Why your AC won't turn on: what to check first

If your air conditioner won't turn on, start with calm, safe steps. Modern central air, heat pumps, and ductless mini-splits include safety interlocks that prevent operation when something is wrong. Bypassing safeties, forcing breakers, or opening panels can be dangerous and may void warranties. Many insurance policies and home warranties require documentation from a licensed HVAC technician for water damage, electrical events, or equipment failure, so keep that in mind as you troubleshoot.

Quick safety checks

• If you smell burning, see smoke, or hear arcing, turn the system off at the thermostat and breaker. Do not reset repeatedly.
• If you see water around the air handler or furnace, shut the system off; a condensate overflow can trigger a float switch and stop the AC from starting.
• Do not open electrical panels or handle capacitors—they can hold a charge even when power is off.

Simple fixes you can try in minutes

1) Thermostat settings and power

• Mode: Set to COOL. Setpoint: 3–5°F below indoor temperature.
• Fan: AUTO for cooling tests.
• Batteries: Replace thermostat or remote batteries. Many no-start calls are dead batteries.
• Time delay: After power loss, many systems impose a 3–10 minute compressor delay. Wait and listen.
• Wi‑Fi/smart stats: Confirm they are online and not in an eco or lock mode.

2) Power path to the system

• Main breaker panel: Look for a tripped AC, air handler, or furnace breaker. Fully switch OFF, then ON once. Do not keep resetting a breaker that trips again.
• Outdoor disconnect: Ensure the pull‑out or breaker near the condenser is fully seated and ON.
• Indoor service switch: Many air handlers have a wall switch that looks like a light switch—make sure it’s ON.
• GFCI: In garages or attics, a tripped GFCI can kill control power.

3) Airflow and condensate

• Filter: Replace a clogged filter. Poor airflow can trip safeties and prevent startup.
• Iced coil: If supply vents are warm or airflow is weak and you see frost, turn system to FAN only to thaw. Ice can trigger a low‑pressure safety and stop the AC.
• Drain line: A full condensate pan can lift a float switch and stop the thermostat signal. Clear the drain if you know how; never cut safeties.

4) Access panels

• Blower door switch: If the indoor unit’s panel is ajar, the safety switch may keep the system off. Secure the panel.

Common technical causes (for awareness)

These items often require tools, meters, or refrigerant handling certification. Treat them as diagnostic clues, not DIY tasks.

Central AC and heat pumps

• Blown fuse at the outdoor disconnect or low‑voltage fuse on the control board.
• Failed run capacitor or contactor—frequent reasons the outdoor unit is silent.
• Bad transformer or control board—no 24V to start the system.
• High‑pressure or low‑pressure switch trip due to airflow issues, dirty coils, refrigerant problems, or fan failure.
• Compressor or fan motor failure, seized bearings, or start relay issues.
• Ambient lockout or crankcase heater problems after cool nights.

Ductless mini‑split systems

Mini‑splits protect themselves with lockouts. Brands like Fujitsu, Mitsubishi Elektrik, Midea, Gree, and Hier use error codes that point to the fault.

• Remote batteries, mode selection (COOL vs DRY vs FAN), or hidden timer.
• Outdoor isolator switch off, tripped breaker, or polarity/communication cable faults.
• Condensate float switch in the wall cassette or a clogged pump.
• Refrigerant or sensor faults leading to no‑start and error codes on the display.

When to stop and call a licensed pro

• Breaker trips again after one reset.
• Burning smell, melted insulation, or loud buzzing at the outdoor unit.
• Repeated short cycling or long delays with no response.
• Ice on lines or coil, or water near the furnace/air handler.
• Error codes on mini‑split indoor or outdoor boards.

Attempting repairs on capacitors, contactors, refrigerant circuits, or control boards can cause injury, create bigger damage, and can jeopardize claims. Many insurers, HOAs, and manufacturers require that a licensed HVAC technician document the fault and repair method.

Prevent it next time

• Change filters on schedule (often every 1–3 months).
• Keep 2–3 feet of clear space around the outdoor condenser; rinse coils gently.
• Flush or vacuum the condensate drain line at the start of cooling season.
• Install surge protection where appropriate to protect control boards and compressors.
• Use reasonable setpoints and avoid rapid swings; enable built‑in short‑cycle protection.
• Get regular maintenance: coil cleaning, electrical checks, refrigerant performance check, and a drain service.

Service area and who we are

#1 AC Guys is a family business in Southern California with 80+ years of engineering experience across four generations. Our mission is simple: healthy air at home for a healthy life. We specialize in commercial and residential HVAC, especially large and complex projects where precise engineering matters.

Our engineers trained at the factory with Fujitsu (Japan), Mitsubishi Elektrik (Thailand), Midea (China), Gree (China), and Hier (China). We serve Los Angeles County and nearby areas including Orange County, Ventura County, and Western Riverside County—covering communities like Los Angeles, Long Beach, Pasadena, Glendale, Santa Monica, Burbank, Anaheim, Irvine, Santa Ana, Orange, Fullerton, Ventura, Oxnard, Thousand Oaks, Riverside, Corona, and Temecula.

Fast reference checklist

1. Thermostat/remote: mode, setpoint, new batteries.
2. Wait 10 minutes for built‑in delays.
3. Check breakers, outdoor disconnect, indoor service switch, and any GFCI.
4. Replace a clogged filter; look for ice; check the condensate pan and drain.
5. Secure the blower door panel.
6. If still dead or if a breaker trips again, stop and call a licensed HVAC technician—both for safety and to protect insurance and warranty coverage.

FAQ

Why does the AC click but not start?

Often a weak capacitor or a stuck/burned contactor. A professional should test and replace these parts safely.

Can low refrigerant prevent startup?

Yes. Low charge can trigger low‑pressure safeties or freeze the coil, stopping the system. Only certified techs should handle refrigerant.

Is it OK to keep resetting the breaker?

No. A breaker trips to protect the circuit. Repeated resets can be dangerous and can complicate warranty or insurance claims.

If your AC still won't turn on after the homeowner checks above, it's time for professional diagnostics. Proper documentation and a safe repair help keep your home comfortable, your equipment protected, and your coverage intact.