No, not that cat!


Catalytic converters - or "cats" - have a nasty habit of failing with age.

What are the symptoms associated with a cat that's gone bad?

Are there any tests that could definitively confirm that a cat has gone bad?

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    Because all the booze is gone and half eaten dead mice are lying around everywhere? Dec 18, 2015 at 4:04
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    If you have pre-cat and post-cat lambda sensors, you could check their readings. The post-cat lambda sensor output should be heavily "filtered" pre-cat sensor output. As in, the post-cat readings should be barely moving around 0.5V or so. If the readings are more or less the same (0-1V oscillation) that means the cat is not doing much at all. Dec 18, 2015 at 8:27
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    When you come home and all of your catnip is gone. Then you know......and must accept......you have bad cats. :)
    – Ppoggio
    Dec 25, 2015 at 5:16
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    @DucatiKiller - good cat = cat kills mice. Bad cat = cat ignores mice. Really bad cat - cat plays cards/gambles with mice. SERIOUSLY bad cat = ...and loses. Megalomaniac cat = cat recruits dogs to form the core of his private army, captures mice who become slave laborers in his secret factories building V-weapons to be used on the humans, and yowls maniacally while readying his plan for world conquest and domination... Mar 10, 2016 at 20:23
  • @DucatiKiller LOL.... the cat is really nasty when it has a little pussy on the side.. no more cat jokes...LOL :)-
    – Old_Fossil
    Apr 11, 2017 at 5:07

3 Answers 3


Cats can stop functioning in a couple of ways:

  • physical deformation due to high temperatures

    Melted catalyst

    According to this article, the catalyst melts at temperatures above 2100 °F (1200 °C).

    In the event of a meltdown, the catalyst gets permanently damaged, at which point it doesn't scrub the nastiness out of the exhaust gases and serves as a major exhaust restriction.

  • inhibition of the chemical reaction, usually due to fouling of the catalyst inside

    Unburnt fuel can accumulate in the cats over time, which inhibits the chemical reaction and increases the restrictiveness of the exhaust. The clogging leads to bigger problems related to engine inoperability.

    A danger associated with built-up hydrocarbons is that if a spark finds its way into the cat, it will turn into a secondary combustion chamber, destroying the catalyst.

Testing cats

There are several ways to assess the health of a cat:

  1. Measure temps

    The catalytic reaction is exothermic in nature, so the exit side the cat should be hotter than the inlet side.

    Paulster2's answer outlines a way to test for the presence of the chemical reaction by measuring temperature across the cat.

    If exhaust side isn't hotter by 200 °F (93 °C), the reaction isn't taking place and the cat isn't doing its job.

  2. Consult the lambdas

    The big caveat with this approach is that it assumes the lambda sensors in question are healthy.

    In narrowband setups, the voltage output of the pre-cat lambda sensor should flit between 0.1 V (lean) and 0.9 V (rich). Since the chemical reaction inside the cat consumes oxygen, the post-cat lambda should show a stable signal which corresponds to a rich condition.

    The image below summarizes the expected results.

    Lambda probe test

  3. Check for restrictions

    Clogging usually occurs due to the accumulation of unburnt hydrocarbons, which is often a sign of bigger problems with engine operability.

    Clogged catalytic converters result in a very high engine backpressure, which results in noticeable loss of power and the engine's refusal to run at higher RPM's.

    A very conclusive test shown by this video involves measuring pressure drop across the catalytic converter. It helps to have pre- and post-cat lambda sensors for this test:

    • unscrew the lambda sensors that sandwich the suspect cat

    • attach a fitting into the bung that allows for a pressure gauge or vacuum gauge to be plumbed in

      In the video, Matt uses some pipe with a 22 mm thread cut into one end, but even a cork could do.

    • measure pressure drop. There should be minimal pressure drop across the cat. Any value greater than 2 psi would be a strong indication of clogging in the catalytic converter.

