ok so i'll just tell it like a story, but im not sure if anybody will be able to tell me what it is just by this, but anyway:

well, i have a '99 civic si stock...i was driving home from work, and it was snowing really bad so i had to go about 40 mph. i warmed my car up 10 mins. before i left, and when i'm about half way home (30 min drive) my cel light comes on out of nowhere. everything was still performing fine, no wierd sounds, temp. was fine, nothing noticeably wrong. now, a week or 2 after i got this car, the cel light came on but i didnt have any money to get it fixed. so i took it to autozone for a code check and i forget the code but it was my o2 sensor. i drove my car with the cel light on for about a week, cuz i was told i didnt have to change the o2 sensor right away. i really needed new brake pads, so i was going to get a new o2 sensor after my brake pads... but right after i got my new brake pads, the cel light wasnt on anymore. so i just didnt worry about it anymore, and now about 3 months later it comes back on. so, i know it could more than likely be my o2 sensor again, but i havent noticed any drop in mpg? any help would be nice

 

Go back to autozone and get the exact code. There are 2 O2s in a OBD2 civic.. the code will tell you what is wrong (heater ckt, low voltage, etc) per bank. The intermittent part of this makes me think you're throwing the slow response code. This one usually means a new O2, but I always check the wiring first. If you know someone with a good OBD2 scanner you can watch the outputs of the O2 and make sure of the issue before you replace it.