# How a catalytic converter goes bad? **URL:** \ **Category:** Factual Questions\ **Created:** [October 4, 2010, 7:31pm UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035 "2010-10-04T19:31:50Z")\ **Posts on this page:** 17\ **Page:** 1
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [October 4, 2010, 7:31pm UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/1 "2010-10-04T19:31:50Z")
Well, how can something with no moving parts go bad? Is it a sensor inside the industry doesn’t care to fix? ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [October 4, 2010, 7:32pm UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/2 "2010-10-04T19:32:54Z")
Besides, isn’t a catalyst suppose to regenerate? ---
**Author:** ![Bill\_Door](https://avatars.discourse-cdn.com/v4/letter/b/50afbb/32.png) [@Bill\_Door](https://boards.straightdope.com/u/Bill_Door)\ **Post date:** [October 4, 2010, 7:40pm UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/3 "2010-10-04T19:40:40Z")
One way is by poisoning. If something coats the catalyst it can’t react with unburned hydrocarbons. Lead, phosphorous, zinc, and I think, silicone, will poison a catalytic converter. Or you could overheat it. If you’ve got a lot of unburned hydrocarbons in your exhaust, like if you were running too rich a mixture, your catalytic converter will run hotter than design and could be damaged by the high temperatures. ---
**Author:** ![Leaffan](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/leaffan/32/299_2.png) [@Leaffan](https://boards.straightdope.com/u/Leaffan)\ **Post date:** [October 4, 2010, 7:42pm UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/4 "2010-10-04T19:42:44Z")
Some just end up hanging out with the wrong crowd. ---
**Author:** ![awldune](https://avatars.discourse-cdn.com/v4/letter/a/c77e96/32.png) [@awldune](https://boards.straightdope.com/u/awldune)\ **Post date:** [October 4, 2010, 7:47pm UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/5 "2010-10-04T19:47:01Z")
What **Bill** said. If the cat on your car has gone bad, it is likely an indicator of another problem with your engine. I know on late-model Saab 9-3s, the ECU will momentarily cut the fuel if the turbo overboosts, which can damage the cat. Again, you probably don’t want to just replace the cat without figuring out what killed it first. ---
**Author:** ![Ludovic](https://avatars.discourse-cdn.com/v4/letter/l/7ab992/32.png) [@Ludovic](https://boards.straightdope.com/u/Ludovic)\ **Post date:** [October 4, 2010, 8:08pm UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/6 "2010-10-04T20:08:34Z")
> [@Leaffan](#): > > Some just end up hanging out with the wrong crowd. And sometimes it get pragnent. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 4, 2010, 8:26pm UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/7 "2010-10-04T20:26:17Z")
The active element in a catalytic converter is sometimes a ceramic honeycomb coated with the catalyst. If the converter gets too hot (if your engine is running rich), the honeycomb can start to break down, and eventually ruin the converter. ---
**Author:** ![GreasyJack](https://avatars.discourse-cdn.com/v4/letter/g/df705f/32.png) [@GreasyJack](https://boards.straightdope.com/u/GreasyJack)\ **Post date:** [October 4, 2010, 8:39pm UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/8 "2010-10-04T20:39:57Z")
A really bad oil burning problem can cause deposits to grow on the catalyst and plug it up too. You can also physically damage the thing-- if you whang it pretty good on a rock or a curb or something the honeycombs can break loose and will sometimes even pass out the tailpipe. ---
**Author:** ![Machine\_Elf](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)\ **Post date:** [October 4, 2010, 8:50pm UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/9 "2010-10-04T20:50:39Z")
> [@beowulff](#): > > The active element in a catalytic converter is sometimes a ceramic honeycomb coated with the catalyst. If the converter gets too hot (if your engine is running rich), the honeycomb can start to break down, and eventually ruin the converter. Even before the ceramic material starts to fail, the heat can damage the catalyst. During manufacture, the ceramic substrate is wash-coated with a suspension of very fine particles of the catalytic material, leaving behind a thin layer of catalyst particles. The fine particle size results in LOTS of surface area that can react with the exhaust as it passes through the cat. If the temperature gets too high, the fine particles undergo[sintering](http://en.wikipedia.org/wiki/Sintering), and adhere/flow together wherever they are touching. The result is a massive reduction in exposed surface