I recently acquired a 1995 civic (4 wire O2 plug) and it has aftermarket header and down pipe and the O2 sensor has been removed (has bung plug or whatever in the hole). Car seems to run fine but has a light surge under acceleration sometime not sure if that has anything to do with it. Gets 40+ mpg. It has p28 ecu and it's not chipped. What I really want to know is do I need the O2 sensor? What's the pros and cons of having it versus not having it. Thanks.

 

Yes, you need to o2. That, long with the MAP tell the ecu how good of a job it is doing tuning the car. Have you checked your timing? Plugs?

 

The ecu is chipped. That's why you have the surge acceleration. O2 sensor is only working at idle and at a cruise (steady throttle such as freeway). If you have no cel for o2 that's a dead giveaway that the ecu is chipped. The o2 is there to control air fuel ratio, and oxidize the cat to keep it functional. And no, you are not getting 40mpg. Especially without the o2.