This is what the second and third wires are for on today's sensors. By the way, the late model Chrysler products are now using 4 wire O2 sensors. Secondly, the computer is programmed not to go into closed loop operation until the coolant temperature sensor tells the computer the engine is warmed up. If the system tries to go into closed loop too early in the warm-up period, the leaning effect of the system would cause driveability problems and pollutants. Thirdly, the computer is also programmed to ignore the O2 sensor at near wide open throttle conditions.
Maximum power requires maximum enrichment.
Also, some manufacturers have a built-in time delay. For example, on some GM models, closed loop is delayed for 1 to 2 minutes every time the car starts. This allows engine stabilization to take place before the engine goes into closed loop. In order to read the O2 sensor, most computers send out a certain voltage to the output terminal of the sensor. This is typically around 450 millivolts.