The xDrive system is a masterpiece of torque management, but it is hypersensitive to wear, mismatched tires, and electronic drift. Trying to diagnose a "4x4" light without an is like trying to read in the dark—you might stumble onto the right part, but you will likely waste time, money, and sanity.
The car sits with its left wheels on polished ice and right wheels on dry asphalt. The tester mashes the throttle to 100%. The goal: Zero steering correction. The XDrive must brake the slipping left wheels and redirect torque right before the driver feels the pull. If the tester has to counter-steer more than 10 degrees, the test fails. xdrive tester