there are two codes, one is a radio code that is transmitted to the car to unlock it (and disarm the alarm). the one that allows you to start the car is a passive RFID chip that is read by a sensor near the ignition switch.

does the LED on the rear view mirror flash when you press the button on the remote? can you open the rear hatch?

have you tried using the key in a different location? you might be getting interference from strong transmitters in the area. plus, if you brought your X5 from the US, i *believe* that frequencies are different so that would increase the chance of you picking up interference.

i was visiting a friend who lived near a TV transmitter and i could not lock/unlock the car with the remote. nothing. zero. as soon as i drove elsewhere, it worked perfectly.