Radiator failed due to decomposition and hardening / cracking of its plastic tanks. It was replaced. The coolant expansion tank also began to leak from cracks. This was also replaced. Radiator was $99 and Coolant Tank $38 from RockAuto. Since most plastic parts which are exposed to hot coolant seem to become brittle and can fail, I also replaced the Thermostat Housing ($62), and the Top and Bottom Radiator Hoses, complete with their Plastic End Fittings and Wire Restraining Clips (about $25 ea). I also replaced the Temperature Sensor in the Bottom Hose ($17). I plan to replace the Coolant Level Sensor, as I've been getting false "Check Coolant Level" messages.
Car overheated do to leak in expansion tank
Expansion tank has got small needle like hole in it on the side (no idea how that happened) I lost some coolant due to this and the fan was working at all times. I suppose the expansion tank needs to be replaced and coolant refilled to it's normal level.
Tank cracked rear tank cap. I don't know what's wrong with a transparent expansion tank far from radiator heat without an electronic floating gauge??? An overengineered system without clear benefits.
The first time I had this problem was just six months ago. I Replaced existing radiator and coolent expansion tank with new units, and now the same problem has reoccurred. I have lost faith in BMW replacement parts!!!