I know the topic of Check Engine (CE) fault codes has been beaten to death, but I have researched this thing over the past 6 months, checked with several MB mechanics, and no one has a solution that has worked. I can't be the only guy that has had this situation come up.

Here is the problem in a nutshell:
-1990 300te, W124 body style.
-CE light, fault code 17 flashes (Oxygen sensor is shorted to positive or ground)
-O2 sensor was replaced
-Codes cleared by holding button down and turning off ignition
-Codes cleared by disconnecting battery and KE control unit
-Code 17 always returns within a week after it is cleared

The wiring diagram below from Braingears.com confirms the following:
-O2 sensor in bottom right hand corner
-Green signal wire to Control Unit (with grounded shield)
-Power and ground wires (for the heating element) to the MAS control unit

Testing Procedures Completed
Car is off. I unplugged the connectors from underneath the passenger floor mat. Pulled out the Ohm meter and first tested the signal wire and the grounded shield for continuity. None was present indicating to me that the wire and its shield were not shorting against each other. Therefore, signal wire OK. (unless it is heating up and then shorting?)

I next ran continuity tests on the power and ground wires. Meter read .002 Ohms downstream (back to the O2 sensor) and .50 Ohms upstream (towards the MAS control unit). I spoke with Bosch and they indicated that there should be a small amount of resistance downstream in the O2 unit since there is a heater coil inside the sensor. They could not comment on the resistance in the upstream direction.

I was able to trace the power wire back to the MAS unit with a toner. I tested the wire with the MAS unit unplugged and it had no continuity with the other lead running to battery ground. I was not able to follow the ground wire since it disappeared under the dash and the toner test did not indicate where it came through the firewall.

I ran tests indicated by the MB 124 workshop manual with the car running and the O2 sensor is performing perfectly.

My bottom line question is this: Is this .5 Ohms of resistance from the plug connector up to the MAS control unit normal? If someone with an ohmeter is willing to check this measurement I would be most grateful.

The procedure to test is to pull up the bottom padded floor mat on the passenger side. There you will see two connectors. The first is a round single wire that is usually green. The second connector is a rectangle with two wires. One should be Black/Red/White (power) and the second is brown (ground). With the key out of the ignition, unplug this rectangle connector and put the leads of the ohmeter in the section that continues away from the O2 sensor (towards the outside of the car). Do you read any continuity?

Thanks in advance, and happy birthday USA.

Attached Images

O2x.bmp (64.9 KB, 103 views)