area, and a corresponding reduction in the effectiveness of a catalytic converter as a whole. “poisoning” is another way to reduce cat effectiveness. A number of substances can do this: -zinc and phosphorus cause problems over the long haul; tighter emission rules have required cats to work better for longer, and this is why [the amount of ZDDP in motor oils](http://en.wikipedia.org/wiki/ZDDP#Applications) has been reduced over the years. -Lead, in addition to being a toxic exhaust constituent all by itself, poisons cats. -Silicone is another bad one. A head gasket that leaks coolant into the combustion chamber can result in silicone poisoning of the cat. For more info, see [Wikipedia.](http://en.wikipedia.org/wiki/Catalytic_converter#Damage) ---
**Author:** ![samclem](https://avatars.discourse-cdn.com/v4/letter/s/a9a28c/32.png) [@samclem](https://boards.straightdope.com/u/samclem)\ **Post date:** [October 4, 2010, 11:50pm UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/10 "2010-10-04T23:50:18Z")
Correct me if I’m wrong—any catalytic converter will be used up/go bad in about 100,000+ miles of normal driving. They just don’t last forever. ---
**Author:** ![racer72](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/racer72/32/3075_2.png) [@racer72](https://boards.straightdope.com/u/racer72)\ **Post date:** [October 5, 2010, 1:01am UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/11 "2010-10-05T01:01:56Z")
> [@samclem](#): > > Correct me if I’m wrong—any catalytic converter will be used up/go bad in about 100,000+ miles of normal driving. They just don’t last forever. I traded in my 93 Toyota pickup last year, it had 255,000 miles and still had the original catalytic converter. I passed the emissions test with flying colors a few months earlier. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [October 5, 2010, 2:53am UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/12 "2010-10-05T02:53:31Z")
> [@beowulff](#): > > The active element in a catalytic converter is sometimes a ceramic honeycomb coated with the catalyst. If the converter gets too hot (if your engine is running rich), the honeycomb can start to break down, and eventually ruin the converter. I know it sound counter intuitive, but rich mixtures run cool, lean mixtures run hot. As has been mentioned, a number of things can kill the catalyst. One more is misfires can melt a catalyst. ---
**Author:** ![Sunspace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/sunspace/32/1250_2.png) [@Sunspace](https://boards.straightdope.com/u/Sunspace)\ **Post date:** [October 5, 2010, 3:15am UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/13 "2010-10-05T03:15:02Z")
And whatever you do, don’t just open the thing up to have a look at it. Because, you know, curiosity killed the cat. :: d&r :: ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 5, 2010, 3:24am UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/14 "2010-10-05T03:24:23Z")
> [@Rick](#): > > > [@beowulff](#): > > > > The active element in a catalytic converter is sometimes a ceramic honeycomb coated with the catalyst. If the converter gets too hot (if your engine is running rich), the honeycomb can start to break down, and eventually ruin the converter. > > I know it sound counter intuitive, but rich mixtures run cool, lean mixtures run hot. > As has been mentioned, a number of things can kill the catalyst. One more is misfires can melt a catalyst. My understanding is that this is true for engine cylinder temperatures, but that the unburned hydrocarbons can over heat the Cat, no? ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [October 5, 2010, 3:39am UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/15 "2010-10-05T03:39:30Z")
Only in the presence of excess oxygen. In the older days cats often had a tube that was fed with air from the air pump. In this case a rich mixture would burn in the cat. Newer cars don’t have air pumps. Now if the mixture is rich, there is no excess oxygen. fuel + no oxygen = no fire in cat. If the mixture is correct, the reducing side of the 3 way cat liberates oxygen so the oxidizing side of the cat can work. Get the mixture too lean the the misfire will cause the cat to melt. (Misfire = unburned HC and a bunch of extra oxygen + heat from the cat = super hot fire inside the cat. The ceramic substrate can melt. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 5, 2010, 3:41am UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/16 "2010-10-05T03:41:30Z")
Makes sense. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [October 5, 2010, 3:42am UTC](https://boards.straightdope.com/t/how-a-catalytic-converter-goes-bad/556035/17 "2010-10-05T03:42:45Z")
😃