O2 sensors should be replaced every now and then since they have a big grey area, as in, at first they work very well, then they enter the grey area where they still work but don't give the perfect readings, and most of the time they stay in that area before they die completely at which point you will probably go bankrupt from gas bills, and change everything else in the car except the O2 in order to fix a stalling, or rough idle problem.
There is a way to clean an O2 sensor, since it is in the exhaust, the only wear and tear it is subjec to is temperature, which it is immune to for the most part, and carbon deposits which cover it bit by bit until it's too covered up to work.
You can clean it in the same manner you clean a muffler for a motorcycle, you simply take a blow torch and burn all the carbon deposits to a crips, in most cases they will catch of fire, then you scrub off the ash with a brush and it's good as new, the only problem is that in order to set the carbon deposits on fire, you have to heat up the poor O2 to a very high temp, at which point you more or less will destroy it unless you have some kind of practice, but how you get this practice I have no clue. And this is why you should replace an O2 instead of cleaning it.
1985 190E 2.3L - a constant project.