I have a 2007 BMW X3 and the temp. gage recently went to red. I turned off air and radio and it returned to normal. I had the car towed for repair and they replaced the expansion tank and thermostat in that. A day later I a driving and the gage goes to red again. I turn off air and radio and it goes not normal. No water is leaking. What else could this be?
It sounds like one of the cooling fans that operate when the air conditioning is turned on is malfunctioning. Have the mechanic check the fan operation on both fans for proper operation.
It could be a fan relay, fuse, poor connection at the fan, poor ground and could also be the fan clutch the list goes on. A certified mechanic or dealership can use a scan tool to help diagnosis the problem.
It could also be a malfunctioning electric coolant pump. There is a service bulletin SI B12 04 05 that could be related to your vehicle. Go to the dealer and have them check out the vehicle with a scan tool and also check the data system to see if this bulletin is related to your vehicle by using the VIN number.