I know there are hundreds of posts on the o2 sensor but, I can't find something with my particular problem. I have a 2000 Honda Civic EX. My CEL came on about three months ago. I drove with it on because there seemed to be no problems with the car at all. I finally got it checked out and it pointed to the secondary o2 sensor (one furthest back). I replaced it with a new BOSCH(spl) sensor but CEL still stays on. I unplugged the battery to resart the computer several times for more than 10 minutes with both terminals unplugged. It seemed to turn off for about 5-10 miles but comes back on. I thought there might be a leak in the exhaust but I dont hear anything coming from the exhaust and I cant feel a leak either. Again, there doesn't seem to be any noticable problem with the car except for the CEL and purhaps maybe, a small difference in gas mileage. Any ideas would be much appriciated.

 

I would recheck the CEL. Did you replace it with an aftermarket sensor? If the CEL point to the same sensor, I would suggest a replacement for a new one.

 

I did check the CEL and it pointed to the same sensor after I replaced it. If Bosch is aftermarket than yes, that's what I used. Are they no good?

 

Also, I thought that it wasnet the new sensor for this reason: Before I replaced the original I unplugged the battery to reset the computer. After I plugged it back up and started the car, the CEL came on imediately. After I replaced the sensor, reset the computer, and turned the car on, the CEL didnt come on until several miles down the road.

 

The service manual details how to check whether the secondary O2 sensor circuit is faulty:

http://hondatech.info/downloads/Auto/Manuals/Civic/

 

I think I had the same problem before. It was awhlie ago, I thought the downstream sensor was bad but it turn to be the primary sensor. I even had an Accord that didn't throw out a CEL and it failed smog because of emissions. It turn out to be the primary sensor that was barely working.

 

I tested both sensors with a volt meter before I did anything and the primary sensor got the right reading and the secondary didnt get anything, so I figured it wasnt that. Could it be a leak? Fuel filter? Gas cap issue? Another bad secondary sensor?

 

What specific CEL code is being thrown?

 

If you get a varying voltage on the output wire of your secondary sensor but the computer still throws the "sensor" code, your catalytic converter may be bad.

 

You should at least get a source voltage, if not check the wiring harness leading to the sensor.