How do i reset the BMW X3 immobilizer?

If You find that Your BMW X3 isn't starting, and You suspect it's due to the immobilizer having kicked in, then the simplest way of resetting it is going to be to use the progRammed keys from the OEM to unlock the doors. Using the key FOB will send the proper signal to the ECU to disable the immobilizer. If Your X3 has been sitting for awhile and the battery's gotten low, it's possible that the system is having difficulties with actually recognizing the signals coming from the remote. In this case, You'll want to charge or replace the battery and then try. For triggered alarms (the horn honking, lights flashing), disabling is typically as simple as inserting Your key into the ignition, turning it to the ON position, and leaving it there for over 10 seconds.