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HVAC Diagnostic Guide HIGH URGENCY

AC Outdoor Fan Not Spinning

Outdoor unit hums but the fan blade isn't spinning, or fan stopped working entirely.

Quick Check Before You Call a Pro

  1. Turn off the AC immediately — running with no fan damages the compressor
  2. Carefully spin the fan blade by hand with a long stick (power must be OFF)
  3. If it spins freely by hand: likely capacitor. If stuck: likely seized motor
  4. Look for visible damage — broken blade, debris jammed in the fan

01 · Most Likely Causes (Ranked)

45%

Failed Run Capacitor

What's happening

The FAN side of the dual run capacitor failed. The motor windings hum (energized) but can't generate enough starting torque to spin. Most common single failure on outdoor AC units.

How to check

Hum from outdoor unit, fan stationary. Push fan blade with a long stick (power must be off first, then briefly turned on) — if it starts spinning, the cap is bad. Test with multimeter — readings more than 6% below MFD = failed.

How to fix

Replace dual run capacitor matching original MFD values (HERM+FAN) and voltage. $15-40 part. Critical: discharge capacitor before touching (insulated screwdriver across terminals).

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20%

Seized Fan Motor

What's happening

Bearings in the fan motor have seized — too much friction for any capacitor to overcome. Common after the motor has been making squealing/grinding noises for weeks.

How to check

Try to spin fan blade by hand (power OFF). If it won't budge or feels rough/gritty, bearings are seized. Replacing capacitor will not help.

How to fix

Replace fan motor. Match HP, RPM, shaft diameter, and rotation direction. $80-250 part. Don't lubricate sealed bearings — replace the motor.

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10%

Broken Fan Blade

What's happening

Fan blade has cracked, broken, or come loose from the shaft. May still be present but unbalanced or unable to rotate. Common after debris strikes (hail, tree branches).

How to check

Visual inspection of fan blade. Look for cracks at the hub, missing blade sections, or wobble when manually rotated.

How to fix

Replace fan blade. Match diameter, blade count, and pitch angle. Make sure setscrew is tight on the shaft. $30-80 part.

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10%

Failed Contactor (No Power to Fan)

What's happening

Contactor doesn't supply power to the fan motor. If contactor is pulled in but burnt/pitted, voltage may not reach the motor.

How to check

Multimeter test: 240V should appear across the contactor load terminals when thermostat calls for cooling. If voltage is missing, contactor is failed or wiring is broken.

How to fix

Replace contactor with matching voltage coil (24V) and amperage rating. $20-50 part.

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8%

Loose Setscrew on Fan Hub

What's happening

Fan blade hub has loosened on the motor shaft. Motor spins but fan blade either doesn't move or slips intermittently. Sometimes drops off entirely.

How to check

Power off. Try to wiggle fan blade on shaft. Looseness = found it. Look for the setscrew (usually one or two on the hub) — should be tight.

How to fix

Position fan blade at correct height on shaft (consult specs — usually centered in the shroud opening). Tighten setscrews with Loctite. Free fix.

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02 · Frequently Asked Questions

Can I run AC if the outdoor fan isn't spinning?
No. The outdoor fan rejects heat from the refrigerant. Without it, head pressure rises rapidly, the compressor overheats, and the high-pressure switch trips. Continuing to run can permanently damage the compressor. Shut off immediately.
Why does my AC fan spin when I push it with a stick?
Classic sign of a failed run capacitor. The motor windings work fine but can't generate starting torque on their own. The push gives them initial momentum, then they self-sustain. Replace the capacitor — your fan motor is still good.
How do I safely spin the fan blade to test the capacitor?
Turn off power at the disconnect. Wait 5 minutes for capacitor to discharge. Use a LONG insulated stick (like a fiberglass golf club shaft) to flick the blade through the top fan grille. Don't reach in with your hand. Then briefly turn power back on while watching from a safe distance.
Will a new capacitor fix any fan motor that won't spin?
No. A new capacitor only fixes weak-starting motors. If the motor bearings are seized (won't spin by hand), if the blade is broken, or if the motor windings are shorted, you need a new motor entirely. Always check whether the blade spins freely by hand first.

03 ·Related Symptoms

AC Not Turning OnAC Blowing Warm AirAC Making Loud Noise
⚠ Safety notice: This guide is for informational purposes only. HVAC systems involve high-voltage electricity, refrigerants under pressure, and natural gas — all of which can cause serious injury, death, or property damage. Refrigerant handling requires EPA Section 608 certification by federal law. When in doubt, contact a licensed HVAC professional.
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