Your AC is running. The thermostat is on. But the air coming out of the vents is barely cool, or not cool at all. Before calling for a service call — and paying the $75 to $200 diagnostic fee — there are 10 things you can check yourself. Several of them take 2 minutes and cost nothing.
Work through these in order. The most common causes are at the top.
Check 1: The Thermostat Settings
This sounds obvious. It's also the cause behind a surprising number of "my AC isn't working" calls.
Confirm:
- The thermostat is set to COOL, not HEAT or FAN ONLY
- The set temperature is at least 2 to 3 degrees lower than the current room temperature
- The fan setting is on AUTO, not ON (the ON setting runs the fan continuously regardless of whether the system is cooling, which pushes room-temperature air through your vents)
If the display is blank or unresponsive, try replacing the batteries first. Most thermostats use AA or AAA batteries and a dead battery is exactly this kind of problem.
If the thermostat is over 10 years old and giving you trouble, replacement is worth considering. A Honeywell T6 Pro or similar basic programmable thermostat runs about $40 and installs in 30 minutes if you're handy.
Check 2: The Air Filter
A dirty air filter is the single most common reason an AC system underperforms. When the filter is clogged with dust, pet hair, and particulate buildup, airflow across the evaporator coil drops significantly. Less airflow means less cooling capacity delivered to the house, and it can also cause the evaporator coil to freeze over (more on that in Check 7).
Pull out the filter and hold it up to light. If you can't see light through it, it needs to be replaced. If you can't remember the last time you changed it, it needs to be replaced.
Standard 1-inch filters should be replaced every 30 to 90 days depending on your home's dust load and whether you have pets. Thicker media filters (4 to 5 inch) can last 6 to 12 months.
Filter replacement costs $5 to $25 depending on the type. It takes two minutes. Do this first, every time.
Check 3: The Circuit Breaker
An AC system actually uses two breakers: one for the outdoor condensing unit and one for the indoor air handler. Both can trip independently. If only one has tripped, the system may appear to be running (you can hear something) but not actually cooling.
Go to your electrical panel. Look for any breaker that isn't fully in the ON position — it may be flipped to OFF, or it may be in a middle "tripped" position that looks almost-on but isn't. If you find a tripped breaker, flip it fully to OFF first, then back to ON.
One important note: if the breaker trips again within a few hours, stop resetting it. A breaker that keeps tripping is telling you there's an electrical problem in the system — either a short, a failing component drawing too much current, or a wiring issue. That requires a technician. Repeatedly resetting a breaker with an underlying electrical fault can damage the equipment or create a fire hazard.
Check 4: The Outdoor Unit (Condenser)
Go outside and look at the condenser — the large unit that sits outside your home. Check two things:
Is the fan spinning? When the system is running, the fan on top of the outdoor unit should be turning. If it's not, the unit may have shut off on a high-pressure or overheat cutout. This can be caused by a failed capacitor (Check 9), a refrigerant issue, or the unit running in extreme heat with restricted airflow.
Is the unit blocked or covered? The condenser needs clear airflow on all sides to reject heat effectively. If shrubs, weeds, or debris are within 18 inches of the unit, that's restricting airflow. Clear anything within 2 feet. Also check whether anything was placed on or against the unit (patio furniture, storage, a cover that didn't get removed in spring).
You can rinse the outer fins with a garden hose on a gentle setting to clear accumulated dirt and cottonwood. Spray from the inside out if possible — or top-down — to avoid pushing debris further into the fins.
Check 5: All Supply Vents Are Open
Walk through the house and check that the supply vents (the ones that blow air into rooms) are open. It's common for a vent in an unused bedroom to get closed and forgotten. Closing more than 20 to 25% of your supply vents restricts system airflow enough to reduce cooling capacity and, in some cases, cause pressure-related problems.
Also check that furniture, rugs, and curtains aren't blocking vents. A couch pushed against a floor vent eliminates that vent's contribution to the room's cooling load.
Check 6: The Return Air Vents
Return vents pull air from the house back to the air handler to be cooled and recirculated. Unlike supply vents, they should never be blocked or closed. A return covered by furniture or a rug creates negative pressure in the system that restricts airflow at the air handler.
Returns are typically larger than supply vents, located on walls or floors, and usually have a filter behind them (which is either the same filter discussed in Check 2, or an additional prefilter). Check that nothing is blocking them.
Check 7: A Frozen Evaporator Coil
If the air coming from your vents isn't cold, or if you notice ice forming on the refrigerant lines running into your air handler (the small copper pipes), you may have a frozen evaporator coil.
The evaporator coil sits inside the air handler and is responsible for absorbing heat from the air. When airflow across it drops too low — from a dirty filter, blocked return, or closed vents — the coil drops below freezing and ice builds up. Once frozen, it blocks airflow almost entirely.
