AC Outside Unit Not Running: Indoor On, Outside Silent

By Ray Bennett · Updated 2 October 2026 · 5 min read · Checked against the sources at the end

AC Outside Unit Not Running: Indoor On, Outside Silent

Short answer

When the indoor fan blows but the outside unit sits silent, either the call for cooling is not reaching it or its power is off. Check the thermostat is on Cool and below the room, and give it five minutes. Then look for a full drain pan tripping the float switch, the breaker and the disconnect by the unit, and the small fuse on the indoor board. A hum with no fan is usually the capacitor.

A central air conditioner has two halves. The indoor unit moves the air, and the outside unit, the condenser, does the cooling. When air blows from the vents but the outside unit stays silent, the house gets no cooler, and the fault is in one of two places: the small signal that tells the outside unit to start, or the power it runs on.

This page works through the call first, because that is where the causes nobody thinks of hide, then the power, then what the unit sounds like. If your system also heats in winter, it is a heat pump, and there is a page written for that: heat pump outside unit not running.

Before you start

What to carry: a torch and a pen, and a multimeter only if you know how to use one safely. Twenty minutes.

Gas or electric? Electric. The outside unit runs on 240 volts. The signal that switches it on is only 24 volts, sent from the thermostat through the indoor unit.

One thing that is normal. Many thermostats hold the compressor off for about five minutes after a change or a power cut, and some flash Cool On while they wait. Give it that long before looking further: the five minute wait on a Honeywell.

What not to touch. 240 volts. The disconnect box on the wall and everything inside the outside unit carry mains voltage. Look, listen and check breakers. Leave the inside of the unit to a technician.

Capacitors hold a charge. Even with the power off, the capacitor in the unit can still give a shock.

Never push the contactor in by hand. It forces the compressor on without its protections, and on a unit with a fault it can finish the compressor off: what the contactor does.

The call that never arrives

Two paths have to be complete: the 24 volt call from the thermostat, and the 240 volt power. A break anywhere on either leaves the outside unit silent.

Check 1

Follow the call for cooling

The thermostat. Set to Cool, and a few degrees below the room. On Auto or Heat, or with the fan simply set to On, the indoor fan can run while nothing asks for cooling: when the thermostat is the problem.

The float switch. Many systems have a small float switch in the drain pan or on the drain line. When the drain backs up, it lifts and cuts the cooling off to stop water coming through the ceiling. The indoor fan can keep running while the outside unit stops. Look for water in the pan: how to clear the drain line.

A condensate pump. If water is pumped out, the pump’s own safety switch does the same job. Do not bypass it: when a condensate pump stops.

The small fuse on the indoor board. Many indoor boards carry a 3 amp fuse on the 24 volt side. If it has blown, a short somewhere in the thermostat wiring blew it.

The wire to the outside unit. The thin thermostat cable runs outside to the condenser. A cable cut by a trimmer or chewed where it runs along the wall stops the call.

The power to the outside unit

Check 2Free, ten minutes

Check the power path

The breaker. The outside unit has its own breaker in the main panel. If it has tripped, reset it once. If it trips again, stop and call a technician: why a breaker keeps tripping.

The disconnect. A box on the wall next to the unit, with a pull out block or a lever. Make sure the block is pushed fully home and the lever is on. Some carry fuses inside.

After work outside. A contractor or a landscaper may have pulled the disconnect and left it out.

What the outside unit sounds like

Stand next to the unit while the thermostat calls for cooling. What you hear tells you which part to look at.

Check 3Free, five minutes

Listen, then match the sound

Silence. Nothing is arriving: go back through the call and the power.

A hum and a fan that does not turn. Power is arriving but the motor cannot start. The capacitor is the usual part: how an AC capacitor fails. Switch it off rather than leave it humming.

The fan turns but the compressor stays quiet. The contactor, the capacitor, or a compressor that has tripped on its own overload after overheating. A dirty condenser coil makes that more likely: how to clean the condenser.

A yellow light blinking on the unit. Some units have a diagnostic light that counts out a code: what the blinking yellow light means.

When it needs a technician

Check 4A service call

Know when to stop

A breaker that trips again. Something is drawing too much current.

A hum that comes back. A new capacitor is a quick job for a technician, and running the unit humming risks the motors.

The call arrives and nothing happens. The contactor or the wiring inside the unit.

The compressor starts, then stops within minutes. A safety is tripping it. On units with a diagnostic light, short cycling has its own code: the Comfort Alert codes.

Questions people ask

Why is my AC blowing but the outside unit is not running?

The indoor fan runs on its own circuit. The outside unit needs both the 24 volt call and its 240 volt power. A full drain pan tripping the float switch, a blown indoor fuse, the breaker and the disconnect are the usual places to look.

Can a clogged drain stop the outside unit?

Yes. The float switch in the drain pan cuts the cooling off when water backs up, so the house does not flood. Clear the drain and it usually starts again by itself.

How long should I wait after turning the AC on?

About five minutes. Many thermostats hold the compressor off that long after a change or a power cut.

What does it mean if the outside unit hums?

Power is reaching it but a motor cannot start, which usually points to the capacitor. Turn it off at the thermostat and have it checked rather than let it hum.

Should I reset the breaker for the outside unit?

Once. If it trips again, something is wrong and it needs a technician.

Can I push the contactor in to start it?

No. It forces the compressor on without its protections, and on a faulty unit it can do real damage.

Sources: Lennox "Outdoor AC unit not turning on", Trane "Troubleshooting the HVAC Air Handler Overflow Switch", Resideo (Honeywell) T6 Pro installation guide 33-00181EFS (five minute compressor protection), Carrier 58SC0A furnace instructions (3-amp fuse), Carrier service manual 24-25-9SM (capacitors, thermal cutout) and the Copeland Comfort Alert user's manual 2006ECT-54 R3