Why is my AC running but not cooling the house?

The most common cause is a bad run capacitor, which prevents the compressor or fan motor from starting properly. Other frequent causes include a failed contactor, a dead condenser fan motor, or a bad thermostat. A capacitor is a quick DIY swap that costs \$10-\$30 versus \$150-\$350 from a technician.

How do I know if my AC capacitor is bad?

Signs of a bad capacitor include the AC humming but not starting, the fan spinning slowly, or the unit shutting off after a few minutes. You can bring your capacitor to Open To Public HVAC Parts in Dallas and we will test it free at the counter. Most capacitors cost \$10-\$30 and take about 15 minutes to replace.

How much does it cost to fix an AC that is not cooling?

It depends on the failed part. A capacitor is \$10-\$30 DIY versus \$150-\$350 from a pro. A contactor is \$18-\$35 DIY versus \$150-\$300 from a pro. A condenser fan motor is \$120-\$180 DIY versus \$600-\$1,200 from a pro. We match and test parts at no charge at our Dallas store.

AC Not Cooling

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Here’s how to diagnose why your air conditioner isn’t blowing cold air.

If your AC is running but not producing cold air, the problem is usually one of a handful of common part failures. Below we walk through each cause from most to least common, with images of the actual parts from our Dallas store.

Common Causes

⚡ Bad Capacitor MOST COMMON

The capacitor stores electrical energy to start and run the compressor and fan motor. If your outdoor unit hums but the fan won’t spin, or the AC runs but only blows warm air, a failed capacitor is the most likely cause. Physically swollen or bulging capacitors are a dead giveaway.

Capacitor
Powers your compressor and fan — the #1 AC failure point
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🔌 Failed Contactor

The contactor is an electrical relay that switches power to the compressor and fan. If you hear clicking from the outdoor unit but neither the compressor nor fan will run, the contactor’s contacts may be pitted or stuck open. Over time, electrical arcing wears the contact surfaces down.

Contactor
Switches power to your compressor — clicking means it’s failing
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🌀 Dead Condenser Fan Motor

If the compressor is running (you’ll hear a loud humming) but the outdoor fan isn’t spinning, the condenser fan motor has failed. Without the fan pushing air over the condenser coil, the refrigerant can’t release heat and the system blows warm air. The compressor will eventually shut down on safety.

Condenser Motor
Spins the outdoor fan blade — if it fails, no heat transfer
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🌡️ Bad Thermostat

Set the thermostat 5°F below the room temperature and wait 30 seconds. If the outdoor unit doesn’t kick on, the thermostat may not be sending the cooling signal. Try replacing the batteries first — this fixes the problem more often than you’d think.

Thermostat
Your AC’s brain — tells the system when to cool
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✅ Check First: Dirty Air Filter

A severely clogged filter restricts airflow and can cause the evaporator coil to freeze. Pull your filter out and hold it up to a light — if you can’t see through it, replace it. This is a free check that can save you from buying parts you don’t need.

Quick Diagnostic Checklist

Before you replace parts, check these:

☑️ Thermostat set to COOL and below current room temperature?

☑️ AC breaker in the electrical panel switched ON?

☑️ Any sounds from the outdoor unit? (Humming, clicking, or nothing?)

☑️ Air filter clean? (Hold it to light — should see through it)

☑️ Supply vents open and not blocked by furniture?

⚠️ Safety Note

Always disconnect power at the breaker before inspecting or replacing electrical components. Capacitors hold an electrical charge even when powered off. If you’re not comfortable working with electrical parts, bring your old part to our store and we’ll help you find the right replacement.