AC Running But Not Cooling? Most Common Causes (Bay Area Troubleshooting Guide)

Your air conditioner is running—you can hear it humming, feel air blowing from vents—but the air isn't cold. The house is getting warmer, and you're starting to worry about expensive repairs.

Don't panic. Many causes of this problem are simple fixes you can handle yourself. This guide walks through the most common reasons your AC runs but doesn't cool, starting with the easiest solutions first.

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Quick Diagnosis Checklist

Before diving into details, check these 5 things first:

1. ✅ Thermostat set to COOL (not FAN or HEAT)
2. ✅ Temperature set 3-5°F below current room temp
3. ✅ Air filter clean (replace if dirty/clogged)
4. ✅ Circuit breaker hasn't tripped (check main panel and outdoor disconnect)
5. ✅ Outdoor unit clear of debris (leaves, grass clippings, bushes)

If all 5 check out and your AC still isn't cooling, keep reading.

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Common Cause #1: Dirty or Clogged Air Filter

Likelihood: 40-50% of AC cooling problems

Why It Happens

Your air filter captures dust, pollen, and debris. When it gets clogged (usually after 1-3 months), airflow drops by 50-70%. This causes the evaporator coil to get too cold and freeze solid, blocking all heat exchange.

Signs You Have This Problem

• Ice buildup on indoor coil or refrigerant lines
• Weak airflow from vents
• AC runs constantly but house doesn't cool
• Water pooling near indoor unit (from melting ice)

DIY Fix

1. Turn off AC at thermostat
2. Locate air filter (usually in return vent or air handler)
3. Remove and inspect - if you can't see light through it, it's clogged
4. Replace with correct size (MERV 11 recommended for Bay Area)
5. If coil was frozen: Wait 4-6 hours for complete thaw
6. Restart AC and monitor

Cost: $15-$30 for quality filters (buy multi-packs)
Time: 5 minutes
Success rate: 40% of cases resolved

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Common Cause #2: Low Refrigerant (Freon Leak)

Likelihood: 25-30% of AC cooling problems

Why It Happens

Refrigerant doesn't "run out"—AC systems are sealed. Low refrigerant always means you have a leak somewhere. Common leak points:

• Evaporator coil (indoor unit)
• Condenser coil (outdoor unit)
• Service valve connections
• Refrigerant line joints

Signs You Have This Problem

• Ice on copper refrigerant lines or indoor coil
• Hissing or bubbling sounds from lines
• AC runs constantly, never reaches set temperature
• Higher-than-normal energy bills
• Outdoor unit struggles to start

Why You Can't DIY This

Refrigerant work requires EPA Section 608 certification. It's illegal for homeowners to purchase or handle refrigerant without certification.

Professional Repair Process

1. Leak detection using electronic sensors or UV dye
2. Repair leak (braze copper joint, replace coil, etc.)
3. Vacuum system to remove moisture and air
4. Recharge with correct refrigerant type (R-410A or R-454B)
5. Test system performance

Cost: $400-$1,500 depending on leak location
Time: 2-4 hours for most repairs
Note: If your system is 12+ years old with major coil leaks, replacement may be more cost-effective

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Common Cause #3: Frozen Evaporator Coil

Likelihood: 20-25% of cases (often related to #1 or #2)

Why It Happens

The evaporator coil absorbs heat from your home's air. When airflow is restricted (dirty filter) or refrigerant is low, the coil temperature drops below 32°F and ice forms.

Signs You Have This Problem

• Visible ice on indoor coil or refrigerant lines
• Water leaking from indoor unit
• No cool air despite AC running
• Indoor unit making gurgling sounds

DIY Fix (Temporary)

1. Turn off AC completely (don't just set to FAN)
2. Let ice thaw for 4-8 hours (don't try to scrape ice off)
3. Replace air filter while waiting
4. Check for other issues (blocked vents, closed registers)
5. Restart after complete thaw

If ice returns within 24-48 hours: You have an underlying problem (dirty coil, refrigerant leak, or blower motor issue). Call for professional service.

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Common Cause #4: Dirty Outdoor Condenser Unit

Likelihood: 15-20% of cases

Why It Happens

Your outdoor unit (condenser) releases heat absorbed from your home. When the coils get clogged with dirt, pollen, cottonwood seeds, or grass clippings, heat can't escape. Result: AC runs but doesn't cool.

Bay Area specific: Coastal areas see salt buildup, inland areas get heavy pollen/dust.

