Outside AC Unit Not Running but Inside Fan Works: What to Check

What it means when only the inside runs

A central air system has two power paths. The outdoor condenser gets 240 V from its own breaker, usually through a disconnect box mounted on the wall beside it. Separately, the thermostat sends a 24 V control signal on the Y wire to a contactor in the outdoor unit; when that coil is energised, the contactor closes and starts the compressor and fan. The indoor blower runs from a different circuit, so it can keep blowing while the outdoor unit sits idle.

That gives you two families of fault: the outdoor unit has no high-voltage power, or it is not being told to run. A third possibility is that it is being told to run and has power, but a part inside it has failed. First, rule out the normal delay: many systems wait 3–5 minutes after a stop before restarting the compressor.

Likely causes, ranked by frequency

1. Tripped breaker or blown disconnect fuse. The outdoor breaker trips, or a cartridge fuse inside the disconnect blows. If it happens repeatedly, see AC breaker keeps tripping.
2. Failed capacitor. The most common part failure on outdoor units. The unit hums or clicks but the fan and compressor do not start, or the fan spins only if nudged.
3. Thermostat not calling for cooling. Wrong mode, dead batteries, a setpoint above room temperature, or a smart thermostat configuration error. See thermostat wiring if it was recently replaced.
4. Condensate float switch tripped. A full drain pan opens the switch, which interrupts the cooling signal while the blower may keep running. See AC drip pan overflowing.
5. Failed or stuck contactor. Pitted contacts, a burnt coil, or insects packed between the contacts.
6. Blown low-voltage fuse. A small 3–5 A fuse on the furnace or air handler control board blows when the thermostat wiring shorts, often where the cable runs outside and has been nicked by a trimmer or chewed.
7. Safety lockout. A high- or low-pressure switch has opened because of a refrigerant or airflow problem, and the control has locked the unit out.
8. Failed compressor or fan motor. Less common, and more costly.

Clues from what you see and hear

Diagnostic sequence, safest checks first

1. Check the thermostat. Mode on COOL, setpoint at least 3 °F below room temperature, fresh batteries, and no schedule or eco hold. Wait five minutes for any compressor delay to pass.
2. Check the breakers. In the main panel, find the two-pole breaker for the AC or condenser. If tripped, switch it fully off and then on, once.
3. Check the outdoor disconnect. The box near the unit may have a pull-out block or a lever. Make sure it is fully seated in the on position. Someone may have pulled it during yard work.
4. Check the condensate float switch on the drain line or secondary pan. If the pan is full, clear the drain first.
5. Look and listen at the outdoor unit while cooling is called. Note whether it is silent, humming, or running only the fan. If it hums without the fan spinning, set the thermostat to OFF to protect the compressor.
6. Inspect the visible thermostat cable running to the outdoor unit for cuts or chewed insulation. A damaged cable is a common reason for a blown control-board fuse.
7. Technician checks. Voltage at the contactor coil and line side, capacitor microfarad test, contactor condition, control-board fuse, pressure switches and lockouts, motor and compressor windings, and refrigerant charge.

Do not open the outdoor unit's service panel or poke the fan with a stick to get it started. The capacitor can hold a dangerous charge with power off, the contactor terminals carry 240 V, and a fan that starts suddenly can cause injury. Fuses inside the disconnect should be replaced only with the power off at the main breaker, by someone who knows how to verify it is dead.

What repairs typically involve

If the compressor has failed, the compressor replacement cost guide lays out when repair still makes sense.

Prevention

• Have the capacitor and contactor checked at each annual service; both wear predictably.
• Protect the thermostat cable from string trimmers and mowers, and keep the area around the unit clear.
• Keep the condensate drain clear so the float switch does not interrupt cooling.
• Clean the outdoor coil each spring so the unit does not run hot and trip its safeties.
• Do not cycle the system off and on rapidly; leave the thermostat's compressor delay enabled.

Frequently asked questions