i recently installed a test pipe in my '99 civic and threaded in the o2 into the test pipe. the cel light is now on and i'm wondering if the cel will throw the fuel trim or anything related into failsafe mode or otherwise affect the performance. some say only the primary o2 will have effect on engine performance. need to know so i wont waste money on an o2 simulator if not necessary.

 

You should get a Code 1 if the O2 does not function properly. I turboed my engine without an O2 sensor and the result was bad gas mileage.

 

As long as you have your primary O2 then you are ok. Thats the one that reads the air fuel, so it can adjust accordingly. Dont worry about the secondary, its sole purpose is to make sure the cat is doing its job. However if you are bugged about the cel always being on, then get the simulator to keep it off.
option #2 Go obd1 and lose the problem completely.

 

Yep, same here. I was getting about 290-300 miles per tank of gas, and the one tank of gas I went through with my CEL on, I got 240 miles, so I definately felt the hit there.