Why Your AC Is Running But Not Cooling: 8 Most Common Causes

When Houston heat hits its peak, an AC that’s running but not blowing cold air is more than inconvenient — it’s an emergency. Fortunately, most cooling failures come from a handful of common issues. Understanding these causes can help you identify the problem early and avoid a full system breakdown.

Here are the 8 most common reasons your AC runs but isn’t cooling — and what you should do next.

1. Dirty or Clogged Air Filters

A clogged filter restricts airflow, making it impossible for your AC to cool properly.
If you haven’t replaced your filter in 1–3 months, this is likely the culprit.

Symptoms:

• Weak airflow

• Warm air

• Longer runtimes

Fix: Replace the filter and see if airflow improves within 30 minutes.

2. Low Refrigerant (Freon) Levels

Low refrigerant usually means a leak, not “normal use.”
When refrigerant drops, the system can’t absorb heat properly.

Signs of low refrigerant:

• Ice on refrigerant lines

• Hissing sounds

• Warm air from vents

Important: Handling refrigerant requires EPA-certified technicians.

3. Frozen Evaporator Coils

Your indoor coil can freeze due to:

• Restricted airflow

• Dirty filters

• Low refrigerant

• Faulty blower motor

A frozen coil = no cooling.

What to do:
Turn off the AC and run the fan to thaw the coil, then call a professional to prevent repeat freezes.

4. Dirty Condenser (Outdoor Unit)

If the outdoor unit is packed with:

• Dirt

• Leaves

• Grass clippings

• Debris

…it can’t release heat, causing warm air inside.

Fix: Gently wash the coils with a garden hose — not a pressure washer.

5. Thermostat Issues

A miscalibrated, dead, or incorrectly set thermostat can trick your AC into running incorrectly.

Check for:

• Wrong mode (set to HEAT instead of COOL)

• Dead batteries

• Wrong sensor placement

If settings look correct, the thermostat may need recalibration or replacement.

6. Electrical or Control Board Problems

If electrical components are failing, your AC may run but never cool.

Common issues include:

• Faulty capacitors

• Damaged contactors

• Control board errors

These require a licensed technician to diagnose safely.

7. Ductwork Leaks

Leaky ductwork can dump cool air into your attic instead of your home.

Signs:

• Certain rooms always hot

• High energy bills

• Weak airflow from some vents

Even small leaks can reduce cooling by 20–30%.

8. An Old or Failing AC System

If your AC is 10–15+ years old, warm airflow may mean it’s reaching the end of its life.

Typical signs of an aging system:

• Frequent repairs

• Rising bills

• Uneven cooling

• Poor airflow

A technician can help determine if repair or replacement is the best option.

When to Call an HVAC Technician

Call a professional if:

• The system freezes repeatedly

• There’s a refrigerant leak

• You hear hissing, grinding, or buzzing

• You notice water pooling around the indoor unit

• Your AC runs constantly but never cools

Quick diagnosis prevents bigger issues like compressor failure.

Prevent the Problem With Routine AC Maintenance

Annual maintenance can prevent 90% of cooling failures, including:

• Low refrigerant

• Dirty coils

• Poor airflow

• Electrical wear

• Drain line clogs

Proper maintenance also improves energy efficiency and extends your system's life.