from what I gather from looking at this the O2 sensor light is activated at a set interval of milage regardless of wether or not it actually needs to be replaced though from my estimation the light comes on somewhere around 60,000-70,000 miles and should cause the owner to bring the car to a dealer for service, where the sensor would be replaced and the light reset via cluster removal all at quite an expense to the owner. The lowest Ive found a direct replacement for the sensor was about 115 dollars this not including diagnostic and labor time and charges that would be incurred at a shop

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1984
190e 2.3L
107000 miles