AC Not Cooling? 7 Common Causes (And How to Fix Them)

Nothing ruins a hot summer day faster than discovering your AC isn't cooling. Before you panic and call an expensive HVAC contractor, let's troubleshoot the issue together. Many causes are simple fixes you can handle yourself.

1. Dirty Air Filter (Most Common Cause)

Symptoms: Weak airflow, ice on indoor unit, system running constantly
Fix difficulty: Easy (10 minutes)
Cost: $8-30 DIY | $40-80 contractor

A clogged air filter is the #1 cause of AC cooling problems. When airflow is restricted, your evaporator coil can't absorb heat efficiently, and may even freeze up.

How to Fix:

1. Turn off AC
2. Locate filter (usually in return vent or air handler)
3. Check if it's visibly dirty/clogged
4. Replace with new filter matching exact size
5. Ensure arrow points toward air handler
6. Turn system back on

Prevention: Change filters every 1-3 months. Set a phone reminder.

2. Bad Capacitor

Symptoms: Outdoor unit won't start, humming sound, unit struggles to turn on
Fix difficulty: Medium (30-60 minutes)
Cost: $15-40 DIY | $150-400 contractor

The capacitor gives your compressor and fan motors the electrical "push" to start. When it fails, your outdoor unit won't run.

How to Diagnose:

• Indoor unit blowing air, but outdoor unit silent? Likely capacitor.
• Outdoor unit makes humming/buzzing but won't start? Probably capacitor.
• You can visually inspect: a bulging or leaking capacitor is definitely bad.

DIY or Pro? If you're handy with electrical work, this is a 30-minute fix that saves $150-350. See our capacitor replacement guide. Otherwise, call a pro.

3. Low Refrigerant (Leak)

Symptoms: Ice on outdoor unit, hissing sounds, unit blows warm air, ice on copper lines
Fix difficulty: Professional only
Cost: $200-1,500 (leak detection + repair + recharge)

Your AC doesn't "use up" refrigerant. If it's low, there's a leak that needs professional repair.

DIY or Pro? Professional only. Handling refrigerant requires EPA certification and special equipment.

4. Dirty Condenser Coils (Outdoor Unit)

Symptoms: Reduced cooling, higher electric bills, outdoor unit running hot
Fix difficulty: Easy (30-45 minutes)
Cost: $15-30 DIY | $120-250 contractor

Your outdoor coils dissipate heat. When clogged with dirt, grass, and debris, they can't release heat efficiently.

How to Fix:

1. Turn off power at disconnect switch
2. Remove leaves and debris
3. Clear vegetation 2 feet around unit
4. Spray coil cleaner on fins (from inside out)
5. Let sit 10 minutes
6. Rinse gently with garden hose (NOT pressure washer)
7. Restore power

Prevention: Clean coils twice yearly (spring and fall).

5. Thermostat Issues

Symptoms: System won't turn on, wrong temperature reading, short cycling
Fix difficulty: Easy-Medium
Cost: $0-250

Quick Troubleshooting:

• Check if thermostat is set to COOL (not heat or off)
• Set temperature 5° below current room temp
• Replace batteries (if applicable)
• Clean dust from inside thermostat
• Check circuit breaker

DIY or Pro? Replacing a thermostat is DIY-friendly (30-60 minutes). See our thermostat installation guide.

6. Frozen Evaporator Coil

Symptoms: Ice on indoor unit, reduced airflow, water leaking
Fix difficulty: Easy (2-4 hours waiting time)
Cost: $0 (if cause is dirty filter)

When airflow is restricted (usually dirty filter), the evaporator coil gets too cold and freezes.

How to Fix:

1. Turn off AC completely
2. Turn fan to ON (to help melt ice)
3. Replace air filter
4. Wait 2-4 hours for ice to melt
5. Check for proper drainage
6. Turn AC back to cool mode

If problem persists: Could indicate low refrigerant or a blower motor issue. Call a professional.

7. Blocked or Leaking Ductwork

Symptoms: Cool air from vents, but house stays warm; some rooms cooler than others
Fix difficulty: Varies
Cost: $0-2,000 depending on issue

Quick Checks:

• Ensure all vents are open and unblocked (furniture, curtains)
• Check attic ducts for obvious disconnections or damage
• Feel for air leaking at duct joints

DIY or Pro? Simple vent blockages are DIY. Major duct repairs require professionals (especially if in walls/ceilings).

How to Determine DIY vs. Professional Repair

Still Not Sure? Take Our Diagnostic Quiz

Our free interactive quiz asks targeted questions about your symptoms and helps you:

• Identify the exact problem
• Learn if it's DIY or needs a pro
• Get fair pricing estimates
• Spot contractor scam red flags

Quick Reference: AC Not Cooling Flowchart

1. Check air filter first (causes 50% of cooling issues)
2. If filter is clean: Check thermostat settings and batteries
3. If thermostat is fine: Go outside and check if outdoor unit is running
4. If outdoor unit isn't running: Likely capacitor or electrical issue
5. If outdoor unit is running but not cooling: Likely refrigerant leak or dirty coils

Bottom Line

Before spending $150-500 on a service call, try these simple troubleshooting steps:

1. Replace air filter
2. Check thermostat settings
3. Inspect outdoor unit for obvious issues
4. Clean outdoor coils if dirty

These 4 steps solve 60-70% of "AC not cooling" complaints and cost under $50.

For persistent problems, use our cost guide to know fair pricing before calling contractors, and watch for common AC scams.