What's inside a Catalytic Converter?
 

I chopped open the catalytic converter on my Corolla to see whats inside and how it works. Here's a video:


The catalytic converter is located between the manifold and resonator in the exhaust system. Its purpose is to convert toxic gasses from the exhaust stream into less harmful gases.

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There's an oxygen sensor behind it for the ECU to monitor catalytic converter performance.

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Grinding it open:

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Inside there is a honey comb shaped ceramic membrane coated with precious metals. That's what makes them so expensive.

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The precious metals act as a catalyst, encouraging a reaction to occur from toxic gasses to less harmful gasses in two stages, reduction and oxidation.

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The ECU controls the air to fuel ratio with the first sensor before the catalytic converter. It must be 14:1 for optimal combustion and catalytic converter performance. The rear oxygen sensor ensures enough oxygen exits the converter and that it is working properly. Otherwise you'll get a P0420 or P0421.

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Those precious metals are usually derived from platinum, palladium, or rhodium. Platinum sometimes costs more than pure gold.

What's going on when there are 4 of them on a car.....(my 335i has 4)...that seems like expense and weight....

edit here's a pic, there are two more cats after the turbos....not shown

http://f30.bimmerpost.com/forums/att...1&d=1339722283

The secondary ones are most likely for reducing startup emissions.

I've never had a CAT fail on me on any car, to include the Maxima being 19 y.o. (it has EGR issues, but not CAT, which is original). I've always felt like that's something you don't want to replace due to cost, and the questionable quality on replacements vs OE.....

Also I'm quite fascinated by the fake dual exhaust phenomenon..when did it start, late 80's? Who started it? I know in the earlier days, they were real, with straight pipes right out of the manifolds of a V8. Imagine that today, you can have dual exhaust on an inline 4? Which of course has nothing more than a y-pipe...also curious if an engineer had free rein, what would they design in this day and age, say out of a V8?

Yep agreed. I too was surprised that gold can be used in the cats, no wonder they're so expensive and used ones are in demand.

All the more reason I wouldn't trust aftermarket (I suppose this is just so you can pass emissions)....when things are "precious," human nature is to cheat. imho the incentive to not cheat on OE is warranty coverage. Put it this way, my 1998 Maxima has the original CAT, yes, but from the CAT back, it's had 2 exhaust systems. The original lasted 12 years on the car in the northeast. The Walker lifetime replacement lasted 30 mos. And nobody incl. Walker would honor the lifetime warranty (not gonna ship the system back to them for inspection lol). Now it has a 2nd Walker and some other muffler forgot the brand but powder coated black.....when they build shoddy exhaust parts, I can only imagine how they would treat CATS....

Gold, though, is usually reserved for the coating of air-bag-sensor components. That's because of gold's resistance to corrosion, and the Federal mandate that says that air-bags have to work reliably for at least ten years.