How to Know If AC Capacitor Is Bad | Signs, Symptoms & Testing
Complete guide to identifying bad AC capacitors. Learn the 7 key symptoms, testing methods, and replacement costs. Portland AC capacitor troubleshooting for homeowners.
Read Time
15 min
Service Area
Portland Metro
License
CCB #187834
Owner & Lead HVAC Technician
AC capacitors are small but crucial components that store electrical energy to help your air conditioning system start and run efficiently. When capacitors fail, your AC may not start at all, or may run poorly and waste energy. In Portland, Oregon, where AC systems face hot summers and long periods of inactivity, capacitor failure is a common issue.
This comprehensive guide covers everything Portland homeowners need to know about AC capacitors: how they work, why they fail, the 7 key symptoms of failure, safe testing methods, and replacement costs. Whether you're experiencing AC problems or want to prevent them, understanding capacitors can save you time and money.
Capacitor replacement costs $150-350 in Portland and is one of the most common AC repairs. Knowing the signs can help you catch problems early before they cause more expensive damage to your compressor or other components.
What Does an AC Capacitor Do?
AC capacitors store electrical energy and provide the extra power needed to start motor-heavy components like the compressor and blower fan. Think of them as electrical booster rockets that give motors the initial push they need to overcome inertia and start running smoothly.
There are two main types: start capacitors (used only during startup) and run capacitors (provide continuous power). Modern systems often use dual capacitors that combine both functions in one unit. Without functioning capacitors, your AC simply won't work properly.
Top 7 Signs Your AC Capacitor Is Bad
AC won't start at all
High - Call technician immediatelySystem is completely unresponsive. Thermostat calls for cooling but nothing happens.
Portland Context: Common in spring when AC hasn't run since previous summer
AC starts then shuts off immediately
High - Capacitor failing rapidlyCompressor kicks on briefly (2-3 seconds) then stops. May repeat in cycles.
Portland Context: Often occurs during first 90°F+ day in Portland summers
Humming noise but fan won't spin
High - Classic capacitor failure signOutdoor unit makes humming sound but compressor or fan doesn't start.
Portland Context: Humidity in Portland can cause capacitor corrosion
Long delay before AC starts
Medium - Early warning signTakes 5+ seconds for AC to start after thermostat calls for cooling.
Portland Context: More noticeable in cooler Portland springs
AC short-cycling
Medium - Capacitor weakeningAC runs for 5-10 minutes then shuts off repeatedly.
Portland Context: Can occur during variable Portland weather
Higher energy bills
Low - Gradual capacitor degradationAC runs longer and less efficiently due to capacitor inefficiency.
Portland Context: PGE bills may show increased usage
Capacitor visibly swollen or leaking
High - Replace immediatelyCapacitor case is bulging or has oily residue around terminals.
Portland Context: Moisture from Portland climate accelerates corrosion
What Causes AC Capacitors to Fail?
Age (5-10 Year Lifespan)
AC capacitors have a finite lifespan due to electrolytic materials breaking down over time. In Portland's climate, capacitors may fail sooner due to heat stress during summer and corrosion from humidity. Most manufacturers recommend replacement every 5-7 years, though some last 10+ years with proper care.
Heat Exposure
Portland summers with temperatures exceeding 90°F put extreme stress on capacitors. Heat causes the electrolytic fluid inside capacitors to break down faster. Poor ventilation around outdoor units exacerbates this problem. Capacitors in direct sunlight or near heat sources fail more quickly.
Power Surges
Electrical spikes from thunderstorms, PGE grid fluctuations, or lightning strikes can damage capacitors. Portland's summer thunderstorm season increases this risk. Surge protectors help, but capacitors are still vulnerable to extreme electrical events.
Poor Ventilation
When outdoor AC units lack proper clearance, heat builds up around capacitors. Portland homes with overgrown landscaping or units installed too close to structures experience accelerated capacitor failure. Always maintain 12-18 inches of clearance around all sides.
Manufacturing Defects
Some capacitors fail prematurely due to manufacturing issues. Cheap or off-brand capacitors may not meet specifications. In Portland, we recommend OEM (original equipment manufacturer) capacitors for reliability. Cheap replacements often fail within 1-2 years.
