Air Conditioner Won't Start — compressor, thermostat

Use only checks that match the exact appliance or system and its official guide.

1. Check 1

   Read the thermostat without removing its faceplate. Record whether the display is on, the mode is Cool, the setpoint is below the room temperature, the fan is Auto or On, and whether a delay, alert, or code appears. If there is no safety warning, make one user-level correction to Cool and a lower setpoint.

   Expected result: The display is blank, the mode or setpoint is wrong, a delay or alert is shown, or the thermostat is correctly calling for cooling.

   Next step: Follow only the thermostat manual's user-level battery or power guidance for a blank display. If the call is correct and the unit remains off, continue with the external checks without removing the thermostat or inspecting wiring.

   Why this matters

   Why: A blank display or absent cooling call can explain a quiet system. A correct call shifts attention toward power, filter, drain safety, indoor response, and outdoor-unit branches; a displayed delay may be normal for that control or may require model-specific service.

2. Check 2

   Set the thermostat to Off before inspecting the accessible air filter. If the model manual allows a user filter change or cleaning, compare the filter with a clean one and restore the correct orientation and fit. Do not open a furnace, air handler, or blower compartment to reach a filter.

   Expected result: The filter is visibly clogged, wet, damaged, clean, or inaccessible without opening equipment.

   Next step: Correct one accessible filter issue as the manual permits and make one normal cooling call. If the system remains off, airflow is weak, the filter is wet, or access requires a cover removal, leave the system off and call HVAC service.

   Why this matters

   Why: A restricted filter can reduce airflow and contribute to freezing, overheating, poor cooling, or a protective shutdown. A clean filter does not rule out a thermostat, compressor, contactor, capacitor, fan motor, control, drain, power, or refrigerant fault. A wet filter points to a moisture problem.

3. Check 3

   During one normal, non-hazardous cooling call, stand at several open supply registers and the return grille. Observe whether the indoor blower produces no air, weak air, or normal airflow, and whether the response begins, stops, or never appears. Keep hands and tools away from registers and equipment openings.

   Expected result: There is no indoor airflow, weak airflow, normal airflow with no cooling, or normal airflow and cooling begins.

   Next step: Record the result and do not repeat a failed start. Request HVAC service for absent or weak airflow, for normal airflow with no outdoor response, or for a recurring start failure.

   Why this matters

   Why: No or weak airflow keeps the filter, return path, blower, control, door interlock, and power path in scope. Normal indoor airflow with no cooling or no outdoor response makes the outdoor power, compressor, contactor, capacitor, fan motor, control, or safety-lockout path more plausible. Normal cooling suggests the initial issue may have been a thermostat or delay state.

4. Check 4

   From a safe standing position, look at the outdoor unit during one cooling call. Without touching the cabinet or approaching the fan, note whether the fan turns, the compressor starts, the unit hums and stops, remains completely silent, or shows visible damage. Never insert an object through the grille and never remove a panel.

   Expected result: The outdoor fan runs, the unit hums without starting, the fan does not run, the unit is silent, or visible ice, damage, smoke, or odor is present.

   Next step: Turn the system off for ice or any warning sign and arrange qualified service. For a quiet but otherwise normal-looking unit, provide the exact sequence to the technician instead of testing the compressor or its electrical parts.

   Why this matters

   Why: A hum or fan/compressor mismatch can involve the compressor, capacitor, contactor, motor, control, or power path. A silent unit can have a missing thermostat call, open protective device, drain safety, control fault, or other condition. Ice, smoke, odor, or damage overrides ordinary observation.

5. Check 5

   Without opening the electrical panel, note whether a clearly labeled HVAC protective device at the household panel or an accessible indoor service switch appears off. Do not reset a device if it is hot, damaged, wet, accompanied by odor, or has opened before. If the equipment manual permits one safe reset, do not repeat it if the device opens again.

   Expected result: The device is on, visibly off, repeatedly opens, or its condition cannot be safely assessed.

   Next step: Leave a repeated or unsafe trip off and arrange HVAC or electrical service. Never replace a fuse, alter breaker size, bypass protection, or open a disconnect enclosure as a troubleshooting step.

   Why this matters

   Why: An open device can interrupt indoor or outdoor equipment. A repeat opening suggests an electrical, motor, capacitor, compressor, contactor, wiring, or other fault; it is not proof that the protective device itself needs replacement.

6. Check 6

   Look from outside closed equipment for water near the indoor unit, a wet filter, or a full or visibly overflowing condensate drain pan. Do not pour chemicals, remove a drain fitting, open a panel, or handle a float switch or safety circuit.

   Expected result: No water is visible, water or a wet filter is present, or water is near electrical equipment.

   Next step: Keep the system off for water, overflow, or a wet filter and request HVAC service. If water is approaching electrical equipment, keep clear and isolate power only from a safe dry location if possible.

   Why this matters

   Why: A plugged condensate path can cause water damage and can stop some systems through a safety switch. Water near electrics makes the electrical safety boundary more important than restarting the compressor or system.

7. Check 7

   Record the timing and sound of the failed call from a safe position: no response, indoor-only response, outdoor hum, rapid stop, repeated attempt, or protective-device event. Also note whether the coil or accessible refrigerant line has frost or ice and whether the filter was dirty. Do not touch refrigerant tubing or chip ice.

   Expected result: The system is silent, hums and stops, starts then stops, cycles repeatedly, or shows frost or ice.

   Next step: Turn the system off for ice, repeated starts, unusual noise, or a persistent no-start. Give the technician the exact sequence and do not order a compressor or electrical part from the sound alone.

   Why this matters

   Why: Timing helps separate a thermostat or control call from a starting component, motor, compressor, airflow, frozen-coil, or refrigerant protection branch. Low refrigerant and frozen coils require professional diagnosis; repeated attempts can further stress equipment.

8. Check 8

   From a dry, safe distance, check for smoke, burning or electrical odor, visible arcing, a hot connection or enclosure, severe vibration, exposed damage, or a shock or tingle. Do not touch a suspect surface, continue a noisy call, use water near a live system, or open a panel to identify the source.

   Expected result: No hazard sign is present, or smoke, odor, heat, arcing, severe vibration, exposed damage, or shock is present.

   Next step: Stop the system and keep clear. Isolate power only from a safe dry location if possible; use emergency help for smoke or fire and qualified HVAC/electrical service for other hazards.

   Why this matters

   Why: A normal visual check permits only the limited thermostat, filter, and airflow observations. Any electrical, fire, water, or mechanical warning sign overrides normal troubleshooting.