My wife just called me from the side of the road using her cell phone (don't get me started on WHY that's now a requirement!!). The car = 1992 w201 190E 2.3 The red O2 sensor warning light just came on in the dash. Car has almost 180,000 miles on it (3X 60,000 miles) so I thought that it's just the speedometer tripping it for a checkup. Just to make sure I'd like to check it out. On a former vehicle my mechanic disconnected the wire leading from the sensor and connected a digital multi-meter to it and read the output value. If I remember correctly he had is set on the DC scale with a small value and it showed .7 volts(?) which he said was "right in there" - no need to replace it. Can someone (MB Doc, Steve, Aaron) tell me how to go about this on this car, what setting to use on my digital multi-meter and what the value should be if it's working correctly? As always, the help I've received over time on this board is nothing short of AMAZING! I so appreciate the people who take the time to give us the feedback we desperately seek. Thanks again. I look forward to the feedback.

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Jay Yambrovich
Scotts Valley, CA.

1993 300 CE Cabriolet (A124) 131K miles
1997 C-280 133K miles

2000 BMW R1100RT 69K miles
1989 300 E 216K miles (sold)

Your VOM is probably an autoranging unit but you are looking to monitor a value that normally oscillates from around 0.2 to 0.9 VDC with a period lasting several seconds. It can be monitored between the elctrode output lead and ground with and without the electrode connected to it's connector in the FI harness. I generally see equivalent results either way when everything is up to snuff. A heads-up clue as to the functionality of the electrode is how the idle changes when it's unplugged. If there is no change whatsoever when it's unplugged, it's output has probably dropped below the point where the default mode takes over. This should store a code in the OBD but I don't know if it trips the Check O2 Sensor light on your car. If you are at an integral multiple of 60K, it's probably just the idiot light reminding you to have it checked.

The way that O2 sensors should be checked is by monitoring the voltage when the vehicle is all warmed up and the ECM is in closed loop mode. Running the engine for 5 minutes will ensure that this is the case. Connect your high impedance voltmeter to the sensor, leaving it connected to the circuit. The O2 sensor voltage should fluctuate fairly rapidly between .4v - .6v. This indicates that the sensor is responding quickly to changes in the mixture caused by it's operation. A faulty sensor will either have a constant voltage output of around .5v, or the voltage fluctuations will be slow (less than one every two seconds). An output on the high side means the engine is running rich, an output on the low side means it's running lean. You can play with the mixture by pulling a vaccuum line or feeding the engine propane from an unlit propane torch to check out the operation. Incidently, there are several sites on the web that have DIY articles on building a LED dash indicator that will display the output of the O2 sensor constantly - it is a very close indicator of the air/fuel ratio, .5v being a 14.0 - 1 mix. One such site is at: http://www.euronet.nl/users/fo_elmo/afdisplay.htm
Richard Wooldridge

Jay,
There's quite a thorough discussion and set of procedures on how to check and diagnose the various elements of O2 sensor operation in an earlier thread:

O2 sensor failure and lambda testing