Your AC is running. The outdoor unit is humming. The indoor fan is blowing. But the air coming out of your vents feels like a warm breath from a tired dog.

Before you pay $150–$250 for a service call, here are the five most common causes — in order of how often they turn up, and how easily you can handle them yourself.

1. Dirty air filter (you can fix this in 2 minutes)

This is the boring answer that turns out to be correct about 40% of the time.

A clogged filter chokes the airflow over your evaporator coil. With reduced airflow, the coil gets too cold, freezes into a block of ice, and stops cooling the air. You won't always see ice — sometimes the airflow is just so weak that whatever cold air is being made never reaches your living room.

Check: Pull your filter out. If you can't see light through it, replace it.

Fix: Buy the right size filter (it's printed on the frame). Swap it in. Done.

2. Outdoor condenser coil is filthy

Your outdoor unit is basically a radiator that dumps the heat from your house to the outside air. If the coil is caked with grass, cottonwood fluff, dog fur, or just years of dust — it can't dump heat, and your system can't cool.

Check: Go outside. Look at the metal fins around the outdoor unit. If they're clogged, this is your problem.

Fix: Turn the AC OFF at the thermostat. Turn the outdoor disconnect (the box on the wall next to the unit) to OFF. Hose the unit down from the inside out if you can access it, or straight through the fins. Use light water pressure — don't bend the fins. Let it dry. Restore power. See if it cools.

3. Condensate drain line is clogged

Your AC makes water (that's the "dehumidifying" part). That water drains out through a small PVC pipe. When it clogs — usually with algae or sludge — the safety float switch shuts your system down to keep your attic or basement from flooding. Symptoms: system running, no cooling, maybe some water stains near the air handler.

Check: Look for a PVC pipe with a visible fitting near your indoor unit. Often there's a T-fitting with a cap on top.

Fix: Pop the cap. Pour a cup of distilled white vinegar in. Wait 30 minutes. The vinegar kills the algae. Or, shop-vac the outside end of the drain line to suck the clog out. Talk to Dale if you want the full walkthrough — he'll customize it to your setup.

4. Thermostat batteries or settings

Sometimes the system isn't running, it just sounds like it. A flaky thermostat can send weird signals. Low batteries in a battery-powered thermostat cause the screen to dim, settings to drift, or the system to cycle off unexpectedly.

Check: Thermostat screen dim, showing "low batt", or behaving strangely? Is it set to COOL (not FAN)? Is the setpoint actually below room temp?

Fix: New batteries. Set to COOL. Set temp 5° below current room temp. Wait 10 min.

5. Low refrigerant (this one is a pro call)

If 1–4 check out and your AC still isn't cold, refrigerant is the usual next suspect. But here's the deal: you cannot legally or safely add refrigerant yourself. EPA Section 608 requires certification to handle refrigerant, and refrigerant under pressure at low temp can cause frostbite or blindness.

Fix: This is a tech call. When you book, tell them: "I already checked filter, condenser, drain line, and thermostat. I suspect a refrigerant issue." That saves them 30 minutes of checks you've already done — and saves you money.

When to stop troubleshooting and call someone

Stop and call a pro immediately if:

You smell something burning from your equipment
Breakers are tripping when the AC kicks on
You see ice anywhere on the refrigerant lines (the copper tubes)
You see standing water around the indoor unit
The outdoor unit is making grinding, clicking, or buzzing noises that weren't there before

The honest take

About 3 out of 4 "my AC is broken" calls are fixable in under an hour by the homeowner. The remaining ones genuinely need a pro — but now you'll walk into that call knowing what's not the problem, which saves you from getting upsold on the wrong fix.

If you want to walk through any of this live, Dale is iHVAC's free AI diagnostic. No signup, no phone number. He'll ask you three questions and tell you exactly what to try first.

This article is advisory only. iHVAC and Dale are not licensed contractors. HVAC work involves electricity, pressurized refrigerant, and sometimes combustion — all of which can cause injury. When in doubt, call a licensed professional.

Before You Call a Tech: 5 Reasons Your AC Isn't Cold