Hello HT. Just out of curiosity, how many of you run your O2 sensor welded to your exhaust without having a cat installed? I have a 99 civic hatch w/ an OBD1 GSR motor. I only have to worry about the primary O2 sensor. My header didn't reach my stock exhaust so I had to go an exhaust shop so they can install some piping to it to make it reach. Since I went this route, I obviously have no catalytic converter. My car gets 24-25 miles per gallon on average under normal driving habits. I was told that I should be near 30mpg and that the O2 sensor should improve this. I do have a CEL for no O2 sensor, and my exhaust has no O2 bung. Will welding an O2 sensor to the stock exhaust improve mpg? I was also told that the O2 sensor welded on to the exhaust might not make a difference since there is no cat installed, and it might still throw off a CEL since the car might be running rich or lean (I forgot which one of those it was).

There is also the "AEM no-weld O2 bung" for an exhaust but I don't want to deal with drilling. Should I go to an exhaust shop to get my O2 welded, or am I forced to get a new catalytic converter? This might have been covered before, so I apologize. Any comments, help, input, will be appreciated. Thanks

 

The primary O2 sensor doesn't care about your cat. The secondary O2 sensor is there to detect catalyst efficiency - it has nothing to do with your fuel trims.

 

What ecu and what code?

 

P72 ECU and CEL 41

 

The primary o2 isn't the one that plugs into the cat? so its no big deal then if the o2 is welded on the exhaust?

 

The primary goes before the cat, the secondary goes after the cat. Neither goes "into" the cat. Although, to the layman, it might appear that the secondary goes "into" the cat...but it's just in the piping after the cat, before the flange.

 

Ah, I see. Thanks for clarifying that