I had similar problems with my '97 HX with the 5-wire sensor costing so much. After reseting the ECU, I would get the o2 sensor code and it would run very rich in the dummy mode-(black gas smelling smoke out the tailpipe even). One day I decided to reduce the idle from like 800 to 600 rpm. I followed the instructions in the Haynes manual and after reseting the ECU and idle...the problem was gone. No black smoke and average mpg went up from 32 to 43mpg. I can't explain it but that's what happened and that was last july.