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How many keys do you have for your 2015 Mustang?
Do both key fobs start the vehicle is you leave one key fob on a bench outside the vehicle and take the other one into the vehicle, will they both start the vehicle?
Do both keys state my key when you start the vehicle with either one of them?
Ok. Both keys are MY KEYS. You will need an ADMINISTRATOR KEY to remove those keys from MY KEY status. See the link below that is from the Ford 2015 Mustang owners manual. You will nee to take it to a dealership and have them reprogram the keys. Since both keys are MY KEYS, you cannot change the MY KEY status without an administrator key.
The MY KEY is designed for parents to control their children's driving habits or company owned vehicle drivers are limited to certain MPH. If they could simply be changed, it would defeat the purpose.
The dealership needs to reprogram both keys which will make them both ADMINISTRATOR KEYS again.