Are you going to tell the boss not to pay you for your time? Is your van going to un-burn the fuel?
It was so simple that a store with 20 employees or more onsite at any given time couldn't figure it out. They called you for your experience, knowledge, and expertise.
Unless your company left the wire loose...
Call it what you want - minimum, truck charge, trip charge, diagnostic... it's due. $1 to throw the switch, $99 for knowing what switch to throw.