kyle97

I always thought the post-cat o2 sensors had nothing to do with fuel delivery, just to set the ses light.

However, after checking AuoTap, it looks like the rear O2s are responsible for determining a short-term fuel trim value. I'm running my car (no cats) w/o anything plugged into the rear o2s and im wondering if this short-term trim can effect the long-term fuel trim values.

Basically -- do i NEED o2 sims, or are those just to make the ses light go away?

Injuneer

How did you come to the conclusion from looking at AutoTap that the after-cat O2 sensors are controlling the short term fuel corrections? Why do you not think the pre-cat sensors are controlling?

The short terms make temporary corrections. The PCM watches the short terms, and if they are always adding fuel, it cuts the long terms until the short terms average 128 over an extended period. Short terms drive the long terms.

cjiaf

you need them casue you wont be able to pass emmisions without them....

Injuneer

The after-cat sensors don't control emissions.... they just tell the PCM whether the cats are there and working. Its possible to pass emissions with O2 sims in place of the sensors. Of course, without cats there will be a bit of a problem.

cjiaf

yeah you do need them so you dont set the ses code cause for 96 and up they do the obd11 plug in instead of the sniffer test and having the ses light on will not pass you for emmissons.....the eye test will fail you too....so you have to figure that one out.....

Honda Hunter

I think he is saying there are plenty of ways to fool them. With ses sims I gather it's like having the stockers there.
Also with a gutted cat youed pass the vis. So if that's all you have to worry about anyone can sneak by.

stingercamaro

Not to hijack your thread but I have a similar question. If I had Ion reprogram my PCM with the rear O2s deleted, do I still need the sims?

kyle97

Yes, I understand that the pre-cat sensors are primarily responsible for trimming fuel delivery. Its just that in AutoTap, they list Short-term fuel trims for BOTH the pre and post cat O2 sensors. I dont actually know whethor or not those post-cat "trim" values are really affecting the fuel delivery, or if it is just a misnomer in autotap -- so thats really what i was hoping to get an answer about.

Dave '97 Z28 M6

It all depends on the testing procedure. Like many people in many states/provinces, I have to pass an emissions test. But it is strictly a sniffer - they do not plug into the OBDII computer to look for codes or readiness signals (good for me, 'cause I'm going OBDI).