Signs You Have This Problem

• Outdoor unit covered in debris or dirt
• Unit runs but feels unusually hot to touch
• Higher energy bills
• AC struggles on hot days but works fine in cooler weather

DIY Cleaning

1. Turn off power at outdoor disconnect box
2. Remove debris from fins (use garden hose, not pressure washer)
3. Spray fins gently from inside-out to push dirt outward
4. Clear 2-foot radius around unit (trim bushes, remove grass clippings)
5. Straighten bent fins with fin comb (optional)
6. Restore power and test

Cost: $0 (DIY) or $100-$200 (professional cleaning)
Time: 20-30 minutes
Frequency: Clean 1-2x per year in Bay Area

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Common Cause #5: Thermostat Problems

Likelihood: 10-15% of cases

Common Thermostat Issues

Wrong Mode: Thermostat set to FAN (circulates air without cooling) instead of COOL

Wrong Temperature: Set too close to current temp (needs 3-5°F difference to trigger cooling)

Bad Placement: Thermostat in direct sunlight or near heat source (gives false readings)

Dead Batteries: Many thermostats stop functioning properly with low batteries

Wiring Issues: Loose wire connections prevent proper system communication

DIY Fixes

1. Verify COOL mode selected
2. Set temp 5°F below current to force cooling cycle
3. Replace batteries if digital display is dim
4. Check for sun exposure (install shade if needed)
5. Gently tighten wire connections at thermostat (power off first)

When to replace thermostat:

• 10+ years old
• Doesn't hold settings
• Displays erratic temperatures
• Upgrade to programmable/smart model

Cost: $50-$250 for new thermostat (DIY install) or $150-$400 (professional)

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Less Common Causes (Require Professional Service)

Compressor Failure

• Outdoor unit hums but fan doesn't spin
• Circuit breaker trips repeatedly
• Burning smell from outdoor unit
• Cost: $1,500-$3,000 repair or system replacement

Blower Motor Problems

• Little/no airflow from vents despite AC running
• Squealing or grinding noises from indoor unit
• Cost: $400-$800 for motor replacement

Ductwork Leaks

• Some rooms cool fine, others don't
• Attic feels unusually cold during AC operation
• Cost: $500-$2,000 for duct sealing

Expansion Valve Stuck

• Ice only on one refrigerant line (not both)
• Intermittent cooling
• Cost: $300-$600 for valve replacement

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When to Call Galaxy Heating & Air

Call immediately if:

• You smell burning or electrical odors
• Circuit breaker trips repeatedly
• Water pooling near indoor unit won't stop
• Ice returns immediately after thawing
• Outdoor unit makes loud grinding/screeching noises

Schedule service within 24-48 hours if:

• DIY fixes didn't work
• You suspect refrigerant leak (ice on lines + hissing sounds)
• System is 12+ years old (may need replacement)
• Energy bills increased 20%+ with no usage change

Routine maintenance recommended:

• Annual professional tune-up before cooling season
• Filter changes every 1-3 months
• Outdoor unit cleaning 1-2x per year

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Cost Comparison: Repair vs. Replace

Age of AC	Repair Cost	Recommendation
0-5 years	Any amount	Repair (likely under warranty)
6-10 years	Under $1,000	Repair
6-10 years	Over $1,500	Consider replacement
11-15 years	Under $500	Repair
11-15 years	Over $1,000	Replace
15+ years	Any major repair	Replace

Rule of thumb: If repair cost exceeds 50% of replacement cost AND unit is 10+ years old, replace.

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Prevention: Keeping Your AC Cooling Properly

Monthly:

• Check air filter, replace if dirty
• Listen for unusual sounds
• Verify all vents are open and unblocked

Seasonally (Spring/Fall):

• Clean outdoor condenser unit
• Check refrigerant line insulation for damage
• Test thermostat accuracy

Annually:

• Professional maintenance tune-up ($150-$200)
• Includes refrigerant check, coil cleaning, electrical testing, airflow measurement

Bay Area specific: Schedule tune-ups in April/May before heat season starts. Coastal homes need more frequent outdoor coil cleaning due to salt air.

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Emergency Cooling Tips (While Waiting for Repair)

If your AC isn't working and you need to stay cool:

1. Close blinds/curtains on sun-facing windows
2. Use portable fans to circulate air
3. Avoid heat-generating activities (cooking, laundry, etc.)
4. Cool house overnight when outdoor temps drop
5. Consider portable AC unit for bedroom (rent or buy)

Bay Area advantage: Evenings cool down significantly. Open windows at night, close in morning to trap cool air.

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The Bottom Line

Most common causes (DIY fixable):

1. Dirty air filter (40% of cases) - Replace filter, wait for thaw
2. Dirty outdoor unit (15% of cases) - Clean with garden hose
3. Thermostat issues (10% of cases) - Check settings, replace batteries

Professional service required:

• Refrigerant leaks (25% of cases) - $400-$1,500
• Frozen coil that won't stay thawed - Likely indicates leak or airflow problem
• Mechanical failures - $400-$3,000 depending on component

When your AC runs but won't cool, start with the easiest fixes first. Check your filter, clean your outdoor unit, verify thermostat settings. These three steps solve 60-70% of cooling problems.

Still not working? Call Galaxy Heating & Air at (925) 578-3293. Our Bay Area technicians provide same-day emergency service and can diagnose any AC issue. We serve all major Bay Area cities including Pleasant Hill, Walnut Creek, Concord, Oakland, San Jose, and San Francisco.