My driver's door doesn't lock when I press the little black square button on the outside door handle. The other doors lock though. From the inside, the driver's door will actuate the other doors as will the button on the dash. I replaced the key fob battery. Is there a simple fix or do I have to take it to the dealer for an expensive repair?

 

Odds on it needs a new door lock module, very common for these to fail on older XFs especially on the driver's door.
Not overly expensive, you can get used ones on Fleabay (but you have to be very sure it is the right type for your particular car eg keyless locking vs otherwise) and it's a doable DIY replacement, check You Tube for videos on it.

 

can be done DIY....however I would try spraying some WD40 in there, maybe its just gunked up. I was having similar issues with my drivers door lock, now works like a charm

 

Where do I spray the WD-40 into?

 

into the door handle / lock mechanism itself. and then manually lock/unlock the door from the handle

 

This seems to have fixed the problem, but it doesn't autolock when I go over 9 MPH like the other doors do. BTW, I spayed from the inside of the door handle.