AC Not Turning On? How to Test and Replace Your AC Capacitor

AC Not Turning On? How to Test and Replace Your AC Capacitor (Step-by-Step) – 2025 Guide

It’s the peak of summer, the temperature is soaring, and suddenly your air conditioner decides to quit. You hear a humming or clicking sound from the outdoor unit, but the fan isn’t spinning, or maybe nothing happens at all. Before you resign yourself to a sweltering house and an expensive emergency service call, there’s one common culprit you might be able to diagnose and fix yourself: a failed AC capacitor.

Capacitors are essential electrical components that provide the necessary jolt of energy to start your AC’s motors (compressor and fan) and keep them running smoothly. Due to constant use, heat, and potential power fluctuations, they are one of the most frequent parts to fail in an air conditioning system. The good news? Replacing a capacitor is often a relatively inexpensive and straightforward repair.

However, this is not a task to be taken lightly. Capacitors store a significant electrical charge even when the power is off, making them dangerous if handled improperly. This guide provides a detailed, step-by-step process for safely testing and replacing your AC capacitor in 2025, but safety must be your absolute priority. If you are uncomfortable working with electrical components or unsure about any step, stop immediately and call a qualified HVAC professional.

Understanding Your AC Capacitor: The Motor’s “Booster Shot”

Think of your AC’s motors (the compressor and the outdoor fan motor) like engines that need a powerful initial kick to get going and a steady supply of energy to keep running efficiently. That’s essentially what capacitors do.

• What is a Capacitor? It’s an electrical component that temporarily stores an electrical charge. In an AC unit, it releases this stored energy to provide the extra torque needed to start the motors and helps regulate the voltage to keep them running efficiently.
• Types of AC Capacitors: You’ll typically find one or two capacitors in your outdoor condenser unit:
  • Start Capacitor: Provides a very brief, high-energy boost just to get the motor spinning from a dead stop. Usually black and cylindrical, often used only for the compressor. (Less common to fail than run capacitors, and sometimes integrated).
  • Run Capacitor: Provides continuous energy to keep the motor(s) running efficiently and smoothly after they’ve started. Usually metal (silver or black), cylindrical or oval.
  • Dual Run Capacitor: The most common type found in modern residential units. It’s a single unit that serves as the run capacitor for both the compressor and the outdoor fan motor. You can identify it by its three terminals on top, typically labeled “C” (Common), “HERM” (Hermetic, for the Compressor), and “F” (Fan). Single run capacitors only have two terminals.
• Why Do They Fail?
  • Age/Wear and Tear: Capacitors have a limited lifespan (typically 5-10 years, but variable).
  • Heat: Excessive heat, especially in outdoor units baking in the sun, degrades capacitors over time.
  • Voltage Issues: Power surges, brownouts, or consistently high/low voltage can damage them.
  • Manufacturing Defects: Less common, but possible.
• Signs of a Failed Capacitor:
  • Humming/Clicking: The motor tries to start but can’t get enough energy, resulting in a humming sound or repeated clicking from the contactor.
  • AC Won’t Start: The outdoor unit remains completely silent, or only the indoor blower runs.
  • Slow/Struggling Start: The fan or compressor takes a long time to get up to speed.
  • Fan Runs, Compressor Doesn’t (or Vice Versa): In units with dual run capacitors, one part might fail while the other still works, leading to the fan spinning but no cold air (compressor failed) or vice versa.
  • Intermittent Cooling: The unit might start sometimes but not others.
  • Visible Damage: The capacitor casing might be swollen or bulging at the top, or leaking an oily substance (dielectric fluid). This is a definitive sign of failure.

If you’re experiencing these symptoms, a faulty capacitor is a prime suspect.

Safety First! Critical Precautions Before You Begin

STOP! READ THIS CAREFULLY. Working inside your air conditioner involves potentially lethal high voltages. Capacitors, specifically, can hold a dangerous electrical charge long after the power has been turned off. Mishandling them can cause severe electrical shock, burns, or worse.

DO NOT PROCEED unless you are comfortable and knowledgeable about electrical safety. If you have any doubts, call a professional HVAC technician. Your safety is paramount.

