BMW Repair: E46 330Ci Convertible 2001, coolant leaks, coolant level

Question
My 330Ci Conv has recently started to overheat. It is fine if I drive normally with the A/C off, but as soon as I turn the A/C on the temperature rises very quickly and the needle hits the max with the red 'overheat' light on.
I have check the coolant level in the expansion tank and it is fine. What should I check ?

Answer
Andy, check auxiliary fan operation (in front of condenser/radiator) for operation. check thermostat operation and water pump. Check for any coolant leaks. I would replace the T-stat after an overheat. DONT overheat the engine. Your M54 all aluminum block and heads are very sensitive to overheating and I have seen many motors detroyed because of owner thinking that he can drive to the next rest stop or similar.
-Evan