# How important is Catalytic Convertor flow direction



## grungti (Jan 14, 2008)

Okay, weird question I know but here's the back story: Bought the car ('00 Passat 4-motion 2.8 ATQ engine) a year ago and it passed the emissions test. 
Now it has 5 codes that say both O2 sensors after the cat's are toast. P1122, P1105, P1114, P1140, P0160 
I crawled under the car to check the wires in case they were burnt. I've seen that before. 
In looking at the rear O2 sensors, I realize that they are UPSTREAM of what I am pretty sure are the cats. Huh?? 

So I checked the Bentley and it indicates the car should have one 3-way cat on each side, with the first or upstream O2 sensors just after the exhaust manifold (these are in the proper location and work fine) and the second or downstream O2 sensors after the cat. 

The reason I want to know about the flow direction is simply that if the direction does not matter, then the repair is to drill new holes after the cats and weld in new bungs. 

If the cats are directional, then the repair is to drop the whole exhaust system, cut it apart and re-weld it back together for the correct flow direction. 

In both cases, I have to put in new O2 sensors but the first option is easier and cheaper and I can handle that in an hour or so. The second option will take me lots longer. 

So, does anybody know if the direction of flow is important in a three way cat? 

Alternately, is there a first cat upstream in the exhaust pipe that looks like a large pipe (about 4 inches in diameter) and it tapers gradually to the 2.5 inches or so inlet to the cat? My other Passat is a 1.8T and the cat on it after the turbo is really obvious. 

Any help/advice is greatly appreciated.


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## 155VERT83 (Aug 1, 2000)

I asked a mechanic the same question. He stated that universal replacement cats can be installed in either direction.


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## Slimjimmn (Apr 8, 2006)

precat o2 sensors are up by the cylinder heads pointing straight up 

the post cat sensors are underneath the car by the end of the trans.


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## O_o (Nov 12, 2009)

Uh... aren't the cats in the downpipes? There's only one way they'll fit...


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## Slimjimmn (Apr 8, 2006)

yea


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