Recently I had to get the catalytic converter in my automobile replaced because it was stolen. The repair shop said that to do it properly, and have the engine light go off, they have to use a more expensive catalytic converter.

Why are cars designed like this when it is just a simple filtration system? Is there a car designed specifically for the cheap catalytic converter in mind?

  • Have you checked the junk yard for a used cat for your car or for a different car with a similar engine size and horsepower? The exhaust shop should weld it in for you. Commented Oct 7, 2021 at 1:29
  • @AlexCannon it’s a Honda Element so I’m pretty sure it’s not an easy replacement
    – William
    Commented Oct 7, 2021 at 2:06
  • The Wikipedia page is a good read - en.wikipedia.org/wiki/Catalytic_converter
    – HandyHowie
    Commented Oct 7, 2021 at 7:57
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    Cats have a lot of expensive metals in them, there's no such thing as a cheap one. It's also a question of location in some cases, the anti-smog laws may be laxer so the cars have less emission control.
    – GdD
    Commented Oct 7, 2021 at 8:10
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    @AlexCannon Can't speak for all junkyards out there but my local junkyard has to take out cats and dispose of them in a fashion inline with regulations. So there are no cats on my junkyard. Commented Oct 7, 2021 at 13:58

2 Answers 2


It is not a simple filtration system. It is, as the name implies, a catalyst system.

In order to work properly, the catalyst must be maintained in the correct condition, which means, being fed the right amount of oxygen relative to fuel.

To know how much that is, the engine computer needs sensors in the cat. It then adjusts engine injection parameters to hit that number.

Without those sensors, the cat just won't work.

What is a cat doing with oxygen? Two different things. First, it wants a lack of oxygen (rich mixture) so it can reduce nitrogen oxides to nitrogen. Then it wants an excess of oxygen (lean mixture) so it can oxidize carbon, unburned hydrocarbons and carbon monoxide into CO2.

Now if you have one of those 80's cars, you have two separate catalyst beds. The engine is run intentionally rich, so the first catalyst bed can reduce nitrogen oxides. Fresh air is then injected, so the second bed can oxidize the hydrocarbons. The fresh air comes typically from a belt driven air pump.

In newer cats, they eliminate the air pump by using a cat with a heck of a trick: it stores oxygen momentarily. However, the computer must wobble the exhaust very precisely between rich and lean, in the right amount and timing to keep the cat stocked but not overwhelmed with oxygen.

It may also help the computer to know the temperature of the cat.

So that's why you need sensors on a catalytic converter.

Have your exhaust shop bolt on a plate of expanded steel mesh across the cat. The cat thief will most likely just leave and steal another car's cat. Thieves don't like variables.

  • Are there any modern cars with passanger air bags and modern features like back up cameras that use cheap catalytic converters(yeah I know this isn’t supposed to be a recommendation site)?
    – William
    Commented Oct 6, 2021 at 22:58
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    @William Modern cats are cheap, considering how long they last, compared with the original ones which self-destructed every 3 or 4 years if you were unlucky.
    – alephzero
    Commented Oct 6, 2021 at 23:06
  • @William I don't know because parts pricing is so capricious from automaker to automaker. Best you could hope for is research several makes of car on your list, get their basic stats, and shop for cats online. Dealer parts departments, would want the VIN. Anyway when someone steals your cat, it's not like you buy a new cat at O'Reilly and done. They probably destroyed a lot of other stuff - exhaust header, post-cat exhaust line, oxygen sensor bungs, the sensors, damaged the wiring harness if they tore it out violently... the cat could be the least of your problems. Commented Oct 6, 2021 at 23:11
  • @William Very unlikely, because the quality of cat affects the emissions standards the car can meet. The only way you're going to get this is with something where the design predates modern emissions regs and is still manufactured, because the regs you have to meet are (mostly - it's country-dependent) linked to the year of design or first sale. The Land Rover Defender was an example of this, with a 33-year production run until the Euro-NCAP pedestrian safety regs finally shut it down.
    – Graham
    Commented Oct 7, 2021 at 12:41

Short answer: Since the OBD-II days there is a second oxygen sensor that is required down stream of the catalytic converter to verify that the catalytic converter is working. If it isn't, you get an emissions code and you fail the smog test. The sensor may screw in to the back of the catalytic converter itself.

  • 1
    Do they make a conversion box that will send the oxygen sensor signal so I can install an older/cheaper one?
    – William
    Commented Oct 7, 2021 at 2:28
  • 2
    @William Heck, you can get an addon that will let you delete the cat altogether and just put a straight pipe in. Just don't do this where there are emissions tests and don't ask me how or where to get one.
    – Glen Yates
    Commented Oct 7, 2021 at 14:53
  • @Wlliam you could buy a new oxygen sensor to replace the stolen one, and then go to the junk yard and find a catalytic converter for a similar sized engine and see if it fits in. You could drill and retap the threads if yours is a little bigger. Did they chop the wires off before the connector? Then see if you can have an online junk place send you part of the wiring harness from a junked car like yours to fix it. Maybe you can get the whole sensor and the wires from them. Commented Oct 7, 2021 at 17:19
  • Or get a nut at the hardware store that fits the oxygen sensor and have someone make a hole just behind the catalytic converter and MIG or TIG weld it in to the hole. Or instead of a nut drill and tap through a small piece of steel. If you want this repair to last more than 10 years consider painting the repaired steel with high temp paint or using stainless steel. Chances are the exhaust place didn't use stainless steel welding metal anyway though. Commented Oct 7, 2021 at 17:24
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    Re: retrofitting read sensor to old/cheap cat... O2 sensors use M18x1.5 threads pretty much universally. A "spark plug antifouler" for the bigger thread spark plugs is one common "couldn't find a proper bung" option. You can also get O2 bungs attached to a hose clamp used for older cars which want to retrofit a sensor for data logging or similar performance-oriented pursuits, where there's no tapping or welding required. Just drill a hole and clamp over the top. summitracing.com/parts/fif-60012, for example.
    – dannysauer
    Commented Oct 8, 2021 at 18:57

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