If you choose to proceed, follow these steps EXACTLY:

1. Turn Off Power at the Thermostat: Set your thermostat to “Off.” This stops it from calling for cooling but does not cut power to the unit.
2. Turn Off Power at the Breaker Panel: Locate your home’s main electrical panel. Find the circuit breaker(s) that control your air conditioner condenser (outdoor unit) AND your air handler/furnace (indoor unit). There might be two separate breakers. Flip them both to the OFF position.
3. Turn Off Power at the Disconnect Switch: Go outside to your AC condenser unit. There should be a separate electrical disconnect box mounted on the wall nearby (usually a small metal box). Open it and turn off the switch or pull out the disconnect block/fuse. This provides a crucial local power cutoff.
4. VERIFY Power is Off: Use a voltage tester (non-contact voltage pen or multimeter) to carefully test the wires coming into the AC unit’s control panel area (where the capacitor is located) to confirm that ALL power is OFF. Do not assume; verify.
5. Wear Safety Gear: Put on safety glasses and insulated electrical gloves or heavy-duty work gloves.
6. Work Deliberately: Avoid distractions and focus on each step.

Locating the Capacitor

With the power completely and verified off, you can now access the capacitor.

1. Find the Access Panel: On your outdoor condenser unit, look for a removable panel, usually on a corner or side where the electrical wiring enters the unit. It’s typically held on by several screws (often 1/4″ or 5/16″ hex head screws).
2. Remove the Panel: Use your nut driver or socket wrench to remove the screws and carefully lift the panel off. Set the panel and screws aside safely.
3. Identify the Capacitor(s): Look inside the electrical compartment. You are looking for one or two large, cylindrical or oval-shaped metal cans.
   • A dual run capacitor (most common) will be a single can with three terminals on top, labeled C, HERM, and F.
   • You might see a single run capacitor (two terminals) for the fan and a separate start capacitor (often black plastic, two terminals) for the compressor, or just single run capacitors.
   • Note the number of terminals and the shape (round or oval) – this is important for replacement.

Tools You Will Need

Having the right tools is essential for safety and success. Gather these before you start:

1. Insulated Screwdriver: A long-shafted screwdriver with a thick, heavy-duty insulated handle is CRITICAL for safely discharging the capacitor. A flathead or Phillips may be needed for panel screws too, but the primary use here is discharging. Do not use a screwdriver with a damaged or thin handle.
2. Nut Driver or Socket Set: You’ll need the correct size (commonly 1/4″, 5/16″, or sometimes 3/8″ hex heads) to remove the access panel screws and potentially the capacitor mounting bracket screw. An adjustable wrench might work in a pinch but is less ideal.
3. Multimeter with Capacitance Setting: Essential for testing. Ensure your multimeter can measure capacitance (marked with µF, MFD, or a capacitor symbol) and has a range high enough for AC capacitors (e.g., up to 100µF). Cheaper multimeters may not have this function.
4. Needle-Nose Pliers (Insulated): Useful for gripping and gently pulling wires off the capacitor terminals if they are tight. Ensure the handles are well-insulated.
5. Voltage Tester: A non-contact voltage pen or a multimeter set to AC voltage. Used to verify power is completely off before you touch anything.
6. Safety Gloves: Insulated electrician’s gloves are best. Heavy leather work gloves provide some protection but are not foolproof against high voltage. Never work without gloves.
7. Safety Glasses: Protect your eyes from potential sparks or debris.
8. Pen and Paper / Smartphone Camera: Crucial for recording the capacitor specifications and, most importantly, documenting exactly which color wire connects to which terminal (C, Herm, F).
9. (Optional) Fin Comb: If the condenser fins nearby are bent, you might want to gently straighten them while the panel is off (be careful!).
10. (Optional) Contact Cleaner / Small Wire Brush: If the wire connectors or capacitor terminals are corroded.

Testing Your AC Capacitor

With the power off and the capacitor located, you can proceed with testing.

Step 1: Visual Inspection

Look closely at the capacitor(s). Do you see:

• Bulging or Swollen Top: The top should be flat. A domed or bulging top is a clear sign of failure.
• Leaking Fluid: An oily substance leaking from the casing or terminals indicates the dielectric fluid inside has escaped – the capacitor is bad.
• Heavy Rust or Corrosion: Especially around the terminals, which can impede connection.
• Physical Damage: Cracks or dents in the casing.

