On my 1995 Honda Passport V-6 3.2L 4WD (also Passports 1994-1997 and similar Isuzu Rodeo's), when the O2 sensor fails, the mixture becomes EXTREMELY rich. So much so that the engine only runs smoothly at idle. Most accelerator pedal positions will cause the rich mixture to burn very poorly and the engine looses 95% of power. Black smoke pours from the tailpipe, the exhaust smells of raw fuel, and mileage drops to about 5 mpg. When pressing the pedal, the engine vibration makes almost a honking noise, and it actually feels like a goose is passing through your cylinders. The primary fix is to simply replace the O2 sensor - any other cleaning or replacement of plugs, filters, wires, injectors, etc. is probably unnecessary and purely optional, as the sensor replacement made the only noticeable difference, and a drastic one at that? Can't these systems be designed fail more gracefully?