Finding out the cause of problems being displayed by your BMW factory radio head unit can be a frustrating guessing game unless you perform proper troubleshooting. Check for more general issues first, and then investigate problems known to afflict your particular head unit's model.
Power the head unit and check that you can obtain both FM and AM radio frequencies. If one frequency works and not the other, or if both are not working, remove the head unit and check its connection to the radio antenna wire.
Determine whether the display reacts to increases in temperature if you are experiencing issues with the radio's LCD display. Over time, these displays can wear down and lose pixels in cold temperatures. If you want to determine if your display issues are temperature-related, try holding a heat gun or hair dryer near the radio to heat up the display. Then power on the display and check if the display's visibility has improved at all.
Insert a CD into your radio's CD player or CD changer. Watch the display for an error message such as "DISC ERROR" or "NO DISC." If you are receiving either of these errors, it is likely that the disc detection mechanism inside your CD player is broken. Check the fuse for your radio or stereo and replace it, if it is blown, before replacing the radio or CD changer altogether.
Look up the type of head unit that your BMW is equipped with on BMWRadioRepair.com, WillmansElectronics.com or a similar site, to obtain a concise list of common issues that plague your head unit's model. There are many different BMW head units available, each of which has its own set of commonly known issues.