Not unusual...I changed the front pads on my former 99 ML320...the sensor only exists on the driver's side caliper.

I suppose the engineers determined that pads would wear evenly on either side, so it was no longer necessary to have sensors on both.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car