Turn off power to the HVAC system at the thermostat and breaker
Check the thermostat settings, batteries, and mode
Replace or clean the air filter
Inspect the circuit breaker and reset if tripped
Check the outdoor unit for debris, dirt, or blocked airflow
Clean the condenser coils if dirty
Make sure vents and registers are open and unobstructed
Inspect the condensate drain line for clogs
Clear any ice from the evaporator or refrigerant lines
Check the blower fan for dust, damage, or failure
Inspect wiring connections for loose or damaged wires
Replace faulty capacitors, contactors, or relays
Check refrigerant levels and look for leaks
Repair or replace damaged ductwork
Test the furnace ignition, pilot light, or heating element
Inspect the flame sensor and clean if dirty
Check the gas supply and gas valve if applicable
Replace worn belts or motor components
Lubricate moving parts if the system requires it
Test system airflow and temperature output
Call a licensed HVAC technician for refrigerant, gas, electrical, or major mechanical repairs