Yes at some point you would have starting problems especially after X has been sitting overnight. You haven't said anything that implicates the battery or alternator as the problem so I wouldn't focus on that this weekend.

As mentioned it could be a fuse. I don't know of ABS or speed sensor fuses. I do know there are DSC fuses. The DSC fuses that could cause your problem are F51,F55 and F63. F51 is also the fuse for fuel pump relay and other areas that would cause other symptoms you don't have. F55 and F63 are dedicated DSC fuses. That said, I don't think it is a fuse since you have a speed sensor error and and no speedo that would not be related to these fuses.

I think a working RR speed sensor harness will solve all your issues. There are speed sensors at each wheel. The speed sensors send input to both the ABS and DSC. The right rear speed sensor is the source for vehicle speed. The logical conclusion is that all your problems are due to RR sensor that is not functioning.

I have never tried it which means I have no idea if it would work, but, I would change the left rear harness to the right rear and see I get the speedo back, assuming both the rear sensors are the same part numbers. If I did I would have confirmation it is the RR sensor harness.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas