Why does the indoor fan run when the AC is broken?

The blower and the outdoor condensing unit are powered and controlled separately. The blower can operate perfectly while the compressor circuit has failed.

Should I turn the fan off?

Yes, switch the system to OFF. Running the blower without cooling just circulates warm air and adds motor heat to the house.

Can I replace a capacitor myself?

No. Capacitors store a dangerous electrical charge even after the power is off. This is a technician task.

How can I tell a capacitor from a compressor failure?

Both can present as a humming unit that will not start. Only electrical testing distinguishes them reliably.

Why Is My AC Not Cooling But Fan Is Running?

Causes, safe checks and when to call a professional

Expert Answer

You have already done the most useful half of the diagnosis: the blower works, so the problem lives in the refrigeration circuit or its power supply. The blower and the outdoor unit are on separate circuits, which is exactly why the fan can run while the compressor is dead. The likely causes, in order, are a tripped outdoor breaker or pulled disconnect, a failed run capacitor, a burnt contactor, a compressor that has failed, or a safety lockout from low refrigerant pressure. All of these are technician repairs.

Check these 3 things first

1Walk outside and confirm whether the condenser fan and compressor are running at all.
2Check the outdoor disconnect pull-out and the AC breaker in the panel.
3Listen at the outdoor unit for a hum or click with no fan movement.

YELLOW — Inspection Recommended

How Serious Is This?

GREEN
Usually Manageable
Often resolved with basic homeowner checks and routine maintenance.
YELLOW — this issue's severity level
Inspection Recommended(this issue)
Worth a professional diagnosis soon to avoid a larger repair.
RED
Potentially Serious
Stop using the system and get professional help right away.

Most Likely Causes

1. Outdoor breaker tripped or disconnect pulled
The most common reason for a live blower and a dead condenser. Service work, a storm, or a nuisance trip can leave the outdoor unit unpowered.
2. Failed run capacitor
Capacitors are the highest-frequency failure in residential AC. Without one, the compressor and fan cannot start — you may hear a hum.
3. Burnt or stuck contactor
The contactor is the relay that switches high voltage to the condenser. Pitted contacts stop the outdoor unit from energizing.
4. Low-pressure lockout from refrigerant loss
Safety switches shut the compressor down when charge is too low, while the indoor blower keeps running normally.
5. Compressor failure
A shorted, open or seized compressor stops cooling entirely. The fan may or may not still spin depending on the failure.
6. Control board or thermostat wiring fault
A broken Y-wire delivers a fan call without a compressor call — the blower runs and nothing else does.

Safe Homeowner Checks

Go outside and confirm whether the condenser is running at all.
Check the outdoor disconnect box near the unit — the pull-out should be fully seated.
Check the AC breaker in the panel; reset it once only if tripped.
Listen for a hum or click at the outdoor unit when the thermostat calls.
Confirm the thermostat is on COOL with the fan on AUTO.
Look for ice on the refrigerant lines, which can indicate a charge problem.

Stop and get help if you notice

• Breaker that trips again immediately after reset.
• Loud buzzing at the outdoor unit.
• Burning smell near the disconnect.

2-Minute Tests You Can Run

Each test isolates one specific cause so you know what you are dealing with before anyone quotes you a repair. Stop immediately if anything feels unsafe.

Disconnect and breaker check
About 5 minutesTools: None
Tests for: Outdoor breaker tripped or disconnect pulled
1. Locate the outdoor disconnect box mounted near the condenser.
2. Confirm the pull-out block is fully seated in the slot.
3. Go to the electrical panel and find the AC breaker.
4. Reset it once if it is tripped, then wait five minutes.
Normal result
The condenser starts within a minute of resetting power.
Problem result
The breaker trips again immediately or the unit stays silent.
What it means: A single clean reset points to a nuisance trip; an immediate repeat trip means a real electrical fault needs a technician.
Listen for a stalled hum
About 3 minutesTools: None
Tests for: Failed run capacitor
1. Set the thermostat to call for cooling and go outside.
2. Stand a safe distance from the condenser cabinet.
3. Listen for a steady hum with no fan blade movement.
4. Turn the thermostat off if you hear this and do not attempt a restart.
Normal result
The fan blade spins and the unit runs smoothly.
Problem result
A hum continues with the fan not turning.
What it means: A humming, non-starting compressor or fan usually means a weak or failed capacitor and needs a technician.
Check for ice on the lines
About 5 minutesTools: Flashlight
Tests for: Low-pressure lockout from refrigerant loss
1. Locate the two copper lines running from the indoor unit to outside.
2. Inspect the larger, insulated suction line for ice or frost.
3. Check the smaller liquid line as well.
4. Note whether the blower keeps running while the outdoor unit is silent.
Normal result
Both lines are clean, cool, and free of ice.
Problem result
Ice coats the suction line or fittings.
What it means: Ice combined with a dead outdoor unit suggests a low-refrigerant safety lockout, which requires professional leak repair.
Confirm the thermostat call mode
About 3 minutesTools: None
Tests for: Control board or thermostat wiring fault
1. Check that the thermostat is set to COOL, not FAN or OFF.
2. Set the fan switch to AUTO rather than ON.
3. Lower the setpoint several degrees below room temperature.
4. Wait two minutes and check the outdoor unit again.
Normal result
The outdoor unit starts within a couple of minutes of the new setpoint.
Problem result
The blower keeps running but the outdoor unit never starts.
What it means: This pattern isolates the fault to the compressor circuit or the wiring that calls for it, separate from thermostat settings.

