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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?

Peter Mortensen
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William
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  • 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. – Alex Cannon Oct 07 '21 at 01:29
  • @AlexCannon it’s a Honda Element so I’m pretty sure it’s not an easy replacement – William Oct 07 '21 at 02:06
  • The Wikipedia page is a good read - https://en.wikipedia.org/wiki/Catalytic_converter – HandyHowie Oct 07 '21 at 07:57
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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 Oct 07 '21 at 08:10
  • Be aware that one of the key ingredients in a catalytic converter is platinum. Yes, that platinum, the very expensive precious metal. There's nothing cheap about them. I'm sure you could install a "cheap" cat in your car, but it might not run as well, and, if you've got emissions testing where you live, it'll probably fail the tests. Then you have the privilege of paying to install the "cheap" one then paying again to install the "proper" one. Sometimes it's best to just bite the bullet up front and do it right. – FreeMan Oct 07 '21 at 13:00
  • Of course, if you don't have any emissions testing at all, all you need is a section of exhaust pipe welded in between the front & rear sections of the exhaust system to ensure the gasses go out the back of the car and don't enter the cabin. Of course, some people might yell at you for harming the environment, but if you don't care, you'll actually get a bit better performance out of the car. NOTE: This is not a recommendation to do this, just a simple statement of fact. – FreeMan Oct 07 '21 at 13:02
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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. – Vanity Slug - codidact.com Oct 07 '21 at 13:58
  • So the engine knows whether it has been stolen. – Eric Hauenstein Oct 07 '21 at 20:49
  • @wha7ever That's because it's illegal to sell used catalytic converters at least in California. – user71659 Oct 08 '21 at 01:57
  • @FreeMan That's if you don't mind the ECU yelling at you because the exhaust system doesn't behave the way it expects, showing a "check engine" light and dropping into emergency mode with vastly sub-optimal performance. (I guess in some cars it might be possible to hack around that, but it's still more complicated than welding a piece of pipe in.) – TooTea Oct 08 '21 at 07:07
  • @TooTea No, fundamentally only the pre-cat O2 sensor is used to regulate combustion. The post-cat sensor, which is usually a cheaper, less precise type, is only there for emissions monitoring, i.e. activating the CEL so that the vehicle fails inspection. Low cat efficiency is not a reason to trigger limp home mode. – user71659 Oct 08 '21 at 08:22

2 Answers2

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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.

Harper - Reinstate Monica
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  • 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 Oct 06 '21 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 Oct 06 '21 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. – Harper - Reinstate Monica Oct 06 '21 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 Oct 07 '21 at 12:41
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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.

Peter Mortensen
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Alex Cannon
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    Do they make a conversion box that will send the oxygen sensor signal so I can install an older/cheaper one? – William Oct 07 '21 at 02:28
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    @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 Oct 07 '21 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. – Alex Cannon Oct 07 '21 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. – Alex Cannon Oct 07 '21 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. https://www.summitracing.com/parts/fif-60012, for example. – dannysauer Oct 08 '21 at 18:57