Damn Eric, are you that disenchanted with me?
Oxygen sensors are exactly what they are called. They sample the exhaust gasses, and tell the computer how much o2 is in the exhaust stream, the computer, decides what to do about this, We are rich ( too much fuel, or lean, not enough fuel ) and makes decisions based on this info. The sensors have progressed over the years and can start making decisions in 30 seconds, and now we have wide band sensors, that are much more sophisticated, they sample ambient o2, to judge against, the o2 in the exhaust stream. These really work, but new o2 sensors can cost 275.00 for the part, + installation, and you may have 4-6 of them. It can be REAL expensive if done right. I did say done right, if your over 90k miles, and only one is acting up, the other 3 -6 are soon to follow, do 1 or do them all, it is your decision.
Now of course this example is WAY over the top. JEEZ. but still not cheap.