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What This Typically Costs

Reset tripped breaker
DIY
$0
Professional
Not needed if it holds
Typical time
5 minutes
Run capacitor replacement
DIY
Not a DIY repair
Professional
$150-$350
Typical time
30-60 minutes
Contactor replacement
DIY
Not a DIY repair
Professional
$150-$300
Typical time
30-60 minutes
Refrigerant leak search and recharge
DIY
Not a DIY repair
Professional
$300-$1,500+
Typical time
1-4 hours
Compressor replacement
DIY
Not a DIY repair
Professional
$1,500-$3,500+
Typical time
3-6 hours

Typical repair costs and time
Problem	DIY cost	Professional cost	Typical time
Reset tripped breaker	$0	Not needed if it holds	5 minutes
Run capacitor replacement	Not a DIY repair	$150-$350	30-60 minutes
Contactor replacement	Not a DIY repair	$150-$300	30-60 minutes
Refrigerant leak search and recharge	Not a DIY repair	$300-$1,500+	1-4 hours
Compressor replacement	Not a DIY repair	$1,500-$3,500+	3-6 hours

Ranges are national estimates for planning only. Local labor rates, equipment access and parts availability change what you will actually be quoted.

Working Through It

Is the outdoor unit completely silent during a cooling call?
Yes: Check breaker and disconnect first; if both are fine, this is an electrical repair.
No: Continue.
Does the outdoor unit hum without the fan turning?
Yes: Classic failed capacitor or seized motor. Shut it off and call a technician.
No: Continue.
Does the compressor start and then stop within seconds or minutes?
Yes: Suspect an overload, a lockout, or a pressure-switch trip.
No: If it runs steadily but there is no cooling, have charge and compressor performance measured.

What the technician will check

Verifies 24V control signal reaching the contactor.
Tests the run capacitor and contactor.
Measures compressor windings and amp draw.
Checks pressure switches and refrigerant charge.
Confirms line-voltage supply at the disconnect.

Repair considerations

• Capacitor and contactor repairs are among the least expensive AC repairs.
• A locked-out system due to refrigerant loss requires leak repair before it will hold a charge.
• Compressor replacement cost depends heavily on warranty status and system age.

What this actually means

In a split system the air handler and the condensing unit have separate power feeds and separate failure modes. A working blower proves the thermostat is calling, the control transformer is alive, and indoor power is fine. That leaves the outdoor half — which is where the cooling actually happens.

When to Call a Professional

The outdoor unit will not run and the breaker is on.
The unit hums, buzzes or clicks without starting.
The compressor starts and stops repeatedly.
The breaker trips more than once.

Preventing a Repeat

• Have capacitors and contactors tested during annual service — they degrade predictably.
• Keep the outdoor unit clear so it does not run hot and stress electrical parts.
• Address slow cooling decline early to avoid a low-charge lockout mid-summer.

Worth doing yourself

• Reset a tripped breaker one time and check the disconnect is fully seated.
• Set the thermostat to COOL with the fan on AUTO.
• Keep the area around the outdoor unit clear of vegetation and debris.
• Shut the system off if you hear humming without a starting fan or compressor.

Do not waste money on

• Do not repeatedly reset a breaker that keeps tripping.
• Do not open the outdoor unit's electrical panel or touch the capacitor.
• Do not keep running the blower for hours with no compressor operation.

Frequently Asked Questions

Guidance this page draws on

ACCA — Guidance on residential split-system diagnostic sequencing.
HVAC manufacturer service literature — Capacitor, contactor and low-pressure switch failure behavior.
U.S. Department of Energy — General residential air conditioning maintenance recommendations.