Can anyone tell me the symptoms of a faulty sensor in the exhaust dump pipe? I've had some flat spots through the rev range for a while now and just couldnt work out the problem.

having fitted a stock 02 sti exhaust recently the sensor cable was too short to reach from the standard length and couldnt be connected. Without the sensor hooked up the car was very smooth and had no flat spots - fantastic actually.

I recently hooked the sensor back up and the flat spots or hesitation is back. Someone mentioned it wasnt wise to drive for long periods without it working. Stock ECU so im wondering:

1. Does anyone have a spare sensor laying around I can use to test?
2. Can the car run without it hooked up?

Much appreciated for any help.