What should you do first?
Check the thermostat display and mode, confirm the breaker is not tripped, and look for a full drain pan or obvious water. Reset a breaker only once. If it trips again, leave it off and request repair.
Safe observations
- Thermostat has power, fresh batteries if applicable, and Cool mode selected
- Breaker position checked with dry hands and safe access
- Furnace or air-handler service switch not accidentally off
- Visible drain pan is not full and filter is not severely blocked
Warning signs and stop conditions
- Breaker trips again after reset
- Buzzing, sparks, smoke, or burning smell
- Water near electrical controls
- Outdoor unit is hot or hums without starting
- Recent storm or electrical event followed by failure
Possible causes
Thermostat or low-voltage control
A blank thermostat, damaged wire, transformer, fuse, float switch, or control board can interrupt the cooling call.
Power or starting component
Breakers, disconnects, contactors, capacitors, motors, and compressors require safe electrical testing.
Condensate safety shutdown
A clogged drain or full pan can open a float switch to prevent water damage.
Professional diagnostic context
- Line and low-voltage power
- Thermostat call and safety circuit
- Drain, float switch, and condensate path
- Capacitor, contactor, motors, and compressor
- Any stored faults or signs of overheating
What an estimate should explain
- The system type, capacity, age, and accessibility
- The failed component and whether compatible parts are available
- Diagnostic or service-call fees in the local market
- After-hours timing and current provider availability
- Refrigerant type, electrical work, and any additional testing required
Repair and replacement decisions
A thermostat, drain switch, or common electrical part may be repairable. A compressor or major electrical damage on older equipment should be compared with replacement using actual findings, warranty status, and system condition.
Questions and answers
Can I reset the AC breaker?
If the panel is dry and safe to access, one reset may be reasonable. Do not keep resetting a breaker that trips again.
Why is the thermostat blank?
Possible causes include batteries, loss of power, a transformer or fuse issue, wiring, or a condensate safety switch.
Can a clogged drain stop the AC?
Yes. Some systems use a float switch that shuts cooling down when water backs up.
Why does the unit hum but not start?
A motor, capacitor, contactor, compressor, or power problem may be involved. Turn it off and avoid opening the panel.