Ok, I did a search on this and couldn't really find and answer. My Hypertech scanner is showing that one of the O2 MV is 8 and is not moving. I changed out the passenger side O2 and changed the O2 sim behind the cat nearest the rear of the car. The car only gets about 140 miles per tank where as a few months ago it got about 240 miles per tank. I'm not getting any service engine lights and the car seems to run fine. Also, I just dynoed and only got 300 rwhp. I got 300 rwhp before my headers a few years ago. And now after headers, still only 300 rwhp. The Dynometers were different models.

 

Your O2 milivolts should bounce around like nuts. If they stay steady or only bounce in a short range they are most likely done.

 

Three are dancing and the one is reading 8. Could it be the computer?

 

Is it the front or rear O2 that is steady?

 

I'm not sure. I think it is the 2nd one on the screen of the Hypertech scan tool.

 

Anyone else?

 

You need to figure out whether it is a pre-cat or an after-cat sensor that appears to be locked at "8". If its a pre-cat, it is affecting performance. If it is an after-cat it is not. In any case, with a defective O2 sensor, you should be getting a code and the SES light.

Will your HPP scan cartridge indicate "closed loop enabled"?

 

Yes it shows closed. I also changed out the other O2 tonight. So both precat O2 sensors are new. It's still locked on 8mv. I even put my old stock computer in and still 8mv. I put a new O2 sim on one side and it still shows 8mv. What gives?

 

Anyone?