AC Compressor Not Working — Diagnose a Dead or Struggling Compressor

Common Symptoms

• Outdoor unit won't run — no fan, no compressor, no sound
• Outdoor unit condenser fan spins but compressor doesn't start (hums then trips breaker)
• Compressor starts then quickly shuts off — short-cycling every 2–5 minutes
• Circuit breaker trips every time the AC runs
• Ice forming on the large refrigerant suction line going into the house
• House isn't cooling despite the outdoor unit running

Most Likely Causes

1. 1

   Failed Dual-Run Capacitor (#1 Cause — Very Common on All Brands)

   The dual-run capacitor provides the electrical boost needed to start and run both the compressor and condenser fan motor. When it fails (capacitors wear out over 5–10 years, faster in hot climates), the compressor hums briefly then trips the internal overload protector or the circuit breaker. A bulging or leaking capacitor is definitely failed — but a capacitor can test bad even if it looks fine. Replacing the capacitor is the first repair to attempt on any compressor that humbles but doesn't start.

2. 2

   Contactor Stuck Open (24V Signal Present, Contacts Not Closing)

   The contactor is the high-voltage relay that switches 240V power to the compressor and condenser fan when the thermostat sends a 24V call for cooling. If the contactor contacts are pitted, burned, or stuck open (common after lightning surges or after years of arcing), no power reaches the compressor — you'll hear the thermostat clicking but nothing happens at the outdoor unit. Replacement contactors cost $12–$25.

3. 3

   Hard-Start Needed (Compressor Struggles on Startup)

   In older compressors or after a power interruption, the startup torque demand can exceed what the run capacitor alone supplies — especially in hot weather when the system shuts off mid-cycle and restarts with refrigerant pressure still equalized. A hard-start kit (Supco SPP6E or equivalent, $20–$35) adds a start capacitor and PTC relay that provide a powerful extra boost at startup, dramatically reducing inrush current. This is often an effective 3–5 year life extension for an older compressor.

4. 4

   Low-Pressure Refrigerant Lockout (Signs: Ice on Line, Short-Cycling)

   When refrigerant charge is low due to a slow leak, the low-pressure safety switch cuts power to the compressor to prevent damage from running in a starved state. Signs: ice forming on the large insulated copper suction line, short-cycling (compressor runs 2–5 minutes then shuts off), and no cooling. Refrigerant work requires an EPA 608 certified technician — never DIY.

5. 5

   Compressor Overload Protector Tripped

   A thermal overload protector inside or on the compressor shell cuts power if the motor runs too hot. On some compressor models there's an external manual reset button; on most it auto-resets after the compressor cools (30–90 minutes). If the compressor trips regularly, it's a symptom — the root cause is usually a weak capacitor, refrigerant restriction, or the need for a hard-start kit.

6. 6

   Compressor Windings Shorted or Open (Replacement Needed)

   If the compressor motor windings have shorted to each other or to ground, or if a winding has gone open (burned out), the compressor cannot start regardless of capacitor or contactor condition. Testing the Common, Run, and Start terminals with a multimeter will confirm this. A failed compressor typically means a $1,200–$2,500 replacement — on units over 10–12 years old, a new system often makes more financial sense.

Quick DIY Checks

1. 1Triage: Determine What's Running. Go to the outdoor unit while the thermostat is calling for cooling. Is the condenser fan spinning on top? Is the compressor vibrating or humming? Or is absolutely nothing running? Also check: is the indoor air handler blowing air? If the indoor blower isn't running, the problem is inside (blower motor, control board), not the compressor. If the outdoor unit is silent with no fan and no hum, check the outdoor disconnect box and breaker before anything else.
2. 2Check the Outdoor Disconnect Box and Circuit Breaker: The outdoor disconnect box (gray box within 3 feet of the condenser) must have the disconnect block fully inserted. At the electrical panel, find the AC breaker (2-pole, typically 30–60A) — a tripped breaker sits in the middle position. Reset it once and test. If it trips again immediately when the compressor tries to start, STOP and call a technician — something is drawing excess current that could damage wiring.
3. 3Capacitor Visual Inspection and Test: Turn off power at the outdoor disconnect box. Open the condenser side panel (4 screws). Locate the capacitor — a cylindrical metal can with two or three terminals (HERM = compressor, FAN = fan motor, C = common). Check the top: if it's bulging upward or has dark oil residue on terminals, it's failed. Even if it looks fine, test with a multimeter in capacitance (µF) mode. Discharge the capacitor first: briefly touch a 20,000Ω resistor (or insulated screwdriver with rubber handle) across HERM-to-C, then FAN-to-C. Test HERM-to-C and FAN-to-C readings. A reading within 6% of the stamped µF value is acceptable — lower indicates a weak or failed capacitor. Replace with an exact µF match (e.g., 45+5 µF at 370V or 440V).

