Code - p0135. Changed o2 sensors, MAF, and plugs. CEL is still on. Data on wires: Grey ground - OK, Black sensor data - OK, White heater 12V -OK, white heater - no ground. Could not find a wire diagram anywhere.
Questions: Is it a wire harness problem? Is there a relay between O2 sensors and ECU? Where it is located. How to test if ECU grounds the sensor?

 

and what engine does that M-Class got?

 

Sorry. ML320, 2000, 3.2 l engine. All 4 Os2 sensors have the same problem - heater. I am not aware of heater circuite diagram. Do the heater wires go directly to ECU or through a relay? Do they go separately to ECU (I doubt that), thus where is the meeting point? Thanks again

 

ok, got it.

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[IMG]local://upfiles/1/2866AA043E9242DA903B7E95D38744A9.jpg[/IMG]

 

Super. Thank you. So no relay, no common wire to the ECU. O2 sensors are working in pairs separately. The ECU transistors govern grounding of O2 sensors. I wonder if the ECU heater switch gets the signal from a temperature gage or from the O2 sensor itself? Can't read the ECU pin numbers, do you have a picture of ECU pins?
Thanks a lot