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As an Oxygen sensor ages, it's chemical reactions that produce the voltage signal begin to slow down and diminish. It will react slower to the mixture changes (which swings back and forth around the sweet spot very rapidly). The diminished output will also mean that it will produce less voltage than what it should be (at that particular mixture), and the false/lower voltage signal will instruct the computer to enrich the mixture to bring it back up to what the computer perceives to be a correct mixture (which is now actually slightly rich).
I can't give you a definate life expectancy of how long your Oxygen sensor will last, but I would assume that you're fuel economy is poorer than what it could be. I personally replace mine every 5 years or 50,000 miles, but that's a bit more often than typically necessary. The service change interval from Volvo is in the neighborhood of 60,000 miles, but I wouldn't be concered if you let it go past 80,000. -definately try to change it before 100,000 miles. Think of it this way, the mild fuel economy benefit from changing the sensor will quickly pay for itself.
The '85 and later Volvos with LH 2.2 and LH 2.4 fuel injection use a 3-wire Bosch sensor (two wires are for the internal heater). Rather than paying $100 for the factory fit sensor (with appropriate connector already on it), I prefer to spend $40 on a Bosch 13913 and then splice it to the old wiring. The trick in this operation is that you have to use physical crimp-splices and seal it with shrink tube over the wires. You can not use solder since it fills in the spaces amid the wire strands, and the Oxygen sensor "breathes" very slightly down the length of the wires to get it's outside air reference sample. A crimp-splice and shrink tube leave this small airway open.
Also, since your sensor has been in your car for so long, it is probably seized tight. You can try to remove it with the special Oxygen Sensor Tool that they rent for free from Autozone, but most likely this tool will be destroied in the process. When I did the first O2 sensor on my '87 240 wagon at 220,000 miles, I gave a mechanic at an exhaust shop $10 to remove it. -and then stood back and waited a half hour while he had to use a torch to extract the old one. Needless to say, putting anti-seize on the threads of the new one is strongly advised. (they often ship with a tiny bit of antiseize on them to start, but I always add more)
God bless, and don't forget to take MPG readings of highway trips before and after the replacement.
Fitz Fitzgerald.
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'87 Blue 240 Wagon, 274k miles.
'88 Black 780, PRV-6, 149k miles.
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