I booked my car for service yesterday, I complained about the radiator fan was turning fast at most of time, BMW pluged into the computer to see if there is any fault. After 2 minutes computer pick up the fault that thermostat is faulty. Then the technician asked me how I feel the power of the car, I said sometimes is good but sometimes is not, then the technician said thermostat sending the signal to computer to say the engine is too hot. Then Computer automatically reduce injecting the fuel. After changing the thermostat, power is back.