croak before the end of the car's life.


My workplace fleet regularly sees the converters outlast the vehicles, 10+ years and 200,000+ miles.

We have seen converter failures, but it's rare, say in the lower single-digit percentage of vehicles (don't have the exact figure). Many of those were from the honeycomb breaking apart due to earlier damage, such as impacts, or being shook during a period of engine misfiring.

So if you had a converter fail at 120,000, it probably didn't just "go bad" with age. Try to find the root cause.


Simple rule is cats don't die they are killed.

They do have a lifespan simply because of the chemical reactions happening inside them eventually the carbon and the nitrogen will use up all the material they need to bond with to neutralize them. This will take a long long time depending on how much material is used it could take longer than any of us are going to live.

That being said the most common causes of cats going bad are:

  • the cat gets clogged up with gunk. They need to get really hot to work and they only work on burnt fuel. If you are burning oil this can stick to the honeycomb ceramic catalyst elements reducing their usable surface area to work their chemical reactions. This can also keep them too cold to change the nitrogen monoxide and co2 into other less harmful gasses. Typically this means you are running too rich or burning oil
  • the cat gets too hot. The catalyst they line the honeycomb with is a metal IIRC and this will melt if it gets to hot. If you melt the catalyst off it will not be able to do its job as a puddle in the bottom of the cat housing. This can be caused by a lean condition in your engine, engine overheating, or mud dirt and grime caked on the outside of the cat.
  • you smacked it on something. this can dislodge the ceramic honeycomb. since the honeycomb has small passages in it if these are not lined up properly the exhaust will not pass through them.
  • the cat is too cold. they will not be able to do there job if they are too cold and an added danger is the co2 instead of changing can stick to the catalyst keeping it insulated and cool so it will never get that small bit of carbon off. This can be caused by a thermostat that is missing or stuck open allowing coolant to be too cool, you may be using the wrong spark plug, or you are running too rich. Typically it is the thermostat or you are in very cold weather. If you are in cold weather use a piece of cardboard to cover a portion of the front grill as the cold air passing through into the engine compartment can chill smaller engines.

one usually good way to check them is use a infrared thermometer and get the temperature before and after. Before should be cooler by 20 degrees or more and after should be hotter.

  • Take the temperature before and after what?
    – John Dee
    Commented Jul 14, 2019 at 17:41

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .