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Compressor Runs Fan Does Not Decision Tree

Troubleshooting Diagnostics Updated Jun 19, 2026 ยท HVAC

Why this matters

A compressor running with a dead condenser fan is one of the fastest ways to kill a system on the truck. Without head pressure relief from the fan, discharge pressure climbs, internal overload trips, and on older units the compressor can lock up before you've gotten the gauges off. This is a 90-second triage call followed by a focused electrical chase, not a tear-down. Get the priority right and a $35 capacitor or a $90 fan motor stays a $35 capacitor or a $90 fan motor, instead of becoming a compressor.

Kill power at the disconnect before touching the contactor or motor leads. Bleed the run capacitor across the terminals with an insulated screwdriver before testing - a stored 370 VAC charge will bite hard. Lock-out / tag-out per OSHA 29 CFR 1910.147 on commercial work.

Symptom presentation

Customer reports outdoor unit "humming" but not blowing air, or you arrive to a unit that is loud, hot to the touch on the cabinet top, with discharge gauge climbing past 400 psig on R-410A. Compressor is running, the contactor is pulled in, but the top fan is motionless. Sometimes the fan twitches at startup then stops. Older systems may have already tripped on internal overload by the time you arrive - the unit is silent but the contactor still energized.

Quick checks (under 2 min)

Step 1: Shut the unit down at the thermostat or breaker. Do not let it run while you diagnose. Discharge pressure climbing without fan flow boils the compressor oil and breaks valves.

Step 2: After lock-out, bleed the capacitor and physically spin the fan blade by hand. Three outcomes:

  • Free spin, no resistance: bearings fine. Electrical fault.
  • Notchy or grinding: bearings shot, motor replace.
  • Frozen solid: motor seized or blade interference. Replace motor.

Step 3: Visual on the capacitor. Bulged top, leaking oil, swollen sides = replace before any further testing.

Step 4: Read the data plate. Note fan motor HP, RPM, voltage, FLA. Note capacitor microfarad rating for the fan side (the C / FAN terminal, not the HERM terminal).

Isolation tree

Branch 1: Fan spins freely by hand, capacitor visibly bad. Replace the run capacitor with an exact microfarad match (within 6% per most OEM specs) and equal or higher voltage rating. Restart. If fan runs, document and exit. Use a known-quality capacitor; the generic cans on every supply house counter fail within 18 months in hot climates and become your callback.

Branch 2: Fan spins freely, capacitor reads within spec on the meter. Move to the motor windings. With power off, disconnect motor leads at the capacitor and contactor. Resistance test:

  • Common to run: should match the motor data plate (typically 3 to 10 ohms).
  • Common to start: higher than C to R but continuous.
  • Any winding to ground: should read infinite (open). Reading of less than 1 megohm = grounded winding, motor is done.

If windings are open or grounded, replace motor. If windings test good, move to Branch 3.

Branch 3: Windings test good, capacitor good, fan spins free. You have a voltage delivery problem. Restore power and meter the fan motor terminals while calling for cool:

  • 240 VAC across the motor leads at the contactor load side: motor itself failed (intermittent, retest under load with clamp meter for amp draw vs FLA).
  • No voltage at motor leads but voltage at contactor line side: contactor failure (welded contact open on the fan leg, or burned terminal). Replace contactor.
  • Voltage at contactor line side intermittent or low: high-voltage feeder problem upstream. Check disconnect fuses, breaker, conductors.

Branch 4: Fan spins free but draws high amps when running and stalls. Bearings on their way out even if they felt fine cold. Replace motor; do not return for a callback in two weeks. Document amp draw vs FLA on the invoice.

Branch 5: Compressor unit is a single-fan condenser with no top access (mini-split outdoor condenser, packaged unit). Same diagnostic logic but you'll find the capacitor inside the electrical compartment under the control cover. PCB-controlled inverter units with DC fan motors throw a fault code instead - reference the OEM service manual for the model and check the DC bus voltage and the fan-drive output before condemning the board.

Confirming the diagnosis

After repair, restart and verify within 10 minutes:

  • Discharge pressure stabilizes within OEM acceptable range for ambient.
  • Fan amp draw within FLA on the data plate.
  • Cabinet top is cool to touch after 15 minutes of runtime.
  • Liquid line temperature within 15 to 25 F of ambient at the service valve.

Repair / remediation

  • Capacitor replacement: match microfarad rating (single-value or dual-run as installed), match or exceed voltage rating, secure with the original strap, tighten terminals to OEM torque (typically 12 to 18 in-lb on quick-connect studs).
  • Motor replacement: match HP, RPM, rotation direction, shaft diameter, mounting style. Many residential condenser fan motors are 1/4 to 1/3 HP, 825 to 1075 RPM. Reuse the correct capacitor microfarad for the new motor (do not assume the old capacitor matches the new motor; read the new motor data plate).
  • Contactor replacement: match coil voltage (24 VAC residential), pole count, FLA rating. Verify pull-in voltage at the thermostat side at 24 VAC nominal.

If you find the compressor has cycled multiple times on internal overload while diagnosing, run megger and amp checks on the compressor before sign-off. A compressor that ran for 20 minutes against high head is not in the same condition it was before the fan failed.

References

  • AHRI Standard 210/240 - Performance Rating of Unitary Air-Conditioning Equipment
  • ASHRAE Handbook - HVAC Systems and Equipment
  • OSHA 29 CFR 1910.147 - The Control of Hazardous Energy (Lockout/Tagout)
  • NEC Article 440 - Air-Conditioning and Refrigerating Equipment

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