I had this same problem, remove your front wheels, and check your front ABS sensors. Chances are that they might be the fault. The rear ones rarely go bad, and they are located at the axel shafts right by the differential. The front sensors see a lot of debris from the road and heat from braking, so they might be the culprit. I changed my front driver side sensor and it cured the problem for me. Here is the way I figured mine out.
Within 5-15 seconds of turning on the car (motor running), press the up on the cruise tab 5 times and then down 5 times, on your cluster you will see a series of numbers pop up on the LCD which states what gear the car is in. Once you have those numbers go to this link
http://www.nicoclub.com/FSM/M/2006_M45-M35/acs.pdf and scroll down to ACTION TEST section which will explain the error code.
And just to make sure, you can try to clear the code while it's still running by pressing the cancel button 5 times and then the distance button if you have the Intelligent cruise control system. If done correctly a number 55 will blink in the LCD section of the gear selector indicator. Turn off the car and re-start it and go for a drive above 19MPH and the lights will turn off and remain off if there is no fault, if the fault is still present the lights will turn back on until the error is fixed. It's how I figured what my problem is.