The measurement can be made with either a Lambda or duty-cycle meter. However, the criteria cited for a good sensor (a 50% duty cycle at idle with less than 10% deviation at 2500rpm) will also pass a BAD sensor. If the fuel mixture is correct and the sensor is bad, you will get a reading of 50% with a variation of perhaps 2% or less at 2500rpm. Also, a good sensor can easily have deviations greater than 10%, thus failing this test!
Mis-diagnosis of sensor operation is apparently common, prompting Mercedes to issue a service bulletin reporting that 60% of sensors returned for replacement from dealers were not defective, and admonishing them to use the correct test procedures. When my mechanic advised that my sensor wasnít working properly and needed replacement, I decided to research the thing and test it myself.
Thanks to MB manuals/service bulletins, other research, and the resources of this great site (Steve and others), here are the recommended tests and diagnostics that I used:
Condition: suspicious O2 sensor
1) Retest (with duty-cycle or lambda meter): heat the sensor by revving to 2500 rpm for 2-3 min; test before engine has idled for more than 1/2 minute. Measure duty cycle between pin 3 and ground, observing the reading while varying engine rpm. If reading does not vary MORE than 5-10% you may need to slightly depress the air sensor plate to force a response. If you still canít get a good reading, donít replace your sensor just yet! And even if you do get adequate meter movement, you may want to skip to step #4 to check the general health and integrity of the sensor.
2) If the duty-cycle reading is not very responsive(and the O2 malfunction light is OFF) then use Steve's test (above) to see if the ECU recognizes the sensor and its basic operation.
3) If the O2 malfunction light does not turn ON, then problem is with the ECU, connections thereto, or the warning circuit; if test is OK then sensor element and heater should be tested. These tests are easily performed using only a multi-meter; wiring is located under the passenger-side carpet.
4) Testing Sensor Element: measure the output voltage of the sensor ceramic (black wire) under full-rich conditions; create this condition by shorting the input to the ECU (green wire). Specified tolerance is 0.45 - 1.0 VDC. Mine measured 0.48, suggesting that the ceramic element is getting weak.
5) Testing Sensor Heater: the KE-Jetronic system uses a heater (3-wire sensor) that should also be tested. Current draw (through either of the white wires) should be between 0.5 Ė 1.3 A (mine was a healthy 1.0).
6) Thereís also a more sophisticated oscilloscope test for the sensor, which fortunately I didnít need to perform. Mine is working fine, for now!
Iím not sure what was more satisfying Ė saving $250 on a replacement sensor, or getting it figured out and being able to test each stage of operation. Thanks to everyone!