I've ignored this problem and now it is doing it again...it looks like the lock itself has a problem...I found the PN on the EPC and found one on ebay for $35 new ($150 MSRP) so I ordered the ebay one and will just replace it but once it is out I can get an idea if it is something fixable.

FYI, mine will work fine with the outside handle once the inside handle is used to open the door, but once you lock it again it will sometimes refuse to open from outside (until you use the inside handle again). Apparently these parts fail quite frequently and dealers keep them on hand.

They are the same part from the C class, but the right and left sides are different PNs.

Left PN 2027300735
Right PN 2027300835

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Marty D.

2013 C300 4Matic
1984 BMW 733i
2013 Lincoln MKz