Mount Pleasant Heating & Air Conditioning · AC Troubleshooting
AC Won’t Turn On? Common Causes
An air conditioner that will not turn on is almost always a power, thermostat, or electrical problem. Work through the checks below — many are free to test — and call a technician for anything involving breakers that will not reset or capacitors.
Quick Answer
If your AC won’t turn on, check the thermostat first (batteries, mode, and setpoint), then check the breaker for the indoor and outdoor units. If the breaker is tripped and will not reset, or the system still will not start, the cause is likely a bad capacitor, a failed contactor, or a wiring issue — a licensed technician should handle those.
Common Causes
What Usually Causes This Problem
Most of these problems come from a handful of causes. Each one links to the service that fixes it.
Dead Thermostat Batteries
A blank thermostat cannot call for cooling. Replacing the batteries is the fastest possible fix.
Tripped Breaker or Blown Fuse
The AC has its own circuit. If the breaker tripped, reset it once. If it trips again, stop — that indicates a real electrical problem.
Bad Capacitor
Capacitors store the jolt needed to start the compressor and fan motors. A failed capacitor is one of the most common no-start causes.
Failed Contactor or Relay
The contactor switches power to the compressor and condenser fan. If it is stuck open or burned, the outdoor unit will not run.
Safety Switch Tripped
Many systems have a float switch that shuts the AC off when the drain pan fills with water. Check for a full drain pan.
Thermostat Wiring Problem
A loose or damaged thermostat wire can stop the signal that tells the system to start.
Try These First
What You Can Check Yourself
Safe, free checks you can do in a few minutes before deciding whether to call.
Check the Thermostat
Replace batteries if needed, confirm the mode is COOL, and lower the setpoint below room temperature.
Check Both Breakers
The indoor and outdoor units may have separate breakers. Reset any tripped breaker once.
Look for a Kill Switch
Many systems have a wall switch near the indoor unit or a disconnect at the outdoor unit. Confirm they are on.
Check the Drain Pan
If the emergency float switch tripped because the pan is full, the AC will not start until it drains.
Listen for a Click
When the thermostat calls for cooling, a click at the condenser usually means the contactor is engaging. No click often means no power or a failed component.
Stop at Repeated Trips
If a breaker trips immediately or repeatedly, do not keep resetting it. That is an electrical issue for a licensed technician.
Get It Fixed
Get It Fixed by a Pro
If the checks above did not solve it, stop guessing. A licensed technician will diagnose the real cause, explain your options, and give you the price before any work begins.
Common Questions
Straight Answers
Quick, honest answers to the questions we hear most from Mount Pleasant homeowners.
Why is my AC not turning on at all?
Start with the thermostat (batteries and settings), then the breakers for the indoor and outdoor units. If power is fine and the system still will not start, a bad capacitor, failed contactor, or tripped safety switch is the likely culprit — all of which are technician repairs.
What does a capacitor do in an air conditioner?
The capacitor gives the compressor and fan motors the electrical jolt they need to start, then helps them run efficiently. When a capacitor fails, the system may hum, click, or do nothing at all when the thermostat calls for cooling.
Is it safe to keep resetting the AC breaker?
No. If a breaker trips once, reset it and watch. If it trips again immediately or repeatedly, something is drawing too much current — likely a shorted wire, a failing compressor, or a bad capacitor. Continuing to reset it risks damage and is a job for a licensed electrician or HVAC technician.
Why does my AC make a humming noise but won’t start?
A hum with no start is a classic symptom of a failed capacitor or a locked compressor. Turn the system off and have it inspected — running it in this state can damage the compressor further.
Ready to Get Comfortable Again?
Call (843) 714-0863 or request service online. We are available 24/7 for emergencies and offer same-day appointments when available.
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