Thanks for input ever_green!
However, with all due respect to your experience, bear in mind that the car in question is 20 y/o and has over 120K miles on it.
There is no CEL, there is only one o2 sensor - in front of the cat. It's not an OBD II car, it has a 4-speed auto transmission with no electronic control whatsoever.
I also have a 25 y/o car with over 400K miles on it and one that is over 40 y/o.
I love older cars and I do my best to keep them in top condition. However, technology advance is now faster than ever. You cannot compare a 40 y/o car with a 20 y/o one and a 20 y/o one with a 10 y/o one.
There's an informative article about o2 sensors here:
Oxygen Sensors & Emissions - Facts & Repair Advice
My sensor is a little bit overdue for replacement.