i went to the auto parts store the other day looking for a 02 sensor for my '95 5.9L and they had 2 different ones, an upflow and a downflow. whats the difference?

 

One is before the cat (upstream), one is after the cat (downstream)

 

ok, thanks!

 

The upstream is what gets you better gas mileage, since it deals with your air/fuel ratio. All the downstream does is measure emissions to make sure your cat is working...don't really need to replace it unless it throws a code for it.

 

well i got under my truck to look at things( got my new headers today REALLY EXCITED FOR TOMORROW!) and come to find out, i have only ONE 02 sensor. its right before the cat(which is norm. i believe) but the one im suppose to have after, is gone and plugged. care to explain anyone?

 

Is there a bung for it to screw into, or is there nothing there?

 

theres a hole for it to screw into, its just got a plug in it. i've never changed anything, so i got the truck like that.

 

Did you check to see if there is an O2 simm plugged into the downstream plug? Someone has either put a simm on it or took it off and plugged the hole. If it doesn't have a simm it's weird that it hasn't thrown a code or CEL...

http://www.thefastman.com/02Simms.asp

 

I didn't realize they made a 02 sensor simm. Is this really preferable over an operating sensor when you still have the cat installed? My aft sensor will throw a code occasionally.

 

It's really made for when you gut or delete your cat...to fool the pcm and emissions tests. But you could run it if you were having constant problems with your downstream sensor acting up.