If you see any of these signs, the capacitor is definitely bad and needs replacement. You might skip the electrical testing (though discharging is still necessary). However, a capacitor can fail electrically without showing any visible signs, so electrical testing is recommended if it looks okay.

Step 2: How to Safely Discharge a Capacitor

THIS IS THE MOST IMPORTANT SAFETY STEP. Even with the power off, the capacitor can hold a powerful, dangerous charge. You MUST discharge it before touching the terminals or disconnecting wires.

1. Grip the Screwdriver Firmly: Hold your insulated screwdriver ONLY by its handle. Keep your fingers well away from the metal shaft.
2. Bridge the Terminals:
   • For a 2-Terminal Capacitor: Place the metal shaft of the screwdriver simultaneously across both metal terminals on top of the capacitor. Hold it there for 5-10 seconds.
   • For a 3-Terminal (Dual Run) Capacitor: You need to discharge each section to the common terminal.
     • First, bridge the screwdriver shaft between the “C” (Common) terminal and the “HERM” terminal. Hold for 5-10 seconds.
     • Next, keeping one end on “C”, move the other end to bridge between the “C” terminal and the “F” (Fan) terminal. Hold for 5-10 seconds.
3. Watch/Listen for a Spark: You might see or hear a small spark when the screwdriver first makes contact. This is normal and confirms the capacitor was holding a charge that is now being safely dissipated through the screwdriver. Even if you don’t see a spark, always perform the discharge procedure.
4. Repeat (Optional but Recommended): Some technicians repeat the bridging process one more time just to be absolutely certain the capacitor is fully discharged.

Never touch the terminals directly until you have performed this discharge procedure.

Step 3: Disconnect the Wires (Document First!)

Before you pull any wires off, STOP! You absolutely must know where each wire goes back. Connecting them incorrectly can damage your new capacitor or other AC components.

1. Label or Photograph: Use your smartphone to take clear, close-up photos showing which color wire connects to which labeled terminal (C, Herm, F). Alternatively, draw a diagram or use masking tape labels. Do not skip this!
2. Remove Wires: Gently but firmly grip the metal connector at the end of each wire (using insulated pliers if needed) and pull it straight off the terminal spade. Wiggle slightly if necessary. Avoid pulling on the wire itself.
3. Inspect Connectors: Check the female spade connectors on the wires and the male terminals on the capacitor for rust or corrosion. Clean them with contact cleaner or a small wire brush if necessary for a good connection later.

Step 4: Test with the Multimeter

Now you can electrically test the discharged and disconnected capacitor.

1. Set the Multimeter: Turn your multimeter dial to the Capacitance setting (µF, MFD, or capacitor symbol). Ensure the range is appropriate (e.g., select a range higher than the highest µF rating on the capacitor).
2. Read the Capacitor Label: Find the specifications printed on the side of the capacitor. You’re looking for the microfarad rating(s) and tolerance.
   • Dual Run Example: “45 + 5 µF ± 6%” or “45/5 MFD ± 6%” (This means 45µF for the compressor, 5µF for the fan, with a 6% tolerance).
   • Single Run Example: “40 µF ± 5%”
3. Test a 2-Terminal Capacitor: Place one multimeter probe firmly on one terminal and the other probe on the second terminal. The polarity doesn’t matter for AC capacitors.
4. Test a 3-Terminal (Dual Run) Capacitor: You need to test both sides:
   • Test HERM Rating: Place one probe on the “C” (Common) terminal and the other probe on the “HERM” terminal. Wait for the reading to stabilize on the multimeter display.
   • Test FAN Rating: Keep one probe on the “C” (Common) terminal and move the other probe to the “F” (Fan) terminal. Wait for the reading to stabilize.
5. Interpret the Results:
   • The multimeter will display a reading in microfarads (µF).
   • Compare this reading to the rating printed on the capacitor label, taking the tolerance into account.
   • Calculate Tolerance: For a 45µF ± 6% rating:
     • Lower limit = 45 – (45 * 0.06) = 45 – 2.7 = 42.3 µF
     • Upper limit = 45 + (45 * 0.06) = 45 + 2.7 = 47.7 µF
     • Any reading between 42.3 µF and 47.7 µF is considered acceptable for the HERM side.
   • Do the same calculation for the Fan side (e.g., 5µF ± 6% = 4.7 µF to 5.3 µF).
   • A capacitor is BAD if:
     • Any reading is outside the calculated tolerance range (e.g., the 45µF side reads 38µF, or the 5µF side reads 6.2µF).
     • The multimeter reads “OL,” “Overload,” or zero (indicating an open or short circuit).
     • The reading fluctuates wildly and never stabilizes.

