AC Fan Running But Compressor Not Starting

Common Symptoms

• Outdoor fan blade spins but no compressor hum or vibration is felt
• System blows warm air — not cooling despite running
• Outdoor unit runs for 5–10 minutes with only the fan, then shuts off
• Compressor hums briefly on startup, then goes quiet — fan continues
• AC worked fine, then stopped cooling after a power outage or surge
• System short-cycles — turns on and off repeatedly without reaching temperature
• Thermostat shows cooling but indoor air temperature keeps rising

Most Likely Causes

1. 1

   Failed Start/Run Capacitor (#1 Cause — Check First)

   The dual run capacitor stores and releases energy to both start and keep running the compressor and condenser fan motors. When the compressor side of the capacitor fails, the fan continues to run (fan winding is usually on a separate terminal) while the compressor cannot generate enough starting torque and simply doesn't turn. The capacitor may look fine externally — a failed cap often shows no visible bulging. Test with a multimeter in capacitance (µF) mode.

2. 2

   Welded or Pitted Contactor Contacts

   The contactor is a relay that switches 240V power to the compressor. Pitted contacts create high resistance that drops voltage to the compressor below what it needs to start. Conversely, contacts that are welded closed can cause the compressor to run continuously — but a contactor with one failed contact allows the fan circuit to energize while the compressor receives insufficient voltage. Inspect the contacts through the inspection window.

3. 3

   Compressor Thermal Overload Tripped

   Every compressor has a built-in thermal overload protector — a bimetallic switch that opens when the compressor motor overheats and resets automatically once the motor cools (usually 20–45 minutes). If the compressor ran hot on the previous cycle, it may sit silent while the fan runs until the overload resets. Thermal overload trips are themselves caused by dirty coils, refrigerant issues, or an aging compressor running too hard.

4. 4

   Refrigerant Pressure Lockout (High-Head Pressure Cutout)

   When discharge pressure rises too high — due to dirty condenser coils, blocked airflow around the unit, refrigerant overcharge, or a non-condensable gas contamination — the high-pressure cutout switch opens and prevents the compressor from starting. The fan continues operating normally. Cleaning the condenser coils and ensuring at least 2 feet of clearance around the unit resolves pressure lockouts caused by restricted airflow.

5. 5

   Low-Voltage Control Circuit Failure

   The compressor contactor coil is energized by a 24V control signal from the thermostat or control board. A blown low-voltage fuse (usually a 3A or 5A fuse on the air handler control board), a broken thermostat wire, or a failed control board will allow the fan capacitor circuit to function while starving the contactor coil of the 24V signal needed to close and send 240V to the compressor.

6. 6

   Dirty Condenser Coils Causing Elevated Head Pressure

   Condenser coils packed with dirt, cottonwood, grass clippings, or pet hair drastically reduce heat rejection from the refrigerant. The result is abnormally high discharge pressure that triggers the high-pressure cutout, preventing the compressor from starting while the fan runs. This is the most common refrigerant-side cause and is fully DIY-fixable with a garden hose.

Quick DIY Checks

1. 1SAFETY CRITICAL — Discharge the capacitor before any internal work: Turn off the outdoor disconnect switch and the circuit breaker at the panel. The run capacitor inside the unit RETAINS A LETHAL CHARGE — typically 370–440V DC — for 5–30 minutes after power is removed. NEVER touch the capacitor terminals without discharging first. Use a capacitor discharge tool (a resistor with insulated leads) or hold the leads of a 20,000Ω (20kΩ) resistor across each terminal pair: HERM-to-C for 5 seconds, then FAN-to-C for 5 seconds. Alternatively, use two well-insulated screwdrivers with a 20kΩ resistor soldered between them. Do NOT short the terminals directly with a screwdriver — this destroys the capacitor and creates a dangerous arc flash.
2. 2Check if the compressor is in thermal overload: If the AC was recently running (especially in hot weather), the compressor thermal overload may have tripped. Turn the system OFF at the thermostat. Wait 30–45 minutes with the system off for the compressor motor to cool and the overload to auto-reset. Then turn the system back on. If the compressor starts and cools normally, the thermal overload was tripped — proceed to Step 3 to find the root cause (usually dirty coils). If it trips again after a short run, the root cause still needs addressing.
3. 3Clean the condenser coils — free fix, do this before anything else: With the outdoor disconnect OFF, use a garden hose to rinse the condenser coils from the inside of the unit outward — push debris out through the fins rather than in. Work around all four sides. Straighten bent fins with a fin comb. Clear debris (leaves, mulch, cottonwood seeds) from within 2 feet of the unit. Restore power and test. Dirty condenser coils are responsible for a large percentage of high-head-pressure lockouts.

