Must the one green/black connector wire located down in the passenger side footwell be disconnected, or must you leave the connection undisturbed, thus splicing into the wire? I unplugged my connection when 'measuring' voltage and got 0v at the green end and .495v at the black end.

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'89 300E - 153k miles - California car - Black/Palomino

The O2 sensor is a sensor so it's signal will vary based on what's being read.
You are better off with a scope to watch any variation as the exhaust passes through. You need to monitor Lambda, there are test ports for that.

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

Google mileliminators while you're at it. Mil eliminators are easily made for the ford taurus and mustang and let you ditch the cat (if its not clogged....and if the o2 sensors are working properly.) Used to add highly modified ehaust systems to performance cars. Sorry for the highjack. You folks who know the proper way to test 02 sensors would probably know about them already.

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2009 E320 Bluetec 117,000
1995 E300D 306,000 Sold
1996 Ford Taurus LX 130,000 Sold
1985 300TD Still 225,000 Sold
2016 Ford Fusion 24,900

DSO/Scope sig monitoring is the ideal way, but for the simple DIY , here is some Basic info to help get around the lack of a scope, while still getting decent test results :

For simplicity, I will use your wiring as the example.

The grn wire is the input signal to the ECU..The Blk wire is the 02 output wire. So, if you leave them plugged into one another at that connector, the ECU responds to the 02 readings.... and if you splice onto that connection, your meter will show a variable voltage as the engine responds to the 02 reading..as the reading goes lean, the ECU corrects rich, and as the reading richens , the ECU corrects lean..
Rich/lean..Rich/lean..at about 1-2 times a second. By constantly changing from rich to lean in this short time frame , the engines a/f mixture is controlled at a mean value. This is where the scope would show a nice wave pattern. [ volts/time]

Now, with only a meter..... if you take that Blk wire off that connector, you can now read the 02 V. sig w/o the engine ECU in the circuit...this means there will be no rich/lean from the ECU b/c it now has no 02 input to respond to.
So , here is the trick...while looking at the 02 sig w/o ecu control, you can change the engines a/f mixture MANUALLY to see if the 02 responds to your manual inputs [ in other words, you are using the engine as a rich/lean a/f mixture generator, except you are the ECU]...so, with engine at idle and up to temp, you pull off a good sized vac line, causing your a/f mix to immediately go lean..02 should dop to a low V , showing it is resonding to your created lean condition.
[ A trick here is to hold your tumb over the created vac leak and open/close it with thumb and watch the meter..it should swing with your changes in inputs, and it should do it fairly fast..if NO, you have a bad or slow sensor.
Same goes for Rich test...there are several ways to enrichen the mix manually [ close off air intake at filter , or some use propane into a vac hose , or pull fuel reg hase off , causing higher pressure , etc]
As said by others ..google or search....

The bottom line is if you do not allow the 02 sens sig to feed back to the ECU for correction by unplugging it, you can manually change the input a/f mix it sees and your meter will clearly show if it responds to those a/f mix changes ..you are looking for a sens that has the capacity to rapidly change from a low V to a high V [ .1-.8 V are about decent for a good sensor..usually ], and one that will not change at all or very slowly..is suspect as NG.
This test is easier on 124s with HFM b/c you can enrichen by simply pulling the FP reg vac hose to do both rich/lean test.
If one does have a sensor that does in fact repsond to your inputs , but the ECU does not make the changes once you hook the grn wire back up, the problem is not the sensor, it is the Engine management not responding to the
sensors output.
... and allways check the 02 sens heater circuit..no/low 02 output can be caused by a bad heater bc they need to be hot to have ANY output ..[ closed loop will never happen with a cold 02 sens]

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A Dalton

Spell check??? That was a really informative post, but nearly impossible to read.

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1987 W201 190D

You want to point those mistakes out ??

How about giving me a min to get them done........

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A Dalton