[puddleglum]

I have no experience with Civic's, but In general, the O2 sensor is primary feedback sensor that the ECU uses to determine mixture in closed loop. Without it you car is probably staying in open loop and running on its default programming like it does on initial start up. Computer can't optimize fuel delivery or timing and you could do damage to other parts like MetroMPG said. I have no idea how much it's hurting your FE but it certainly is. I assume your CEL is on. If money is really tight you could maybe try one from a wrecker if you can find one with a good return policy. If you put it in and it works (no CEL) your ahead, if not take it back. Just a thought. I should ask, is it the front or back one that is disconnected? The back one usually just monitors the cat, but the front one is really important.