I've had this issue on occasion and it ended up being the voltage reg on the alt. If the voltage drops below threshold, it will not unlock the doors. You can actually see and hear when it will happen. When you unlock when everything is normal, you can hear the actuator on the rear doors engage. When hitting the button again, it will pop the lock. But when the system is low on juice, the rear doors will not function properly. I ended up replacing my voltage reg and for months, the problem hasnt reared its head again. I confirmed the issue by doing a volt test... Whenever the problem occurred, I check the voltage and it was at 9.5 volts. After opening the door, resetting the system and locking the car again... unlock and all doors unlocked, checked voltage and it was at 12.4v. So if this is the problem you are experiencing, check the voltage.

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2011 E70 50i M Sport