Record equipment identity
Model, serial, age if known, fuel/energy source and prior repair history are useful.
Describe the symptom
No operation, short cycling, unusual noise, uneven temperature and loss of airflow are different clues.
User-serviceable maintenance stays within the manual
Accessible filter inspection or replacement can be appropriate when the manufacturer's instructions identify it as a user task.
Electrical and refrigerant service is not DIY territory
Internal testing, refrigerant handling and powered-component service require appropriate qualifications.
Safety boundary. General planning information only. Do not open, dismantle or work on hazardous electrical, gas, structural, fire-protection or contaminated-water systems unless you are appropriately qualified and authorized.