Interested to find out the answer to this as my wife's car has the same problem.

Both key fobs open the doors remotely, one key starts the car every time, the other key is temperamental, it will work for a couple of weeks then won't start the car at all. Then a couple of weeks later it will start the car again?

Is it something to do with the coded chip inside the fob?

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E90 318i 2007 N46 2.0 Manual
E53 X5 2004 M54 3.0 Sport Auto
E83 X3 2004 M54 2.5 Auto