When your air conditioning (AC) unit won't turn on, the issue often stems from common electrical, mechanical, or maintenance-related problems that can be systematically diagnosed. The most frequent causes include tripped circuit breakers, thermostat malfunctions, clogged air filters, faulty capacitors, or power supply interruptions. Many of these problems can be resolved without professional help by following structured troubleshooting steps.

Key findings from the sources reveal:

• 80% of summer AC service calls are due to bad capacitors in the condensing unit ^[9]
• Thermostat issues (low batteries, incorrect settings, or wiring problems) account for a significant portion of no-power scenarios ^[4]
• Tripped breakers or blown fuses are among the first components to check, as they cut power to the entire system ^[2]
• Clogged condensate drains can trigger safety switches that prevent the AC from operating ^[4]

Step-by-Step Troubleshooting for an AC That Won’t Turn On

Electrical and Power Supply Checks

Power-related issues are the most immediate culprits when an AC fails to start. Begin by verifying the system’s electrical connections, as these are often the simplest to diagnose and fix. A methodical approach ensures you don’t overlook basic problems before moving to more complex components.

Start with the circuit breaker panel: Locate the breaker labeled for your AC unit and check if it’s in the "ON" position. If it’s tripped, flip it fully to "OFF" before resetting it to "ON" to ensure a proper connection. Breakers can trip due to power surges, overheating, or electrical faults, and resetting them resolves the issue in many cases ^[2]. If the breaker trips repeatedly, this indicates a deeper electrical problem requiring professional attention.

Next, inspect the indoor and outdoor shutoff switches:

• The indoor shutoff switch is typically a standard light switch near the furnace or air handler. Ensure it’s in the "ON" position, as it can be accidentally turned off during maintenance or cleaning ^[4].
• The outdoor shutoff switch is a weatherproof box mounted on the exterior wall near the condensing unit. This switch may have been turned off during lawn care or storms. Flip it to the "ON" position if it’s off ^[4].

Check for blown fuses in the air handler or furnace panel. Some systems use fuses in addition to or instead of breakers. A blown fuse will appear blackened or have a broken filament. Replace it with a fuse of the same amperage rating ^[7].

Finally, verify the thermostat’s power source:

• Replace the batteries if the display is blank or unresponsive. Low batteries are a common oversight ^[4].
• Ensure the thermostat is set to "COOL" mode and the temperature is set 3–5°F below the current room temperature to trigger the AC ^[2].
• If the thermostat is hardwired, check for loose or damaged wiring behind the faceplate. Turn off power to the HVAC system at the breaker before inspecting ^[9].

Mechanical and Component-Specific Issues

If electrical checks don’t resolve the issue, the problem likely lies in mechanical failures or maintenance-related blockages. These require closer inspection of the AC’s physical components, some of which may need cleaning, repair, or replacement.

• The outdoor unit hums but the fan doesn’t spin ^[3].
• The fan spins slowly or struggles to start ^[1].
• Visible bulging, leaking, or burnt marks on the capacitor ^[5].

To test the capacitor, turn off power to the unit, then use a flathead screwdriver to gently spin the fan blades. If the fan starts and continues running, the capacitor is likely faulty and needs replacement ^[3]. Replacing a capacitor is a common DIY fix, but always discharge stored energy and match the microfarad (µF) rating of the new capacitor to the old one ^[5].

• A clogged air filter restricts airflow, causing the system to overheat and shut down as a safety measure. Replace disposable filters or clean reusable ones every 30–60 days, especially during peak usage ^[2].
• A clogged condensate drain triggers a safety switch that cuts power to the AC. Use a shop vacuum to clear the drain line or pour a mixture of bleach and water (1:1 ratio) to dissolve algae buildup ^[4].

• Debris like leaves, dirt, or grass clippings blocking airflow ^[2].
• Ice buildup on the refrigerant lines, which indicates low refrigerant or restricted airflow. Turn off the AC and allow the ice to melt before restarting ^[4].
• Bent or damaged fan blades, which can prevent the motor from spinning ^[1].

If these steps don’t restore function, the issue may involve refrigerant leaks, faulty contactors, or motor failures, all of which require professional diagnosis ^[7].