AC Not Cooling? 7 Things to Check Before Calling an HVAC Tech

It sounds almost insultingly obvious, but 15–20% of HVAC service calls are resolved with thermostat corrections. Before any tech does anything diagnostic, they check the thermostat first. So should you.

Verify all of these:

• Mode is set to COOL, not HEAT or OFF
• Fan is set to AUTO, not ON. When set to ON, the fan runs continuously — even when the AC isn't actually cooling. This circulates warm air and feels like the AC isn't working.
• Set temperature is below the current room temperature. If it's 78°F inside and you set it to 78°F, the AC won't run. Try 72°F.
• If you have a smart/programmable thermostat: check that there's no schedule override, vacation mode, or app-set hold that's overriding manual settings.

If you have a smart thermostat (Nest, Ecobee, Honeywell Home T9), check the app. Look for "hold" modes or schedule conflicts. Sometimes a thermostat firmware update can reset settings.

If the thermostat screen is blank: Replace the batteries first. Smart thermostats can also lose power if their C-wire connection failed — check that the "C" terminal wire is seated firmly on the back of the thermostat.

Most central HVAC systems have two separate breakers: one for the air handler (inside unit) and one for the condenser/compressor (outside unit). If either is tripped, part or all of your AC stops working — and it won't always be obvious which part failed.

How to check:

1. Go to your electrical panel (usually in the garage, utility room, or basement)
2. Look for breakers labeled "AC," "Air Conditioner," "HVAC," "Air Handler," or "Condenser" — you may have two separate ones
3. A tripped breaker is in the middle position — not fully on, not fully off. It may not be obvious at first glance.
4. To reset: switch the breaker fully to OFF first, then back to ON

This is the single most common cause of AC underperformance — and the most preventable. A clogged air filter restricts airflow so severely that it causes the evaporator coil to freeze over, which blocks cooling entirely. It also makes your system work harder, shortening the compressor's lifespan.

How to check:

1. Find the filter — it's usually in the return air vent (a large grille, often in a hallway ceiling or wall), or in the air handler itself (a slot in the unit)
2. Pull the filter out. Hold it up to a light. If you can't see light through it, it's too dirty.
3. Replace it. Most standard filters cost $8–25. For most homes, 1-inch filters should be replaced monthly during heavy use; 4-inch filters every 3–6 months.

What MERV rating to buy: MERV 8–11 is the sweet spot for most homes. MERV 13 and above are excellent at filtering but restrict airflow enough to stress some systems — check your system's manual if you're unsure. For most homes, MERV 10 is the right call.

If your AC is blowing warm air (or barely any air) and the air coming from vents feels slightly cool or damp, you may have a frozen coil. The evaporator coil — the part inside your air handler that actually cools the air — can ice over when airflow is restricted or refrigerant is low.

Signs of a frozen coil:

• Frost or ice visible on the refrigerant lines running to the outside unit (look at the larger insulated copper pipe)
• Water leaking from the air handler or drip pan
• Reduced airflow from vents despite the system running
• Warm air from vents when the thermostat is set to cool

How to thaw it yourself:

1. Turn the thermostat to FAN ONLY mode (not cooling). This runs the fan to push warm air over the coil and thaw it faster. Or, turn the system off completely.
2. Wait 2–3 hours for the coil to fully thaw. Don't rush this — running the compressor on a frozen coil can damage it.
3. Put down towels around the air handler — the melt water needs somewhere to go.
4. After thawing, replace the air filter (see Check #3), then turn cooling back on.

If the coil freezes again within a few hours: The problem isn't just the filter. A coil that keeps freezing likely has low refrigerant — which requires an HVAC technician to diagnose and recharge.

The outdoor unit (the big box with the fan) does the heat rejection side of the cooling cycle. If it's clogged, obstructed, or struggling, the whole system underperforms — or trips a safety shutoff.

Things to check and fix yourself:

• Clear debris: Leaves, grass clippings, cottonwood fluff, and dirt accumulate on the condenser coil fins (the metal fins around the outside of the unit). Use a garden hose to rinse from inside out (spray through the fins from the fan side, not into the fins from outside). Don't use a pressure washer — it bends the fins.
• Clear the area around it: The unit needs 18–24 inches of clearance on all sides for airflow. Overgrown bushes, a fence too close, or a tarp thrown over it will cause it to overheat and shut down.
• Check that the fan is spinning: Stand near the unit while it's running and listen/look. The fan on top should be spinning. If it's not, or it's spinning slowly, the fan motor or capacitor may have failed — this requires a tech.
• Check if the unit is running at all: If the outside unit is completely silent and not running while the thermostat calls for cooling, you may have a failed capacitor. Capacitors are a common failure point, especially in hot climates. This is a tech job, but it's a relatively cheap fix ($150–$350 parts + labor).

Refrigerant — the substance that makes cooling possible — should never need to be "refilled" in a healthy system. Unlike car oil, refrigerant doesn't get consumed. If your system is low on refrigerant, it has a leak.

Signs of low refrigerant:

• AC runs continuously but never reaches the set temperature
• Ice on the refrigerant lines or coil (but filter is clean)
• Higher-than-normal electricity bills during cooling season
• The outdoor unit is running but the air isn't getting cold
• Hissing or bubbling sounds from the refrigerant lines (pressurized leak)

Why you can't DIY this: Handling refrigerant requires an EPA 608 certification. It's illegal to purchase or handle HVAC refrigerants without it. More importantly, simply "topping off" refrigerant without finding and fixing the leak is throwing money away — it'll leak right back out.

A proper refrigerant repair involves: finding the leak (usually with a UV dye or electronic detector), fixing it, and recharging the system to the correct pressure. Total cost varies widely — leak location matters a lot. A simple service valve leak is $200–400; a leaking evaporator coil can be $1,000–3,000 depending on the coil.

As your AC cools air, it removes moisture from it — this is why you feel dehumidified air from your vents. That water drips off the evaporator coil into a drip pan, then drains through a condensate drain line to the outside or to a floor drain. When this line gets clogged with algae, mold, or debris, water backs up into the drip pan and eventually overflows into your ceiling or walls.

Signs of a clogged drain:

• Water stains on the ceiling below your air handler
• Water dripping from the air handler or visible in the drip pan
• System shutting off unexpectedly (many systems have a float switch that cuts off the AC when the drip pan is full)
• Musty smell from vents

How to clear it yourself:

1. Find the condensate drain line — it's a PVC pipe (usually 3/4" diameter) running from the air handler to the outside or a floor drain. There's typically a cap on the line near the air handler (the cleanout port).
2. Remove the cap. Pour 1 cup of white vinegar into the line. Let it sit for 30 minutes — the acid kills algae and loosens buildup. Flush with water.
3. Alternatively, use a wet-dry vacuum to suction the clog from the outside end of the drain line. Seal the vacuum hose tightly around the pipe end and run it for 2–3 minutes.
4. To prevent future clogs: pour 1/4 cup of white vinegar down the drain line every month during cooling season.

If the drip pan has standing water and you can't drain it: Use a wet-dry vacuum to remove the water before it overflows. Then address the clog. If the pan itself is cracked, it needs replacement — this is a tech job.