I was getting O2 codes, can't remember just what they were - changed all plugs and plug wires and the problem went away. (I knew the plugs and wires were bad, the car had 101,000 on it.)

SOMETIMES the code does not mean a bad sensor but that the sensor's data is out of bounds. Question is, is it out of bounds due to a bad sensor, or is the sensor telling the truth and something ELSE is causing the out of bound condition? In my case, the O2 level was showing too high because it WAS too high, the engine was missing and therefore passing unburnt O2 thru.....