What's the BMW fault code 5df4?

It's important to note that BMW fault code 5DF4 is usually thrown in tandem with at least one other fault code -- You could be looking at 5E43, 5E21, or others. This is an electrical code that points, typically, to a wheel speed sensor. The technical read out will be "electrical system under voltage" -- but the true cause is much more specific. Is the code also accompanied by an ABS warning light on Your dashboard This almost certainly means that it's a speed sensor. Alternatively, it could be a tone wheel, which is what Your speed sensor reads the data from. This is a bit more of an expensive fix, since You've got to swap out the entire wheel bearing. Speed sensors, though, are relatively cheap and don't take more than an hour or two at most to swap out.