One other consideration; from a recent thread here; somebody with intermittent start problems; it worked out to be a faulty key; the immobilization circuitry probably had a bad solder joint so sometimes it was working sometimes it wasn't. Now i don't think the car will crank with the immobilization circuit non operational but i would definitely try your other key, especially right after you get a non-start situation. if you can start up first try with the other key, good chance the key's immobilization circuit is not talking to the car.

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