What's the 2ef5 BMW fault code?

The 2ef5 BMW fault code means DMW - 2EF5 DME: Map thermostat, activation - Open circuit error code. This means that the thermostat on the car is not reporting to the CPU correctly. If this happens then the car will not know how to regulate itself, for instance, it will not be able to use the correct amount of coolant. This will make it very likely that the car is going to overheat. While an overheating car is not fatal by itself it is bad. It warps the head gasket, which will eventually fail. The more warped it is, the quicker it will overheat. Having this fixed is a multi-thousand dollar job, so in short get the thermostat fixed, which is a couple hundred, and save Yourself the pain.