Just an added note on the O2 sensor testing:
If the voltage sweeps appear to be consistent using the correct diagnostic tool (say Hurray for OBDII diagnostic tools to read the O2 feedback real time on cars equipped; can't do easilly or w/o an oscilliscope on the LH x.x cars) the difference in amplitude of a old 'weak' sensor sweep compared to a new sensor will not be obvious to the untrained technician. You certainly cannot tell the condition of the signal sweep on a DVM.
O2 sensors get 'lazy' and the timing response as well as amplitude of the signal get weaker with age, until they essentially flatline and give a more or less constant low voltage output. When the ECU sees this type of signal it is read as if the car is running leaner than it should and the ECU will enrichen the mixture. The ck engine light won't light until the sensor is nearly completely toasted, so you could be running on the rich side of tune for years and not know it.
When in doubt swap it out. O2 sensors are like spark plugs, 60k and toss em even if you think they still work. It is far easier to not guess (or buy diagnostic equipment)and be sure it gives a good signal.