I recently bought a 1997 honda civic DX. It failed smog and gets poor mileage (29 mpg). Check engine light came on, O2 sensor slow to respond bank 1 sensor 1. Replaced O2 and no difference. Took the car to Honda and they said bad O2 sensor, so I purchased one from them and installed it. Still the same problem. The car also seems to bog down between 2500-3500 rpms which the smog shop also noticed while it was on the machine. The smog guy commented that when the engine bogged down the CO went up considerably. I have also changed out the map sensor and spark plugs and that did nothing. Any help or ideas would be appreciated.

 

did u replace the correct 02 sensor?

 

Yes, I first replaced it with a bosch unit and took it to Honda and they said it was bad, so I bought a genuine Honda O2 and installed it but that did not work either.

 

I'm not sure what could cause it exactly, but maybe the sensor is working, and there is something wrong with the a/f ratios that it is reading to cause the fault. Which could explain your bad gas mileage. Although, 29 mpg isn't too shabby IMO...especially on a 12 year old motor.

 

Sensor 1 is the one closest to the engine, the one before the catalytic converter, that's what jprommel meant by "replacing the right sensor."

There is a problem with 96 and 97 model years where the wiring behind the engine can gets frayed and cut power to the sensor. This usually causes codes for both sensors as well as some other problems though. Check fuse #15 under the dash. Make sure there is 12 volts going to the heater circuit of the sensor and the sensor ground wire is grounded.

And like cvcrcr suggested, if your mixture is off (typically due to the wrong fuel pressure) you will get bad gas mileage, fail smog, and quite possibly an O2 sensor code even though the sensor is OK. Check that the vacuum line on the fuel pressure regulator is hooked up properly. It would be good to check the fuel pressure with a gauge.