JDN902

I noticed what look to be exhaust smoke coming from under/around my heat shield so i'm assuming i have a crack in my manifold (might be just making reasons to install my header also). Basically I want to know if i replace my manifold with the header and don't install a new cat am i going to affect the cars preformance? I know i'll have a CEL, but i can live with that for awhile.

BayTib13

Unless you've removed the cat after the flex pipe, you should be fine. Just will have to move the 2nd 02 sensor to behind the cat. I didn't do this, and I'm gettin a CEL and so my car is in some kinda safe mode or somethin, and runnin really rich in high rpms.

koreancorvette01

There is something you can d,o however it kills the o2 after time. I know this because I did it in my car for a longtime. If you leave the second o2 out of the exhaust pipe just hung up out of the way of things in open air you wont have a cel and the ecu will think the cat is ok. The front o2 is where the adding and subtracting of fuel is calculated from . I did this when I ran the factroy ecu on my turbo setup so I would get the least o2 corerection and less fighting from the safc and the ecu. I had good luck with this I think it may benefit you so it may be worth a try.

SpideyShark

our cars have a "safe mode" when the secondary o2 sensor is detached?

Stocker

When something funny is detected (MAFs backwards, O2 sensor failure) they dump fuel in like it's going out of style, run conservative timing, generally causing poor performance. We call it "limp mode" usually but it's supposed to be safe to drive, even if it doesn't sound very nice.

DTN

There's a few options
1. reposition 2nd sensor past 2nd catalytic converter (cali emissions vehicles)
2. add a catalytic converter, and relocate 2nd o2 sensor (non cali vehicles)
3. add a spark plug non-fowler to position the o2 sensor out of the exhaust stream, but only slightly so that it still reacts.
4. do what was suggested earlier and leave it hanging out to air.