I'm trying to understand what O2 sims do as far as relaying fuel/air mixture back to the PCM. I'm installing a Random Tech dual cat y-pipe and headers on my 97 SS, and was wondering if replacing the stock after cat 02 sensors with 02 sims would be of any benefit. Does an 02 sim relay back to the PCM that the air/fuel mixture is at perfect nominal conditions? Would there be any negative effects of adding 02 sims?

 

The after cat O2s are there to tell the pcm how well the cats are functioning. Basically, a watchdog for the cats.

 

O2 sims genrate a voltage that rapidly fluctuates in the 200-800mV range. The computer interprets this as "stoich" and does not make any fueling changes based on the input. Will it benefit you? I don't know, depends what you are trying to accomplish.

Rich Krause

 

So their pretty much there to send a signal to the pcm if the cats are starting to plug up? The o2 sims are cheaper than getting 2 new 02 sensors; thats why I was thinking about adding them. I guess I was taking the cheaper way out
Has anyone ever had an 02 sim go bad?
If there aren't any negative effects on fuel mixture; I'll probably go with them.

 

Usually you only use sims when you're removing the cats and you don't want the computer to throw an SES light all the time. All they do is fool the computer into thinking it's getting an ok reading from the back two sensors. I don't see how it would benefit your application.

 

The rears don't affect fuel calculations, unless they indicate a problem with the cat for an extended period of time (closed loop only). The pcm will then adjust fuel delivery slightly.