AC Not Cooling?
Complete Troubleshooting Guide – 8 Common Causes & Fixes
For homeowners and HVAC technicians · Updated March 2026
Complaint
5-Minute Quick Checklist – Do This First
Before calling a technician, run through these five checks. They resolve about 40% of "AC not cooling" calls at zero cost.
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1
Thermostat setting
Confirm mode is COOL (not FAN or HEAT), set point is at least 3°F below current room temperature, and batteries are fresh.
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2
Air filter
Pull out the filter and hold it up to the light. If you cannot see light through it, replace it immediately. A clogged filter is the single most common cause of poor cooling.
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3
Circuit breaker
Check the electrical panel for a tripped breaker labeled "AC," "Condenser," or "Air Handler." Reset once if tripped. If it trips again, stop and call a technician.
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4
Outdoor condenser unit
Go outside and verify: (a) the unit is powered on, (b) the fan on top is spinning, (c) there is at least 2 ft of clearance on all sides, (d) no ice is visible on refrigerant lines.
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5
Supply vents
Walk through the house and confirm every supply vent is fully open and unobstructed by furniture, rugs, or curtains.
8 Reasons Your AC Is Running But Not Cooling
Each cause below is paired with its difficulty level and whether a homeowner or professional should handle it.
Dirty or Clogged Air Filter
Most common cause · Easy DIY fix · Free or under $30
A clogged filter chokes airflow over the evaporator coil. Without adequate airflow, the coil cannot absorb heat efficiently and the system blows barely cool or even warm air. Severe restriction causes the coil to ice over, making things worse.
Symptoms
- Weak airflow from vents
- Warm or slightly cool air
- Ice on the indoor coil or lines
- Visually grey/brown filter
Fix
- Replace with a new filter (correct MERV rating)
- If coil is frozen: run FAN ONLY for 1–3 hours first
- Schedule filter changes every 1–3 months
Thermostat Issues
Wrong settings, dead batteries, faulty sensor
A thermostat set to FAN, HEAT, or AUTO with the wrong schedule will never signal the compressor to run. Dead batteries in wireless models can cause the unit to lose its settings or stop communicating with the air handler.
Check
- Mode: must be set to COOL
- Set point 3°F+ below room temp
- Replace batteries (even if display works)
- Disable any "vacation" or "away" schedule
Smart thermostat note
Check the app for any cooling lockout schedules, geofencing issues, or connectivity errors that could prevent cooling cycles from starting.
Frozen Evaporator Coil
Blocked by ice – causes warm air output
When the evaporator coil temperature drops below 32°F, moisture in the air freezes on it. Ice acts as an insulator, blocking heat transfer entirely. The three root causes are low airflow (dirty filter or blocked return), low refrigerant charge, and a dirty coil.
Frozen Coil Diagnostic Flow
Thaw procedure (DIY)
- Turn system to FAN ONLY
- Place towels under the air handler
- Wait 1–3 hours for full thaw
- Replace filter, restart in COOL
Tech note: Superheat target
After thaw, measure suction line superheat. Target: 8–12°F for TXV systems, 12–20°F for fixed orifice. Low superheat = low charge or metering device issue.
Refrigerant Leak
Requires EPA 608 certified technician
Refrigerant does not get "used up" – a low charge always means there is a leak somewhere in the system. Adding refrigerant without finding and repairing the leak is a temporary fix at best and is illegal to vent refrigerant intentionally.
Warning signs
- Ice on suction line or coil
- Hissing or bubbling sound
- Warm air despite compressor running
- Oil stains near fittings or connections
- System runs constantly but never cools
Tech diagnostic
- Measure suction & discharge pressure
- Check superheat and subcooling values
- Electronic leak detector on all joints
- UV dye or nitrogen pressure test
- Repair leak BEFORE adding charge
Failed Capacitor or Compressor
Electrical fault – compressor or condenser fan not running
Capacitors provide the startup boost for the compressor motor and condenser fan motor. A weak or failed capacitor causes one or both motors to fail to start. You may hear a humming sound from the outdoor unit but see the fan or compressor not turning.
Capacitor failure signs
- Outdoor unit hums but fan does not spin
- Compressor not running (no vibration)
- Visible bulge or leakage on capacitor
- System trips breaker on startup
Compressor failure signs
- Compressor hard-starts or trips
- High amp draw on phase measurement
- Suction and discharge pressures equalize
- Scroll or piston mechanical noise
Dirty Condenser Coil
Outdoor unit cannot reject heat
The condenser coil (outdoor unit) rejects heat absorbed from the house. When it is coated with dirt, grass clippings, or cottonwood seeds, it cannot dissipate heat. High head pressure results, which reduces system efficiency dramatically and can cause compressor overheating and shutdown.
DIY cleaning
- Turn off power at disconnect
- Rinse coil fins from inside out with garden hose
- Remove debris from coil base
- Restore power and test
Tech measurement
Measure condensing temperature (liquid line subcooling). For R-410A: subcooling should be 10–15°F. Higher discharge pressure with low subcooling indicates fouled condenser coil.
