I just had this problem as well. I drove my car to the gym and when I was finished there, the XF wouldn't start... Just like every other XF with this problem, it would crank and crank but not ignite. I waited about 15 minutes, and it started right up, only to die a couple lights down the road. Luckily she started again before the light turned green or that would have been embarrassing! The check engine light turned on at that point.

Anyways, I looked up the problem when I got home and came across this post. I purchased the crank shaft position sensor immediately on ebay for $45 (free shipping) and it came in about four days.

BTW some users mentioned that a 1/4 12pt socket (star) is required. This is true. However a 1/4 6pt socket will work as well, as long as the bolt isn't screwed in too tight.

Also, some users might have trouble disconnecting the wire from the sensor. Look at how some of the wires connected to where the spark plugs are. These clips are the same design as the one used to hold the wire to the sensor. Just push down on the near side of the clip, and the opposite side will rise up.

After some stretching to get my body to contort into the awkward position required to unscrew the bolt, I replaced the sensor and she's running like a champ again. Thank you to everybody who posted helpful comments on this problem!