Understanding Catalytic Converters
 

I'm trying learn more about the catalytic converters on our cars, and have a few questions.

Why are there 2 on each bank? Does the integrated cat function the same as the downstream cat?

Why is the integrated cat called the precat, even though it's the one monitored by the O2 sensors? Why have two but only monitor one?

Since they don't do the sniff test anymore, the downstream cat shouldn't make a much of a difference in terms of getting tested, right?

the pre cat is closer to the engine and smaller, so it gets hot really quick on a cold start, lowering starting emissions.

Once the main or three way cat is hot, it has much greater conversion capacity than the pre cat, so it is the only one monitored.

Removing the main cat will lead to problems in two areas: The car will code as the O2 sensors have nothing to go on. With your CARB inspections, the cats have to physically be there, they actually look.

Just redid my homework, and I think I made a mistake. Haven't been down under the car for a while, but looking at the diagram from ********************************************, there are actually three O2 sensors for both the starter cat and the main cat on each bank, correct?

The location of the sensors depends on the year. On my '01 S, there are 2 sensors for each bank, one before and one after the pre-cat, nothing attached to the 2nd cat. Often diagrams are drawn/labelled to cover more than one model and may show various possibilities.

No, there are only two sensors on each bank, one ahead of the three way cat, and one behind it.

Just want to make sure we're talking about the same things. You were saying the starter/pre cat gets hot quicker because it's closer to the engine, and once the main cat (closer to muffler) gets hot, it'll convert more than the starter cat. But isn't the pre cat monitored by the computer with two sensors? There isn't another O2 sensor after the main (2nd) cat to monitor its performance, so how does the computer know it's there?

the sensor before the pre-cat is monitoring combustion so as to control engine fueling. the sensor after the pre-cat is monitoring whether the pre-cat is working. there is no sensor for the main cat.

the pre-cat, and secondary air injection system on the engine, are north America specific systems due to higher start-up emissions standards. you typically don't find these systems on 'rest of world' cars. that's why one of the tricks if putting catless headers on your car is to get a 'row' flash, because the computer no longer monitors whether the pre-cat is working or not.

It also depends on what year your car is.

What year is your car?

2001, c.a.r.b.

Yours should be like what NewArt posted. Nothing is monitoring the back cats.