Dead key on BMW 325? Try this first.
My key died, and naturally, I searched E-46 for advice. Instead of the $240 new key from BMW, the thread suggested opening the key and replacing the battery. (FWIW, I did this on my wife's Mazda with a battery from CVS, a screwdriver and DIY from the owner's manual)
Kinda buried in the thread was the procedure to resynch the key.
So before getting out the X-acto and soldering iron that's what I did.
1) Insert the key in the ignition to position II and remove it quickly, within 5 seconds or so.
2) Hold down the unlock button on the key, and press the lock button three times within five seconds.
3) The doors then lock and unlock to confirm the process.
In short, while the battery DIY is great if the battery really is dead, re synching the key is a lot easier. Try it first.
FWIW, it didn't work the first time, for me, but I tried it again and had success. Thanks for a clue to an easy fix.