Is it permissible for one to take his car key with him to the synagogue for Arbit at the end of Yom Tob so he could drive immediately after the end of Yom Tob? For example, if a person’s car is parked at the synagogue and, for whatever reason, he does not have a key in the synagogue, may he bring a key from home when he goes for Arbit so he can drive his car home, given that carrying is permissible on Yom Tob?
Some authorities rule that it is forbidden to carry a car key to the synagogue on Yom Tob because it is Mukseh. Since it is designated for a forbidden use, it is considered Mukseh and may not be handled on Yom Tob. One might, however, argue that since it is possible to arrange the car’s system such that no lights or other electrical devices are activated when one unlocks the car, the key has a permissible function and should therefore not be regarded as Mukseh. In fact, Rav Shelomo Zalman Auerbach (Jerusalem, 1910-1995) ruled that even if the lights do go on automatically when one unlocks the door, the car key is nevertheless not Mukseh because its essential function is not to turn on the lights, but rather to unlatch the lock. The activation of the lights is an unintended consequence of unlocking the doors, and it is not the primary function of the key, and it should thus not be considered Mukseh.
In truth, however, this discussion is irrelevant nowadays, when virtually all car locks are electronic. Car keys nowadays unlock the door through an electronic mechanism, and thus regardless of Rav Shelomo Zalman’s line of reasoning, car keys would be considered Mukseh and are thus forbidden to be handled on Shabbat or Yom Tob.
Moreover, even though carrying is generally permissible on Yom Tob, it is forbidden to make preparations on Yom Tob for after Yom Tob. One is allowed to carry on Yom Tob only something that he needs on Yom Tob, but not in preparation for something he plans to do after Yom Tob ends. Therefore, it is forbidden to carry on Yom Tob not only car keys, but even ordinary keys, and thus if one wants to go open his office or store immediately after Arbit at the end of Yom Tob, he may not bring his key with him to the synagogue.
This is the ruling of Hacham Ovadia Yosef, as recorded in Yalkut Yosef (p. 233 in the English edition; listen to audio recording for precise citation).
Summary: It is forbidden to bring a key to the synagogue for Arbit at the end of Yom Tob so one can drive or open his office after Yom Tob, as it is forbidden to carry on Yom Tob in preparation for after Yom Tob.
