Socaleuro, the O2 sensor is a voltage generator depending on engine condition, as Prof says. Readings from a correctly-functioning O2 range between 0v and 1v. Lots of good material posted in the archives, particularly several posted by Steve Brotherton (stevebfl). Search for those.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't