HVAC Capacitor Failure — Symptoms, Testing, and Replacement

Common Symptoms

• AC or heat pump outdoor unit hums but won't start
• Fan blades spin slowly or only start with a manual push
• System starts but shuts off after a few minutes on thermal overload
• High electricity bills with reduced cooling output
• Visible bulging, leaking, or burn smell from the capacitor can

Most Likely Causes

1. 1

   Dual-Run Capacitor Failure (Most Common)

   Most outdoor units use a single dual-run capacitor that serves both the compressor and the condenser fan motor. It has three terminals: HERM (hermetic compressor), FAN, and C (common). When it fails, the system may hum without starting, or the fan may run while the compressor won't start, or vice versa.

2. 2

   Heat Degradation

   Capacitors degrade faster in hot climates or inside sun-exposed outdoor units. Every 10°C rise in operating temperature roughly halves capacitor life. Units that regularly see ambient temps over 90°F need capacitor checks every 3–5 years.

3. 3

   Power Surges and Voltage Spikes

   Lightning strikes and utility power surges can instantly destroy a capacitor — or degrade it enough that it fails within weeks. If your capacitor failed shortly after a storm, check for other electrical damage in the outdoor unit.

4. 4

   Normal Age and Wear

   Most capacitors last 10–20 years, but the rated life is often 10 years. On any unit over 10 years old showing symptoms, testing the capacitor is the first diagnostic step — it's fast and often conclusive.

5. 5

   Start Capacitor Failure (Hard-Start Issue)

   Some units have a separate start capacitor (hard-start kit) that provides extra torque to start the compressor. When it fails, the compressor may draw locked-rotor current for several seconds before starting or fail to start entirely, eventually tripping the breaker.

Quick DIY Checks

1. 1Turn off the thermostat and the outdoor unit disconnect switch. Wait 15 minutes for the capacitor to naturally discharge. Open the outdoor unit access panel (usually 2–4 screws on the side). You'll see a cylindrical or oval metal can with 3 or more wire terminals — that's the capacitor.
2. 2Visually inspect the capacitor: the top dome should be flat, not bulging. Look for any oil leaking from the bottom, burn marks, or a blown rubber vent plug. Any of these means the capacitor has failed.
3. 3Test with a digital multimeter set to capacitance mode (μF). Discharge the capacitor first by touching a 1k ohm resistor between each terminal pair (HERM-C, FAN-C). Then measure each terminal pair against the capacitor's rated MFD. Within 10% of rated value = good. More than 10% low = replace.

1. 4Note the capacitor specs from the label: MFD rating (e.g., 45+5 MFD) and voltage rating (e.g., 370V or 440V). The replacement voltage rating can be equal to or higher than original; MFD must match within ±5%.
2. 5Install the new capacitor: connect each wire to the matching terminal — HERM wire to HERM, FAN wire to FAN, common wire to C. Press connectors on firmly with needle-nose pliers. Restore power and test.

Repair vs Replace

Recommended Tools & Parts

• Dual Run Capacitor (45+5 MFD, 370/440V)

  Most common size for residential HVAC. Serves both compressor and fan motor. Match MFD and voltage to your unit's data plate before ordering.

  $12–$25

  Buy on Amazon →

• Digital Multimeter with Capacitance Mode

  Required for testing capacitor MFD values. Also useful for testing motor windings, contactors, and voltage throughout the HVAC system.

  $20–$40

  Buy on Amazon →

• Hard-Start Kit (Supco SPP6E or Equivalent)

  Adds start-assist capacitance to reduce compressor startup current. Helps aging compressors start reliably and reduces breaker trips.

  $25–$45

  Buy on Amazon →

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

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