My AC Is Running But Not Cooling

You can hear the system running, you can feel air coming from the vents, but it's warm. On a hot day, this can go from annoying to miserable in a hurry. The good news: roughly half of the causes are things you can check and fix yourself in the next 10 minutes.

Quick Diagnosis (30-Second Checks)

Before you do anything else, check these three things right now:

1. Thermostat setting — Is it set to COOL (not HEAT or FAN ONLY)? Is the set temperature lower than the current room temperature? Is it in AUTO fan mode? If the fan is set to ON, you'll feel air blowing even when the compressor isn't running, and it will feel warm.
2. Air filter — Pull it out and hold it up to light. If you can't see light through it, it's too clogged. A dirty filter is the #1 cause of AC problems and the easiest to fix.
3. Outdoor unit — Go outside and look at it. Is the fan spinning? Is it making any noise at all? Is it buried in leaves, grass clippings, or debris?

If the thermostat is correct, the filter is clean, and the outdoor unit is running, keep reading.

Common Causes (Ranked by Likelihood)

1. Dirty Air Filter (Most Common)

A clogged filter starves the system of airflow. The evaporator coil gets too cold, ice forms on it, and then it can't absorb heat at all. You end up with a system that runs constantly and cools nothing.

Fix: Replace the filter. If you see ice on the indoor coil or refrigerant lines, turn the system to FAN ONLY for 2-4 hours to let it thaw, then restart in COOL mode with the new filter.

2. Thermostat Misconfigured or Malfunctioning

The fan running on ON instead of AUTO is the classic culprit. You feel air, but the compressor isn't engaged, so it's just room-temperature air blowing around.

Fix: Switch to AUTO fan and COOL mode. Set the temperature 5 degrees below room temp. Wait 5 minutes. If cool air starts flowing, you found it.

3. Dirty Condenser Coil (Outdoor Unit)

The outdoor unit dumps heat from your home. If the coil fins are packed with dirt, cottonwood fluff, grass clippings, or pet hair, it can't release heat efficiently.

Fix: Turn off the unit. Gently spray the coil fins from inside out with a garden hose (not a pressure washer — you'll flatten the fins). Let it dry. Restart.

4. Frozen Evaporator Coil

If the indoor coil is encased in ice, airflow is blocked and no cooling happens. Causes: dirty filter, low refrigerant, dirty coil, blower motor failure, or closed/blocked supply vents.

Fix: Turn system to FAN ONLY. Let it thaw completely (2-4 hours). Check and replace the filter. Make sure all supply vents are open. Restart. If it freezes again within a few days, call a technician — you likely have a refrigerant leak.

5. Refrigerant Leak

Your AC doesn't "use up" refrigerant — if it's low, there's a leak somewhere. Signs: ice on the refrigerant lines, hissing or bubbling sounds near the indoor unit, the system runs constantly but never reaches set temp.

Fix: This requires a licensed HVAC technician. They'll locate the leak, repair it, and recharge the system. Do not use "recharge kits" — adding refrigerant without fixing the leak is throwing money away.

6. Capacitor Failure (Outdoor Unit)

The capacitor gives the compressor and fan motors the electrical kick they need to start. When it fails, the outdoor fan may spin slowly or not at all, and the compressor may hum but not engage. You might hear a clicking sound.

Fix: Call a technician. Capacitors store dangerous electrical charge even when power is off. A tech can replace one in 15-30 minutes. Cost: $150-$400 including labor.

7. Compressor Failure

The compressor is the heart of the system. If it fails, the outdoor unit may hum or trip the breaker but produce no cooling. This is the most expensive repair scenario.

Fix: Technician diagnosis required. If the compressor is under warranty (typically 5-10 years), you'll pay labor only ($500-$1,000). Out of warranty, a new compressor costs $1,500-$3,000 installed — at which point replacing the whole outdoor unit often makes more sense.