Corrosion from Moisture
Portland's high humidity (40-60% average) and 165 rainy days create moisture that corrodes capacitor terminals and connections. Moisture ingress through vent holes or damaged cases causes internal failure. This is more common in older units or those exposed to weather.
DIY Capacitor Testing (Safe Methods Only)
⚠️ SAFETY WARNING: Capacitors Store Lethal Voltage
AC capacitors can store enough electricity to kill you, even when the power is off. Discharge time can take 5-10 minutes. Never attempt to test or replace capacitors unless you're experienced with high-voltage electrical work. In Portland, we strongly recommend professional service for any capacitor work.
Visual Inspection (Safe)
- •Check for swelling or bulging of the capacitor case
- •Look for oily residue or leaks around terminals
- •Inspect for corrosion on wires and connections
- •Check if the capacitor is securely mounted
- •Note any burn marks or discoloration
Listen for Issues (Safe)
- •Turn AC to ON and listen for humming from outdoor unit
- •Check if fan spins freely when gently pushed
- •Listen for clicking sounds without motor engagement
- •Note if system tries to start but immediately stops
- •Check for unusual buzzing or electrical sounds
When to Stop DIY and Call a Professional
- • Any electrical testing or measurements
- • Removing or opening capacitor cases
- • Working on live electrical circuits
- • If you're unsure about any safety procedures
- • For any actual repairs or replacements
Professional Capacitor Testing & Diagnosis
Multimeter Testing Procedure
Professional technicians use digital multimeters to test capacitor capacitance in microfarads (μF). They check if readings match manufacturer specifications. For dual capacitors, they test both start and run circuits separately.
- • Discharge capacitor safely first
- • Test capacitance values
- • Check for shorts or opens
- • Verify voltage ratings
- • Test under load conditions
What Techs Check During Service
During capacitor diagnosis, technicians perform comprehensive system checks to ensure proper operation and identify any related issues.
- • Full electrical system inspection
- • Compressor and fan motor testing
- • Wiring and connection integrity
- • Voltage supply verification
- • System performance under load
Why DIY Testing Is Risky
Capacitor testing requires specialized equipment and knowledge. Incorrect testing can damage the capacitor further or create safety hazards. In Portland, licensed HVAC technicians have the proper tools and insurance to handle capacitor work safely. The small cost of professional testing is worth avoiding potential electrical hazards or voiding your AC warranty.
AC Capacitor Replacement Cost in Portland
Run Capacitor
$20-$75
Part only
- • Continuous operation capacitor
- • Smaller size (round or oval)
- • Lower voltage ratings
- • Most common type
Start Capacitor
$25-$100
Part only
- • Startup boost capacitor
- • Larger size (cylindrical)
- • Higher voltage capacity
- • Used with start relays
Dual Capacitor
$30-$120
Part only
- • Combines start and run
- • Most common in modern AC
- • Oval or round shape
- • Two terminals plus common
Total Replacement Costs
Labor Costs: $100-$200
- • Diagnosis and testing: $75-$125
- • Capacitor replacement: $50-$100
- • Electrical safety testing: included
- • Portland market rates apply
Total Investment: $150-$350
- • Standard replacement: $150-$250
- • Complex installations: $250-$350
- • Emergency service: +$100-$150
- • Same-day service available
Cost-Saving Tips
- • Annual AC tune-ups catch capacitor issues early
- • Maintenance plans include capacitor replacement discounts
- • OEM parts last longer than cheap alternatives
- • Regular filter changes reduce capacitor stress
How to Prevent Capacitor Failure
Annual AC Tune-Ups
- •Professional capacitor testing during spring maintenance
- •Clean electrical connections to prevent corrosion
- •Check capacitor age and recommend replacement
- •Verify proper ventilation around outdoor unit
- •Catch capacitor weakening before complete failure
Proper Maintenance
- •Keep outdoor unit clean and clear of debris
- •Change air filters every 30-60 days
- •Ensure proper electrical supply (no voltage fluctuations)
- •Install surge protectors for electrical protection
- •Replace capacitors proactively at 7-8 years old