I'm not sure about the battery disconnection idea, as SpannerJuggler pointed out on another thread, for quite a few years now ECUs have stored their fault codes on EEPROMs (like Flash Memory) which don't require a poser supply to preserve their contents.
Plugging in a code reader to find out what the problem is is the only real option, but as the car's under warranty, then this is probably best done by the dealer.
