I have a 1988 CRX Si with a Hedman Chikara header (part # 37020). Other than that it is stock. Car has 208,000 miles on it and the header was installed at approximately 200,000 miles. A few months ago my O2 sensor went bad (at 206,000 miles). The only O2 sensor I could get quickly was a generic one from a local auto parts store. Once this new one was installed the code went away from the ECU and I was fine.....for about a day. The code came back and the car ran poorly between 2,000 and 2,500 rpms once it was warm. I had to drive the car like this for about 1,000 miles.
I got my hands on an OEM O2 sensor and again the ECU code went away. All was well until now, approximately 4 weeks and 1,000 miles later, the code is present again. Fortunately, other than an annoying flash from the ECU's LED, the car is running very well.
The original O2 sensor was fine with the header for approx 6,000 miles so I
wouldn't think the header has an effect on it. Any thoughts?

 

I have this same problem to. It gets so annoying! my car runs like crap. i bought a 02 sensor to and it did the same thing. I fixed it it thou. Sometimes when you clip it in you have to clip it in really hard(use some muscle). Mine went away then. 2 weeks ago i had emmissions done, and its giving me the same code so i checked the connection again and its still doesn't work, next thing i'm going to check is the wire(maybe that might help you) i just haven't got to it, to cold outside(and lazyness doesn't help either) hope the clicking thing helps you

 

If all the other connections are good, my only other guess is that something else is making the car run rich enough to set the O2 code...Generally this does not happen, but every so often we see one.. Did you check the exhaust when the light comes on, any black smoke or anything?? You said the car is running well and the MPG is back up since the O2 was replaced with the OEM one...Maybe the circuit in the ECU is cooked due to increased resistance in the O2 circuit for so long....Haven't seen that on too many older cars, but on the newer ones...seen it alot.. Have you double checked all the connections at the entire lenght of the O2 wire?? There are 2 splices in it for the header extension..One under the distriibutor and another near or at the factory O2 connector... Make sure they are both tight and not corroded... If you have a service manual peek in it and see what the resistance in the wire at the O2 should be.. This may help in diagnosing it....

Hope this helps some more...

Jeff