DIY Fixes

• Replace the air filter (check monthly in summer, replace every 1-3 months)
• Verify thermostat settings (COOL mode, AUTO fan, temperature below room temp)
• Clean the outdoor condenser coil with a garden hose
• Clear vegetation and debris at least 2 feet (60 cm) from the outdoor unit
• Make sure all supply and return vents inside are open and unobstructed
• Thaw frozen coils by running FAN ONLY mode for 2-4 hours
• Check the circuit breaker — sometimes the outdoor unit has a separate breaker that's tripped

When to Call a Pro

• Ice keeps coming back after thawing and replacing the filter — likely a refrigerant leak
• Outdoor unit doesn't turn on at all — could be capacitor, contactor, or compressor
• Strange noises from the outdoor unit — grinding, screeching, or loud buzzing
• Breaker trips repeatedly when the AC tries to start — electrical issue, possibly compressor
• Refrigerant line is hissing — active leak, needs professional repair
• System is more than 15 years old and has never had major repairs — may be time for replacement

Pro Detail

Diagnostic Procedures

1. Verify thermostat call — Check for 24V signal at Y terminal on air handler board
2. Check capacitor — Measure microfarads with a multimeter; compare to rated value on capacitor label. Replace if more than 10% below rating.
3. Check contactor — Inspect for pitting on contact points. Measure voltage across line and load side.
4. Measure superheat and subcooling — Superheat above 15F on a TXV system suggests low charge. Subcooling below 8F suggests low charge. High superheat + high subcooling suggests a restriction.
5. Check compressor amperage — Compare to RLA on nameplate. Above RLA suggests mechanical issues. Locked rotor = bad compressor or failed start components.
6. Inspect evaporator coil — Remove access panel and check for ice, dirt buildup, or damaged fins.
7. Verify duct static pressure — Should be below 0.5" WC total. High static = restricted airflow.

Common Failure Modes

| Component | Avg Lifespan | Failure Signs | Repair Cost | |-----------|-------------|---------------|-------------| | Capacitor | 5-10 years | Humming, slow fan, clicking | $150-$400 | | Contactor | 5-10 years | Buzzing, intermittent operation | $150-$300 | | Fan motor | 10-15 years | Squealing, overheating, slow rotation | $300-$700 | | Compressor | 12-20 years | Not starting, tripping breaker, low cooling | $1,500-$3,000 | | TXV valve | 10-15 years | Inconsistent cooling, icing | $400-$900 |

Prevention

• Replace filter monthly during cooling season (or at minimum every 90 days)
• Annual professional tune-up in spring before cooling season starts
• Keep outdoor unit clear — trim vegetation 2 feet back, never stack anything against it
• Don't close more than 20% of supply vents — restricts airflow and causes problems
• Install a smart thermostat — monitors runtime patterns and alerts you to anomalies
• Schedule condenser coil cleaning annually if you have pets, cottonwood trees, or dusty conditions

Cost Guide

| Service | Typical Cost | Notes | |---------|-------------|-------| | Air filter replacement | $5-$30 | DIY | | Condenser coil cleaning | $100-$200 | Professional | | Capacitor replacement | $150-$400 | Parts + labor | | Refrigerant leak repair + recharge | $500-$1,500 | Depends on leak location | | Evaporator coil replacement | $1,000-$2,500 | Often covered under warranty | | Compressor replacement | $1,500-$3,000 | Consider full unit replacement | | Full system replacement | $4,000-$12,000 | Depends on size and efficiency |

Shipshape Integration

SAM can help prevent most AC-not-cooling situations before you even notice a problem:

• Temperature monitoring detects when indoor temps rise despite the AC running, triggering an alert before the house gets uncomfortable
• HVAC runtime tracking flags when the system runs significantly longer than normal for the current weather — an early warning of declining performance
• Filter reminders based on your system type and local air quality, not just a generic timer
• Energy usage anomalies catch refrigerant leaks and dirty coils early — these problems show up as increased runtime before they show up as discomfort
• Seasonal maintenance reminders prompt you to schedule a spring tune-up before cooling season begins
• Dealer coordination — if SAM detects a problem, your Shipshape dealer can proactively reach out with a diagnosis and appointment, often before you've noticed the issue