I have seen this problem with the C-class and it was actually a mechanical problem inside of the gauge. I took out the gauge and disassebled it. I found that the paint on the end of the peg had actually sweated some sort of sticky residue that would hold down the needle on occasion. I cleaned it off and the problem was fixed!!

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Benzmac:
ASE CERTIFIED MASTER AUTO TECHNICIAN
LEAD TECHNICIAN FOR 14 BAY FACILITY
MERCEDES SPECIALIST 8 YRS
PARTNER IN MERCEDESSHOP.COM