    Note -

    Since the post-cat exhaust pressure is almost atmospheric, it is possible to do this with just the pre-cat O2 sensor bung, with one caveat that restrictions may be caused by other downstream elements such as mufflers.

  4. Use an OBD-II scan tool

    The OBD-II protocol assigns P0420 and P0430 for Bank 1 and Bank 2 cat efficiencies respectively.

    The exact logic by which the engine computer sets these codes will vary between manufacturers and models, but they will invariably involve assessing the signal coming from the pre-cat and post-cat lambda sensors, as outlined in 2.

    Once again, this assumes fully-functional lambda probes, so a faulty lambda sensor may trigger a false-positive P0420 code.

  5. Propane test

  • A pressure gauge pre cat works just fine. My cutoff pressure 1.25 psi. The intake manifold vacuum is also a good clue. If it is not close to the idle reading at 3000 rpm suspect an exhaust restriction. Dec 17, 2015 at 20:46
  • @FredWilson do add that as an answer. The more ways the better. Note that there are other ways for cats to fail as well.
    – Zaid
    Dec 17, 2015 at 20:50
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    Antidepressants - Common antidepressants such as Prozac, Zoloft, Effexor and Cymbalta can be poisonous to cats. Cats are drawn to certain antidepressants more than others, particularly Effexor, which contains a smell and flavor that is seemingly appealing to cats. If ingested, symptoms can include anorexia, lethargy, vomiting, tremors, seizures, hyperthermia, and diarrhea. While antidepressants are sometimes used in veterinary medicine to treat behavioral problems, caution should be taken, as even therapeutic doses moderate to severe clinical signs can occur. Dec 18, 2015 at 0:14
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    @DucatiKiller you forgot antifreeze :)
    – Zaid
    Dec 18, 2015 at 4:17
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    @endolith I added the details of the propane test in another answer and linked the entry here to that.
    – Zaid
    Nov 8, 2016 at 19:39

Even before a cat plugs, you can test your cat using a laser thermometer. You are looking to compare the inlet and outlet temps on your cat. Do the following:

  • Run the engine up to operating temperature (at least ten minutes so the cat should be fully warm)
  • Check and record the inlet temperature. You're looking to measure the temperature right where the pipe goes into the cat.
  • Check the outlet temperature in the same manner.

If the inlet temp is around the same temp or higher than the outlet, the cat is bad. A good running cat could have an outlet temp at least a hundred degrees F hotter than the inlet (should possibly be closer to 200). If you aren't seeing the temperature difference, it's most likely bad.

NOTE: Before you go believing the cat is bad due to an OBD-II check light (P0420), do this test to see if it's actually the cat or if it's the O2 sensor. The computer is fooled quite often by a bad sensor.

  • Yes, I should add an answer that explains how the P0420 code is set
    – Zaid
    Dec 18, 2015 at 4:17
  • @Zaid - Actually I can only tell you what it is and not how the computer thinks it needs to be tripped. Dec 18, 2015 at 4:18
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    Yeah, I'll add it, don't worry
    – Zaid
    Dec 18, 2015 at 4:19
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    Finally, a reason to buy a laser thermometer!
    – bishop
    Mar 10, 2016 at 19:23
  • you mean an infrared thermometer?
    – endolith
    Oct 20, 2016 at 3:44

If an exhaust restriction is suspected a pressure gauge pre-catatlyst works just fine. My maximum cutoff pressure is 1.25 psi but most good systems will be well below that. The intake manifold vacuum is also a good clue. If it is not close to the idle reading at 3000 rpm suspect an exhaust restriction. In the case of a restriction the vacuum reading will be lower than normal. Also, the sound coming from the intake changes to a louder, deeper sound upon revving the engine.

  • How do you measure the pre-catalyst pressure? Drill a hole, then plug it afterwards?
    – dlu
    Jul 14, 2016 at 19:32
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    @dlu It is usually done by removing the oxygen sensor and measuring it there. Jul 14, 2016 at 20:51

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