Bad Contactor
Outdoor unit does not receive the start signal
The contactor is a high-voltage relay inside the outdoor unit that switches 240V power to the compressor and fan motor when the thermostat calls for cooling. Pitted or burned contacts prevent the outdoor unit from starting even though the indoor air handler may be running fine.
Diagnosis
With power off, inspect contacts for pitting or carbon buildup. With power on (caution – high voltage), measure 24VAC across the contactor coil when thermostat calls for cooling. No voltage = control board or thermostat issue. Voltage present but contacts open = failed contactor.
Faulty Expansion Valve / Metering Device
TXV stuck open or closed
The thermostatic expansion valve (TXV) or electronic expansion valve (EEV) meters refrigerant flow into the evaporator. If it sticks closed, the evaporator starves and superheat skyrockets. If stuck open, the evaporator floods, superheat drops, and liquid refrigerant can reach the compressor.
Stuck closed symptoms
- Very high superheat (>20°F)
- Low suction pressure
- Warm air, system short-cycles
Stuck open symptoms
- Very low or negative superheat
- Liquid slugging noise in compressor
- High suction pressure
DIY vs. Professional – Know the Boundary
Homeowner Can Do
- ✓ Replace air filter (every 1–3 months)
- ✓ Change thermostat batteries and settings
- ✓ Thaw a frozen evaporator coil (FAN ONLY mode)
- ✓ Rinse condenser coil fins with garden hose
- ✓ Clear vegetation/debris around outdoor unit
- ✓ Open all supply and return vents
- ✓ Reset a tripped breaker (once only)
- ✓ Replace a programmable thermostat
Call a Certified Technician
- ✗ Any refrigerant work (EPA 608 required)
- ✗ Replacing capacitors (240V shock hazard)
- ✗ Replacing or testing the contactor
- ✗ Compressor diagnosis or replacement
- ✗ TXV / EEV replacement or adjustment
- ✗ Evaporator coil chemical cleaning
- ✗ Control board diagnosis or replacement
- ✗ Repeated breaker trips (wiring fault)
When to Call a Professional Immediately
Some situations require urgent professional attention. Do not delay if you observe any of the following:
Breaker trips repeatedly, burning smell from unit, visible scorch marks, or sparks when the system tries to start.
Hissing/bubbling sounds, visible ice on lines even after filter replacement, or system loses cooling rapidly over a short period.
Loud clanking or grinding from outdoor unit, compressor that hums loudly but will not start, or no vibration/sound from compressor at all.
Significant water pooling under indoor unit, water on ceiling or walls near air handler – indicates blocked condensate drain or frozen coil melt-off.
Indoor temperature exceeds 85°F with vulnerable occupants (elderly, infants, medical conditions). Prioritize safety first.
System is 12+ years old and requires a major repair. A cost-benefit analysis from a technician may favor replacement over repair.
Prevention – Keep Your AC Cooling All Summer
| Task | Frequency | Who | Benefit |
|---|---|---|---|
| Replace air filter | Every 1–3 months | DIY | Prevents frozen coils, maintains airflow |
| Rinse condenser coil | Every spring | DIY | Reduces head pressure, saves 10–15% energy |
| Clear condensate drain | Every 6 months | DIY | Prevents water damage and system shutdown |
| Annual tune-up | Once/year (spring) | Pro | Catches issues before peak cooling season |
| Check refrigerant charge | If symptoms appear | Pro | Identifies leaks early before compressor damage |
| Trim vegetation around unit | Monthly (summer) | DIY | Ensures adequate airflow to condenser |
| Program thermostat schedule | Seasonal | DIY | Reduces runtime and energy costs |
Frequently Asked Questions
The most common reasons are a dirty air filter restricting airflow, a frozen evaporator coil, low refrigerant charge, or a failed capacitor. Start by checking and replacing the air filter, then inspect the outdoor unit for ice or debris.
Check five things in order: (1) thermostat set to COOL and below room temperature, (2) air filter – replace if clogged, (3) circuit breaker – reset if tripped, (4) outdoor condenser unit – clear any obstructions and confirm the fan is spinning, (5) all supply vents are open and unblocked.
Signs of a refrigerant leak include ice forming on the refrigerant lines or evaporator coil, a hissing or bubbling sound near the unit, warm air from the vents despite the system running, higher-than-normal electricity bills, and oil stains around refrigerant line connections.
Yes, homeowners can thaw a frozen coil. Turn the system to FAN ONLY (do not run COOL) and let the ice melt completely – this takes 1–3 hours. Then replace the air filter and restart in COOL mode. If it freezes again, the cause is likely low refrigerant or a dirty coil, which requires a professional.
A properly sized and functioning system should lower indoor temperature by 1–3°F per hour. On a mild day the house should reach the set point within 2–3 hours. If it runs for more than 3 hours without reaching the set point on a normal day, something is wrong.
Costs vary widely: replacing an air filter is $5–$30 DIY; cleaning a condenser coil runs $75–$200; recharging refrigerant is $150–$500 depending on type and amount; replacing a capacitor is $75–$200; a bad contactor is $100–$250; a failed compressor replacement is $800–$2,500+. Always get a diagnosis before authorizing repairs.