My AC Won’t Turn On: Troubleshooting Before You Call a Technician
My AC Won’t Turn On: Troubleshooting Before You Call a Technician When your AC system won’t turn on at all — no fan, no compressor, no response to thermostat changes — the cause is either an interruption in the electrical supply reaching the system (tripped breaker, blown fuse, safety switch activation, thermostat failure) or a failure of a startup component within the system itself (dead capacitor, burned contactor, failed control board, or seized compressor). About 30% of “AC won’t start” calls that Coastal Carolina Comfort responds to across South Carolina are resolved by the homeowner checks described below. The remaining 70% require professional diagnosis and repair. Coastal Carolina Comfort provides same-day diagnostic service for systems that won’t start across Summerville, Charleston, Columbia, and the entire Lowcountry and Midlands. Our NATE-certified technicians carry the common startup components — capacitors, contactors, control boards — needed to restore your system in a single visit. Call (843) 708-8735. Homeowner Troubleshooting: 6 Checks Before You Call These checks are safe for any homeowner to perform and take less than 10 minutes total. Work through them in order. Check 1: Thermostat Power and Settings Start at the thermostat — it’s the most common cause of a system that appears dead. Verify it has power. If the display is blank, the thermostat has lost power. Check whether it uses batteries (replace them) or is wired to the system (a tripped breaker or blown fuse at the air handler may have cut power to the thermostat). Verify the settings. Ensure the system is set to “cool” (not “off,” “heat,” or “fan only”), the set temperature is at least 3–5°F below the current room temperature, and any scheduling or away modes aren’t overriding your settings. Try a hard reset. Turn the thermostat completely off, wait 30 seconds, then turn it back to cooling mode. For smart thermostats, a power cycle or factory reset can resolve software glitches that prevent the system from responding. If the thermostat appears to be working correctly — display is on, settings are correct, it shows “cooling” — but the system doesn’t respond, the issue is downstream. Check 2: Circuit Breakers Your AC system typically uses two circuit breakers in your main electrical panel: one for the indoor air handler and one for the outdoor condenser unit. Check both. Look for a tripped breaker. A tripped breaker sits in the middle position — not fully “on” and not fully “off.” Push it firmly to “off” first, then back to “on.” If the breaker holds and the system starts, monitor it. If the breaker trips again within minutes, do not reset it a third time — the system has an electrical fault that requires professional diagnosis. Check for a blown fuse at the air handler. Some air handlers have a small fuse (typically 3–5 amp) on the control board that protects the low-voltage circuit. If this fuse blows, the thermostat loses its ability to communicate with the system. This fuse is accessible inside the air handler’s front panel — but if you’re not comfortable opening the panel, skip this check and call for service. Check 3: Emergency Shutoff Switch Many AC installations include a wall-mounted emergency shutoff switch that looks like a standard light switch — typically located near the indoor air handler or in the utility closet. If someone accidentally flipped this switch off (during maintenance, painting, or cleaning), the entire system is disabled. Check for a switch near the air handler that might be in the “off” position. Flip it on and see if the system responds. Check 4: Condensate Drain Safety Switch Many South Carolina HVAC installations include a float switch on the condensate drain line that shuts the system down when the drain line clogs and water backs up. This is a safety feature designed to prevent water damage — but it also means a simple drain clog can make your entire AC system unresponsive. If your system has a visible float switch on the drain line (a small device with a float attached to the PVC pipe near the air handler), check whether standing water is visible in the drain pan. If the pan is full, the float switch has activated. Clearing the drain line may restore system operation. Our guide on AC leaking water inside your house covers drain line clearing in detail. Check 5: Outdoor Disconnect Box Near your outdoor condenser unit, there’s a disconnect box — a small metal box mounted on the wall within a few feet of the unit. This box contains either a pull-out fuse block or a breaker that supplies power specifically to the outdoor unit. Check that the disconnect is in the “on” position and that the fuses (if applicable) haven’t blown. A blown disconnect fuse is a common cause of a system where the indoor fan runs but the outdoor unit doesn’t respond at all. Check 6: Wait 5 Minutes After Any Reset Modern AC systems have built-in time delays that prevent the compressor from restarting too quickly after a shutdown — rapid cycling can damage the compressor. If you’ve reset a breaker, flipped a switch, or changed the thermostat, wait at least 5 minutes before concluding the system isn’t responding. The delay is protecting your equipment. If the Homeowner Checks Don’t Work: Common Professional Repairs When all homeowner-accessible checks fail to restore the system, one of these mechanical or electrical failures is typically the cause. Failed Capacitor The run capacitor provides the electrical boost the compressor and fan motors need to start. When it fails, the motors can’t overcome their starting inertia. You might hear a clicking sound from the outdoor unit as the contactor engages, followed by a hum and then silence — the system is trying to start but the capacitor can’t deliver. Capacitor failure is the single most common reason an AC system won’t start. It’s also one of the most affordable repairs: $150–$300 including parts and labor. In South Carolina’s long cooling season, capacitors endure more