Both of my keys acted similarly to your symptoms. Resolved for a couple years with a reflow of most of the solder joints then after a few years of random drops the key I didn't lose on a road trip started doing it again then finally stopped.

I had an even weirder situation: it would work if I slammed the tailgate so I was pretty sure it was wiring or related to the amplifier in the tailgate.

I replaced my key with a new one and once synced it works perfectly. The problem was entirely the key.

I haven't had time to fix the solder in the key yet but that would probably bring the key back to life.

I used an online dealer to get the key enough of a discount that I got a non remote valet key I can leave hidden in the car for emergencies included for the same price as just the key from local dealer.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)