You have to check the thin wire at starter . It hase to have power in it while you start engine. If there is no powerin it it colud be the ignition switch or the key. I would try the spare key and see if same problem is present?These keys hase built in battery that charged trough the ignition switch . The battery in key is not replaceble if battery goes bad in it you have to replace it at BMW dealer.Both key needs to be used to keep battery charged in keys.