1. 4Inspect and test the run capacitor: Turn off the outdoor disconnect and circuit breaker. Wait 15 minutes. Open the condenser access panel (2–4 screws on the side panel). Locate the capacitor — a cylindrical metal can with two or three terminals labeled HERM (compressor), FAN, and C (common). Discharge as described in Step 1. Visually inspect the top dome — any bulging, cracking, or oil residue indicates a failed capacitor. Test with a multimeter in capacitance (µF) mode: probe HERM-to-C for the compressor winding µF value and FAN-to-C for the fan µF value. Any reading more than 6% below the stamped µF value means the capacitor has failed and must be replaced with an exact µF and voltage match.
2. 5Inspect the contactor for pitted or welded contacts: With the capacitor discharged and power off, locate the contactor — a rectangular relay with two sets of power terminals and a small coil. Look through the small plastic inspection window: contacts should appear flat, silver, and smooth. Pitting (small craters), blackening, or burned residue indicates a worn contactor. A bad contactor costs $12–25 and takes about 20 minutes to replace — photograph wiring before disconnecting anything.
3. 6Install a hard start kit if the capacitor tests good: If the run capacitor tests within spec but the compressor still won't start, add a hard-start kit (Supco SPP6E or equivalent, $20–35). Wire it in parallel with the existing capacitor — one lead to HERM, one to C. The hard-start kit provides a powerful extra burst of starting torque for 500–800ms that can allow a compressor struggling against equalized refrigerant pressure (common after a short shutdown) or with slightly degraded windings to spin up. This is a low-risk, low-cost next step before calling a technician.
4. 7Check low-voltage control circuit: Turn off power. Locate the air handler or furnace control board. Look for a small blade fuse (3A or 5A) labeled 'LOW VOLTAGE' or 'CONTROL' — a blown fuse here prevents the thermostat signal from reaching the outdoor contactor coil. Replace a blown fuse and test. If the fuse blows again immediately, there is a short in the low-voltage wiring — call an HVAC technician. Also check that the 24V thermostat wires (typically Y, G, R, C) are firmly connected at both the thermostat and the air handler control board.
5. 8Test for 24V at the contactor coil (requires multimeter — power ON): With the unit disconnect restored and the system calling for cooling (thermostat set below room temperature), set a multimeter to AC voltage (50V range). With extreme caution — power is live — probe the two small terminals on the contactor coil (not the large power terminals). You should read approximately 24–28V AC. If you read 0V, the control signal is missing (bad thermostat, blown fuse, failed control board) — the contactor cannot close and the compressor cannot start. If you read 24V but the compressor still doesn't start, the contactor contacts are likely failed or the compressor itself has a problem.
6. 9Test compressor motor windings if capacitor and contactor are both good: Turn off all power and discharge the capacitor. Locate the compressor terminal block under the protective cover at the compressor body — three terminals: C (common), R (run), S (start). Disconnect the wires. Set multimeter to resistance (Ω) mode. Measure C-to-R, C-to-S, and R-to-S. The sum of C-to-R plus C-to-S should approximately equal R-to-S — this confirms intact windings. Then test each terminal to the bare metal of the compressor shell (earth ground) — any reading other than OL (open/infinite resistance) means the windings are shorted to ground and the compressor has failed and must be replaced.
7. 10Call a licensed HVAC technician for: refrigerant pressure checks (high-head pressure lockout from overcharge or non-condensable gas requires manifold gauges and EPA 608 certification), shorted compressor windings, or any situation where you're not confident working around live high-voltage components. Present your findings — capacitor test results, contactor condition, 24V control voltage presence — to get an accurate repair estimate rather than a blanket 'needs a new compressor' quote.

Repair vs Replace

Recommended Tools & Parts

• Dual Run Capacitor (matched µF and voltage rating)

  Most common first repair. Read the µF and voltage rating from the sticker on the existing capacitor — the HERM value and FAN value are listed separately. Replace with exact µF values; voltage rating can be equal or higher (440V caps work in 370V applications).

  $10–$25

  Buy on Amazon →

• Hard-Start Kit (Supco SPP6E or equivalent)

  Start capacitor plus PTC relay wired in parallel with the existing run capacitor. Provides an extra burst of starting torque for 500–800ms. Effective on aging compressors, post-shutdown pressure equalization issues, and marginal winding conditions.

  $20–$35

  Buy on Amazon →

• HVAC Contactor (2-pole, 30A or 40A)

  Replacement contactor for the outdoor condensing unit. Match amperage rating and coil voltage (24V coil is standard) to your unit's original. A contactor with pitted contacts drops voltage to the compressor and prevents starting.

  $12–$25

  Buy on Amazon →

• Capacitor Discharge Tool

  Safety tool with built-in 20kΩ resistor and insulated leads for safely discharging capacitors before any internal work. Mandatory for safe diagnosis inside the outdoor condensing unit.

  $10–$20

  Buy on Amazon →

• Digital Multimeter with Capacitance Mode

  Required for testing run capacitor µF values, compressor winding resistance (Ω), and 24V control voltage. Essential for accurate diagnosis — guessing wastes time and money.

  $25–$55

  Buy on Amazon →

• Non-Contact Voltage Tester

  Confirm power is truly off before opening the outdoor unit. An essential safety tool every homeowner doing HVAC work should own.

  $15–$25

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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