If the capacitor tests bad (or showed visual signs of failure), it’s time to get a replacement. If it tests within spec, your AC problem likely lies elsewhere (contactor, motor, wiring, refrigerant, etc.), and you should call a professional HVAC technician for further diagnosis.

Choosing the Right Replacement

Selecting the correct replacement capacitor is not just important – it’s absolutely critical for the proper and safe operation of your air conditioner. Using the wrong capacitor can prevent your AC from working, damage expensive motors, or create a safety hazard.

You MUST match the specifications of the old capacitor precisely. Look at the label on the failed part:

1. Capacitance Rating (µF or MFD): Match EXACTLY.
   • Dual Run: The replacement MUST have the same two numbers. If the old one is 45/5 µF, the new one must also be 45/5 µF. You cannot substitute a 40/5 or a 45/7.5.
   • Single Run: The replacement MUST have the same single number. If the old one is 40 µF, the new one must be 40 µF.
   • Tolerance (±%): The replacement should ideally have the same or a tighter tolerance. If the original was ±6%, a replacement rated ±6% or ±5% is acceptable. Do not use one with a looser tolerance (e.g., ±10%).
2. Voltage Rating (V or VAC): Match or Go Higher.
   • This indicates the maximum voltage the capacitor can safely handle. Common ratings are 370VAC and 440VAC.
   • The replacement capacitor’s voltage rating MUST be equal to or GREATER THAN the original.
   • Example: If your old capacitor is rated 370VAC, you can replace it with either a 370VAC or a 440VAC capacitor.
   • Example: If your old capacitor is rated 440VAC, you MUST replace it with another 440VAC capacitor.
   • NEVER use a capacitor with a LOWER voltage rating than the original (e.g., do not replace a 440VAC with a 370VAC). This creates a serious risk of premature failure or even explosion.
   • Using a higher voltage rating (e.g., 440V instead of 370V) is perfectly safe and often recommended, as higher-voltage capacitors tend to be built more robustly and may last longer. They do not “send” more voltage; they simply tolerate more.
3. Physical Shape and Size:
   • Capacitors come in Round and Oval shapes.
   • Try to match the shape (round or oval) so it fits correctly in the existing mounting bracket.
   • Check the diameter and height. The replacement should be physically similar enough to be securely mounted. A slightly shorter or taller capacitor might work if the bracket can still hold it firmly. A loose, vibrating capacitor can lead to premature failure or wiring issues.
4. Terminal Type:
   • Ensure the replacement has the same number of terminals (2 for single run, 3 for dual run) and that they are labeled correctly (C, Herm, F for dual run).

Where to Buy Replacement Capacitors:

• Local HVAC Supply Stores: Often have knowledgeable staff but may primarily sell to licensed contractors. Call ahead to see if they sell to the public.
• Appliance Parts Retailers: Stores that specialize in appliance parts often carry common capacitor sizes.
• Online Retailers (Amazon, eBay, etc.): Wide selection and competitive prices. Be cautious: Verify the seller’s reputation and ensure you are buying a quality brand (Genteq, Titan PRO, Packard, Nucap are reputable aftermarket brands). Avoid suspiciously cheap, unbranded capacitors, as quality can be poor.
• Specialty Electronics/Capacitor Websites: Offer a wide range of brands and specifications.

Tip: Take the old capacitor (or clear photos of its label) with you to the store or use the exact specifications when ordering online to ensure you get the correct replacement.

