Tenant says AC not working AC Contractor says otherwise

[Dave Mills]

We have a single floor 1300 sqft house with 6 year old Trane hvac system. Yesterday, Tenant messaged me and stated that the system was not working around 3AM, so we dispatched a AC contractor to check the system that same day, they found nothing. I was told the system is in good condition and functioning properly. Today tenant messaged again about the same problem, I asked AC contractor to take another look. 

I know as Landlord we are required to make and pay for any repairs to make the unit livable that are not caused by the tenant. If ac contractor finds no issue with the system again, who would be responsible for the payment? Landlord or Tenant?