Purpose of downstream pre cat O2 sensor

[wantmorepower]

Is it just me or does it seem that there is no purpose to the 02 sensor that is just ahead of the catalytic convertor. There are 2 one in each manifold. Then one right before the convertor. Finally one after the Cat.

Someone in the know could you explain the reason for the downstream pre cat o2 sensor?

[wantmorepower]

BTW this is a 96.... just had to replace. (Surprisingly simple considering)

[wantmorepower]

Anyone Anyone....... ?

[MrCoffee]

http://www.zzperformance.com/products1.php?id=179

[cboney2003]

THERE IS A PURPOSE!!! DO NOT USE AN O2 Simulator on the PRECAT Sensor!!!

The sensors and their functions:

Manifold Sensors - One per exhaust manifold or header - Used for air/fuel trims

Precat Sensor - One before the stock cat - USED FOR AIR/FUEL TRIMS

Postcat Sensor - One after the stock cat - only used for catalytic efficiency test, you can use an O2 sim for this one.

If you need new O2 sensors like I did, go on ebay and check this out, I just got three brand new GM factory sensors for $38 a piece for the manifold ones and $35 for the precat one, if you don't see them all there, email the guy, he has them and my car runs PERFECT now that i replaced mine. They list for $60+ a piece on gmpartsdirect not to mention shipping and wait times, get them off ebay!!!

Guy's name is Kevin Simmons, email is iceberg117@hotmail.com. His ebay handle (seller name on ebay) is: global-automotive

Manifold Sensors: Delco O2 Sensor AFS-109
Pre and Postcat Sensors: Delco O2 Sensor AFS-97

Both the manifold sensors are the same and both the precat and the postcat sensors are the same. For a highflow cat setup or no cat setup, get two new manifold sensors, one precat sensors, and an o2 simulator, and enjoy a good running engine! I had SIX engine codes (too lean both sides, o2 heater malfunction, mult/random misfire, etc etc) until I changed all mine and now, NONE. Best fix I have ever done to the car. Hope this helps! (instructions are for a 95-97 3.8, not sure of differences for the newer ones)

[AZ3.8Camaro]

Are you sure that the one before the cat controls a/f trims?

[cboney2003]

Yes, I have confirmed it with two mechanics that work on GM's that I trust and also, if it only checked for catalytic efficiency, me having no cat and no o2 sim on that one would set off a light right? Well, I have an o2 sim on the postcat but not the precat, and no SES [img]smile.gif[/img] Trust me, I've been running without a cat for a while now and haven't had one problem or SES light since I replaced my o2 sensors and had the o2 sim on the postcat.

[wantmorepower]

Will have to save that info.... i just paid 47 for the manifold 02s and almost 60 post the downstream sensors..... TY for the heads up i'll prob be giving him my business for the other sensors when needed.

I still cant really see the reason for the downstream sensor.... it would seem to me to be used to help guage cat performance and thats it.... espically since it's identical to the after cat sensor ... hmmmmmm

[AZ3.8Camaro]

Originally posted by cboney2003:
Yes, I have confirmed it with two mechanics that work on GM's that I trust and also, if it only checked for catalytic efficiency, me having no cat and no o2 sim on that one would set off a light right? Well, I have an o2 sim on the postcat but not the precat, and no SES [img]smile.gif[/img] Trust me, I've been running without a cat for a while now and haven't had one problem or SES light since I replaced my o2 sensors and had the o2 sim on the postcat.

I always thought that the o2 sensors in the manifolds checked a/f trim. The pre cat and post cat sensors were for cat eff. I don't understand exactly why they would use 3 sensors to andjust fuel trim.

[F1GT]

Originally posted by AZ3.8Camaro:
</font><blockquote>quote:</font><hr />Originally posted by cboney2003:
Yes, I have confirmed it with two mechanics that work on GM's that I trust and also, if it only checked for catalytic efficiency, me having no cat and no o2 sim on that one would set off a light right? Well, I have an o2 sim on the postcat but not the precat, and no SES [img]smile.gif[/img] Trust me, I've been running without a cat for a while now and haven't had one problem or SES light since I replaced my o2 sensors and had the o2 sim on the postcat.

I always thought that the o2 sensors in the manifolds checked a/f trim. The pre cat and post cat sensors were for cat eff. I don't understand exactly why they would use 3 sensors to andjust fuel trim. </font>[/QUOTE]He is right, all 3 of them are to adjust for air fuel ratio. I read that somewhere, maybe if I "can" find it, i'll post it.

And ZZ Performance sucks donkey balls. i tried ordering their $30 "emulator" last night and it kept saying my cart is empty, so I said Ok, maybe I need to sign up first, so I did. Everytime I log on, it brought me back to the log on screen. I got frustrated and bought from ThunderRacing instead. Though it cost 10 bucks more. :rolleyes:

[Snuffalofogus]

is there a precat sensor for 99+?

I thought it was only for earlier years

[98Camaro3.8]

Originally posted by Snuffalofogus:
is there a precat sensor for 99+?

I thought it was only for earlier years

98+ don't have a pre-cat sensor.

[Turbo V6 Camaro]

ehhh

i replaced my pre cat with a WB02 [img]graemlins/banana.gif[/img]