1. 4Contactor Inspection and Test: With power off, inspect the contactor contacts through the small inspection window on the side of the contactor block. Pitted, burned, or discolored contacts mean replacement is needed. To verify the thermostat is sending a 24V call: restore power, set thermostat to COOL (unit calling). With a multimeter on AC voltage, test the two small control wires connected to the contactor coil — you should measure 24V AC. If 24V is present but the contactor hasn't pulled in (no click, contacts not closed), the contactor has failed. If 24V is absent, trace back to the thermostat or control board.
2. 5Hard-Start Kit Installation: If the capacitor tests good or is newly replaced but the compressor still hums briefly and trips, install a Supco SPP6E hard-start kit ($20–35). Wire the two leads in parallel with the existing capacitor — one lead to the HERM terminal, one to C. The kit adds a start capacitor and PTC relay that energize only at startup and automatically drop out once the compressor is running. Installation takes about 10–15 minutes and requires no tools beyond needle-nose pliers.
3. 6Compressor Overload Protector Reset: If nothing else is obviously wrong, the internal overload protector may have tripped due to heat. Shut the unit off completely and let it sit for 60–90 minutes (allow it to cool). Then try restarting. If it starts and runs, a weak capacitor or hot ambient temperature was the cause. Check for vegetation blocking airflow around the condenser unit and trim back to 18 inches clearance on all sides.
4. 7Compressor Windings Test (Advanced — requires multimeter): Turn off power. Remove the terminal cover on the compressor body. Identify the three terminals: C (Common), R (Run), and S (Start). Test resistance between each pair. The R-to-C reading is typically the highest (2–10Ω depending on tonnage), S-to-C slightly less, and R-to-S should approximately equal R-to-C plus S-to-C. Any reading of OL (open circuit) = that winding is burned out. Any reading near 0Ω = short circuit. Also test each terminal to the bare metal compressor shell — you must read OL on all three; any continuity to ground indicates a shorted winding. Either failure type means the compressor needs replacement.

Repair vs Replace

Recommended Tools & Parts

• Dual-Run Capacitor (Match µF Rating Exactly)

  Replacement dual-run capacitor for AC condensers. Must match the HERM µF and FAN µF values printed on the old capacitor (e.g., 45+5 µF) and voltage rating (370V or 440V). A 440V capacitor can replace a 370V-rated one, but never the reverse. Do not substitute a different µF value — even 10% off can cause motor damage.

  $15–$40

  Buy on Amazon →

• HVAC Contactor (24V Coil, 30A or 40A)

  Replacement single-pole or two-pole contactor for AC condensers. Most residential systems use a 24V coil, 30A or 40A contactor. Check the original contactor's amperage rating and coil voltage before ordering. A two-pole contactor breaks both legs of 240V — preferred for safety.

  $12–$25

  Buy on Amazon →

• Supco SPP6E Hard-Start Kit

  Universal hard-start capacitor kit for AC compressors 1–5 tons. Adds a start capacitor and PTC relay in parallel with the existing run capacitor to dramatically reduce compressor start current and inrush amperage. Installs in 10–15 minutes without disconnecting existing wiring — two-wire installation.

  $20–$35

  Buy on Amazon →

• Non-Contact Voltage Tester

  Essential safety tool for HVAC work. Confirms power is off before opening the condenser unit. Beeps and lights when AC voltage is present — test it on a known live outlet first to verify the tester itself is working before relying on a 'no beep' reading.

  $15–$30

  Buy on Amazon →

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

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