My engine had been cutting out a bit under acceleration and studdering occasionally for a while. When the check engine light came on I took it to AutoZone (thanks for the tip) and their reader said the primary oxygen sensor was the problem. I replaced it with a Bosch, the light stayed off after disconnecting the battery and stayed off. The car ran wonderful for a week, and then the light came back on again. The car still felt like it was running perfectly this morning, but the light persists even after pulling the negative lead. I just checked the code myself by counting flashes etc. as shown in a different thread. Just one long flash, then nothing. If I did this correct, and truly have just one flash, that is the code for the primary oxygen sensor again. What's going on!? Those things are expensive. What would cause a brand new o2 sensor to fail? Again, the engine feels perfectly smooth. 2000 Civic LX...and thanks.

Edit: Upon the recommendatino of folks at AutoZone I ran some fuel system cleaner and a tank of high octane through the sytem prior to changing the original bad sensor. Otherwise, nothing has been modified to the vehicle or exhaust anytime recently. No custom exhaust or other changes. I put on new plugs and wires several months ago, and new exhaust from the (resonator?) on back to the muffler about 6 months ago. 92,000 miles on the car. Got 42 mpg last week, was down around 36 before I put the new o2 sensor on.

 

If you have a test pipe or a removed cat, i know the secondary sensor will thorw signals just because of the failed emissions, but i'm not sure why the primary would do that. Do you have a header on your car? If you do it could be becasue of the increased exhasut flow you're o2 sensor might not know whats going on. I think an o2 Simulator could easily fix that though. Idk man, there are people on here that know alot more than me, just wait, they'll respond. Good luck


EDIT: Also check where the wires from the o2 sensor itself connect to the little gray harness thing in the engine bay. That could be bad too.

 

You didnt' connect the wire backwards did you? i accidentally did that on my old corolla and the CEL cept comming back on at random times, and after i reset the ecu it would go away most of the time.

 

No, the harness can only go one way. Besides, things were fine for a week. Also, why is it running perfectly now if the sensor is actually faulty?

 

Could you maybe have read the code wrong?

 

I don't think I had pulled the negative terminal long enough to reset the system last time I tried...I did that, and now the light is off?? I don't know why it ever came on, but the car is running great and for now I have no more worries.