Symptom: Front-passenger door lock fails to operate remotely or when manually engaging and disengaging door locks. Other three door locks and hatch-lock operate just fine.

I suspected a defective door-lock assembly, and removed it from door. I found that the large and small spring were both okay
I then removed the internal DC drive-motor after cutting and removing the portion of the plastic cover that covers the motor.
I then tested the DC motor with both a 1.5 and 9 VDC battery; shaft rotated both directions when battery polarity was changed.

Is the problem perhaps just lubrication of the door-lock assembly or is the right-front SAM controller perhaps the issue?

2006 ML350 W164 - 115,000 miles