Bad AC Capacitor Symptoms and Replacement Guide

The run capacitor is one of the most frequently replaced AC components — and one of the easiest for a DIY-confident homeowner to swap. A failed capacitor can cause the compressor or fan motor to not start at all, to hum without spinning, or to run slowly and struggle to cool. Learn the symptoms, safety steps, and replacement procedure.

Try the AI Diagnosis Tool

AI Repair Tools

Photo Diagnosis Scan Label Wiring Scan Ask AI

Common Symptoms

AC won't turn on — outdoor unit is silent or hums without starting
Outdoor fan spins slowly or struggles to get up to speed
AC takes a long time to start ('hard start') or needs multiple attempts
AC cools less effectively than it used to on the same thermostat setting
Capacitor canister is visibly bulging, leaking oil, or has a domed top

Most Likely Causes

1
Capacitor Natural Failure with Age (Most Common)
Run capacitors degrade with heat exposure over time. Units in hot climates or in direct sun experience faster capacitor degradation. Most capacitors last 10–20 years, but units in hot attic air handlers or uncovered southern-facing outdoor units often fail sooner.
2
Power Surges
A power surge or lightning strike can damage or destroy a capacitor instantly. If your AC stopped working after a storm or power restoration, check the capacitor first before assuming the compressor has failed.
3
Running AC with a Dirty Filter
A clogged air filter causes the blower motor to run at higher current draw to push air through restriction, shortening capacitor life over time.

Not sure if this is the right fix for your exact model?

Upload a photo of your appliance label — Repair Scout will identify your exact unit and tailor the diagnosis.

Diagnose My Appliance →Already a Pro member? Sign in →

Quick DIY Checks

Safety Warning

DANGER: AC capacitors store a lethal electrical charge — up to 400+ volts — even after the unit has been turned off and unplugged. ALWAYS discharge the capacitor using an insulated screwdriver or proper discharge tool before touching any terminals. Never touch capacitor terminals bare-handed.

Caution

Turn off power at both the thermostat AND the outdoor disconnect box. A double shutoff ensures the capacitor circuit is de-energized before you open the panel.

1Turn off the AC at the thermostat AND at the outdoor disconnect box (a weatherproof box on the wall near the outdoor unit). Flip the disconnect lever or remove the pull-out block.
2Wait 15–30 minutes after shutting off power — a charged capacitor can retain a lethal voltage even with power disconnected. Do NOT skip this wait time.
3Remove the outdoor unit side panel (usually 4–6 screws). The capacitor is a cylindrical metal can, typically silver or gray, with 3–5 wire terminals on top. Look for bulging, oil stains, or a domed top — all signs of failure.

Want the complete repair checklist?

See all 5 steps + safety warnings + tool list + cost estimate. Save $150+ vs. a service call.

Review Pro — $1.00/mo

30-day money-back guarantee · Cancel anytime

4Discharge the capacitor before touching any terminals: use an insulated screwdriver to short each terminal pair briefly (a small spark is normal). Do NOT touch the terminals bare-handed.
5Note the capacitor ratings: the MFD (microfarad) values and voltage rating are printed on the label (e.g., '45+5 MFD 440V'). Purchase a matching replacement. Install the new capacitor by transferring wires one at a time to the same terminals.

Stop Paying $150+ Per Service Call

Get instant AI diagnosis for any home repair — plumbing, electrical, HVAC, appliances, and more.

✓Unlimited AI repair diagnoses
✓Step-by-step fix guides with tool lists
✓Cost & time estimates before you call a pro
✓1,000+ appliance & system repair articles
✓Emergency repair guidance, 24/7

$1.00/mo — less than one service call trip charge

Review Monthly Pro — $1.00/mo

$1.00/month · Checkout continues to Stripe · Cancel anytime

Most homeowners save $150+ on their first repair

Repair vs Replace

✓ Worth Repairing

A capacitor is one of the cheapest AC components to replace. A matching run capacitor costs $15–$40 online. Even a HVAC technician's visit for a capacitor swap typically runs $150–$300 (mostly labor). If the capacitor tests fine but the AC still won't start, the compressor may have failed — a much more expensive diagnosis.

Est. Repair Cost

$15–$60 in parts (DIY)

Est. Replacement Cost

$3,500–$8,000 for a new AC system

Recommended Tools & Parts

AC Dual Run Capacitor
Dual-run capacitor for compressor and fan motor (most residential AC units). Match MFD ratings and voltage exactly (e.g., 45+5 MFD 440V).
$15–$40
Buy on Amazon →
Capacitor Discharge Tool
Insulated resistor tool for safely discharging AC and HVAC capacitors before service. Essential safety equipment.
$10–$20
Buy on Amazon →
Hard Start Kit
Capacitor add-on kit that helps struggling compressors start ('hard start'). Reduces startup current draw and extends compressor life.
$25–$50
Buy on Amazon →

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

Still stuck? Let AI take a look.

Describe your problem or upload a photo — get a diagnosis in seconds.

Start Free Diagnosis →— or —Review Pro for $1.00/mo →

Related Repair Guides

This section combines editorial `relatedSlugs` from the article source with metadata-based matches so each guide links deeper into the repair library.

HVAC Capacitor Failure — Symptoms, Testing, and Replacement

AC won't start or fan spins slowly? A failed run capacitor is the #1 HVAC repair — and a $15–40 part you can replace yourself.

Read guide →

Swollen AC Capacitor: Signs, Symptoms, and Replacement Guide

Bulging top, oil leakage, or a reading far below rated MFD — a swollen AC capacitor is a $20 fix if caught early. Here's how to test, safely discharge, and replace a failed run or start capacitor.

Read guide →

Understanding HVAC Capacitor Ratings: MFD, VAC, Run vs Start Caps Explained

Capacitors are one of the top 3 HVAC failure causes — knowing how to read MFD, VAC, and run vs start capacitor ratings means you buy the right part the first time and replace it safely.

Read guide →

Heat Pump Capacitor Diagnosis — Run, Start, and Dual-Run Capacitor Testing

Diagnose heat pump run capacitor, start capacitor, and dual-run (3-terminal) capacitor failures. Covers MFD tolerance testing (±6%), mandatory discharge procedure, hard start kit installation, and exact replacement specs for 45+5MFD/440VAC dual-run caps.

Read guide →

ECM vs PSC Blower Motor: Diagnosis & Replacement Guide

HVAC blower not spinning or running at wrong speed? Identify whether you have an ECM or PSC motor, test the capacitor or module, and determine if you need a $80 module swap or a $300+ full motor replacement.

Read guide →

Burnham Boiler Troubleshooting: Error Codes, Pilot Outage, Pressure Relief Valve & Thermostat Wiring

Burnham boiler showing fault codes, pilot won't stay lit, or pressure relief valve dripping? Covers Alpine flash-code diagnostics, BTH/V-6 standing-pilot thermocouple replacement, PRV weeping causes and replacement, heat exchanger descaling for ES2/V-6 models, and 24V thermostat wiring problems that prevent heat calls.

Read guide →