Installing the New Capacitor (Step-by-Step)

Once you have the correct replacement capacitor:

1. TRIPLE-CHECK POWER IS OFF: Before proceeding, verify again at the breaker and disconnect switch that all power to the AC unit is OFF.
2. (Optional but Recommended) Discharge the NEW Capacitor: While new capacitors shouldn’t hold a charge, it costs nothing to be extra safe. Briefly discharge it using the same insulated screwdriver method described earlier before handling the terminals.
3. Mount the New Capacitor: Remove the old capacitor from its mounting bracket (usually held by a metal strap with a screw or bolt). Place the new capacitor in the bracket and tighten the strap securely so it doesn’t move or vibrate. Ensure the terminals are accessible.
4. Reconnect the Wires: This is where your photos or diagram are essential. Carefully and correctly connect each wire to its corresponding terminal on the new capacitor.
   • Match the wire colors/labels from your notes to the C, Herm, and F terminals.
   • Push the female spade connectors firmly onto the male terminals until they seat securely. A loose connection can cause overheating or malfunction.
5. Tidy and Double-Check: Gently tuck the wires back into the control panel area, ensuring they aren’t pinched or rubbing against sharp edges or hot pipes. Visually double-check that each wire is on the correct terminal according to your reference.
6. Replace the Access Panel: Put the panel back in place and securely fasten all the screws you removed earlier.

Restoring Power and Testing Your Work

The moment of truth!

1. Restore Power (Reverse Order):
   • First, push the disconnect block back in or flip the switch ON at the outdoor unit’s disconnect box.
   • Second, go to your main electrical panel and flip the AC condenser and air handler breakers back to the ON position.
2. Set the Thermostat: Go inside to your thermostat. Set the system mode to COOL and lower the desired temperature several degrees below the current room temperature to ensure it calls for cooling.
3. Listen and Observe:
   • Go back outside near the condenser unit. Within a minute or two (some thermostats have a delay), you should hear the contactor click, followed by the relatively smooth startup of both the compressor (a lower hum) and the fan motor.
   • The loud, struggling hum or repeated clicking should be gone.
4. Check for Cold Air: Go back inside and feel the air coming out of your supply vents. It should start feeling noticeably cooler within a few minutes as the refrigerant cycle gets going.
5. Monitor Performance: Allow the system to run through a full cooling cycle (until it reaches the set temperature and shuts off). Listen for any unusual noises and ensure it seems to be cooling effectively.

If the AC starts up normally and blows cold air, congratulations! You have successfully replaced your AC capacitor.

When to Call a Pro

While replacing a capacitor can be a manageable DIY task for some, never compromise your safety. Call a qualified HVAC technician if:

• You are uncomfortable or uncertain about ANY part of this process, especially electrical safety or discharging the capacitor.
• You cannot definitively verify that the power is completely off.
• Replacing the capacitor does not solve the problem. This indicates the issue lies elsewhere (e.g., failed fan motor, bad compressor, faulty contactor, wiring issue, low refrigerant, control board failure), requiring professional diagnosis.
• You visually identified the old capacitor as bad (bulging/leaking), replaced it correctly, but the unit still doesn’t work.
• You see burned wires, melted connectors, or other signs of electrical damage inside the unit.
• The circuit breaker trips immediately after restoring power post-replacement (leave it OFF!).
• You don’t own a multimeter with a capacitance setting or are unsure how to use it correctly.
• The capacitor is in a very difficult-to-reach location.
• The capacitor fails repeatedly (e.g., you replace it, and it fails again within weeks or months), suggesting a deeper underlying electrical problem putting stress on the capacitor.

Professionals have the training, tools, and safety equipment to handle these situations correctly and diagnose the root cause if the capacitor isn’t the only issue.

Conclusion: A Cool Fix, Handled Safely

A failed AC capacitor is a common headache during hot weather, often preventing your air conditioner from starting or cooling effectively. Fortunately, for homeowners comfortable and knowledgeable about electrical safety, testing and replacing this component can be a cost-saving DIY repair.

The keys to success are prioritizing safety above all else (disconnecting power, verifying it’s off, safely discharging the capacitor), meticulously documenting wire placement, and choosing an exact-spec replacement (matching µF ratings and meeting or exceeding the voltage rating).

However, remember that working inside an AC unit carries significant risks. If you encounter unexpected issues, are unsure about any step, or if the new capacitor doesn’t resolve the problem, do not hesitate to call a licensed HVAC professional. Sometimes, what seems like a simple capacitor issue might mask a more complex problem requiring expert diagnosis. By following safety protocols and knowing your limits, you can effectively tackle this common repair or make an informed call for professional help, getting your cool air back on track. Consider scheduling regular professional AC maintenance to help catch failing components like capacitors before they leave you sweating.