What to do: Turn the system to FAN ONLY mode (not OFF, and not COOL). This runs the fan without activating the cooling, which allows the coil to thaw. Give it 2 to 4 hours. Then replace the air filter, check your vents, and restart the system in COOL mode.
If the coil freezes again after you've addressed the airflow issues, the cause is likely low refrigerant — which is a leak that requires a technician to find and repair. Don't keep running the system through repeated freeze cycles; it can damage the compressor.
Check 8: Signs of a Refrigerant Leak
Refrigerant doesn't evaporate or get consumed — if your system is low on refrigerant, it's because it's leaking somewhere. Signs that point to a refrigerant leak:
- Ice forming on the refrigerant lines or outdoor unit (despite clear airflow)
- The system runs continuously but barely reduces the indoor temperature
- Hissing or bubbling sounds from around the refrigerant lines or air handler
- Significantly higher electric bills with no change in usage patterns
You cannot diagnose or repair a refrigerant leak yourself. The refrigerant handling requires EPA 608 certification, and the leak itself needs to be located and sealed. Running the system with a known refrigerant leak can damage the compressor.
Turn the system off and call a technician for this one.
Check 9: Capacitor Symptoms
The run capacitor helps start and run the compressor and condenser fan motor. It's a common failure point, especially in hot climates where systems run hard all summer.
You can't diagnose a failed capacitor without a multimeter and some knowledge of what you're measuring — but you can recognize the symptoms. If you hear the outdoor unit making a humming sound but the fan isn't spinning, or if the system starts briefly then shuts down, a failed capacitor is a leading candidate.
Do not attempt to replace a capacitor yourself unless you know how to safely discharge it first. Capacitors store a substantial electrical charge that can cause serious injury even when the power is off. This is a $150 to $400 repair that a technician can complete in 20 to 30 minutes — it's worth the service call.
Check 10: Ductwork Leaks or Disconnected Ducts
If some rooms are cooling well and others are barely getting any airflow, the problem may be in the ductwork rather than the HVAC system itself. In attics, duct connections can come loose over time, spilling conditioned air into the attic instead of into the room it was meant to serve.
Check any visible ductwork in your basement, crawl space, or attic for obvious disconnections or large gaps. A disconnected duct will be visibly separated from the trunk line or the boot at the register.
Duct sealing with mastic or metal tape is a legitimate DIY repair for accessible sections. Ductwork in finished walls or ceilings requires a technician with diagnostic equipment to locate and repair.
When to Stop Troubleshooting and Call a Tech
Stop working through this list and call a technician immediately if:
- You smell burning, electrical, or chemical odors from the system
- The system is making loud banging, grinding, or screeching noises
- You see or smell refrigerant (an ether-like sweet chemical smell)
- The system is sparking or tripping breakers repeatedly
- The frozen coil returns after you've addressed the airflow issues — this indicates a refrigerant leak
- The system runs but can't bring the indoor temperature below 85°F even overnight
For any of these situations, turn the system off at the thermostat and the breaker, then call for service. Running a system with a serious underlying problem typically makes the repair more expensive.
What to Tell the Technician
When you call, give them a clear description of what the system is doing (or not doing), what you've already checked, and your system's approximate age. This helps them show up with the right parts on the truck and shortens the diagnostic time — which saves you money on labor.
Useful information to have ready:
- System brand and model (usually on a label on the outdoor unit and air handler)
- Approximate age of the system
- When the problem started
- Whether you've noticed anything unusual recently (ice on the lines, unusual noises, higher electric bills)
If you've already replaced the filter and confirmed the thermostat settings, say so. It saves the tech from starting their checklist at the beginning.
If you've worked through this list and the system still isn't cooling, post a service request on HVACJobs.IO. Describe what you've checked, what the system is doing, and your location — licensed techs in your area will respond with availability and quotes.
Frequently Asked Questions
Why is my AC running but not cooling the house?
The most common causes are a clogged air filter, a frozen evaporator coil, a failed capacitor on the outdoor unit, or low refrigerant from a leak. Start with the filter — it's the most frequent culprit and takes two minutes to check. If the filter is clean and the system still isn't cooling, the outdoor unit is the next place to look.
Can I fix my AC not cooling myself?
Some causes are DIY-friendly: replacing the air filter, resetting a tripped breaker, clearing debris from the outdoor unit, and thawing a frozen coil. Anything involving the refrigerant circuit, electrical components like capacitors, or the internal coils requires a licensed technician.
How long does it take for a frozen AC coil to thaw?
Switch the system to FAN ONLY (not off) and allow 2 to 4 hours. Running the fan speeds up the thaw by pushing warm air over the coil. Once thawed, check the filter and vents before restarting in COOL mode. If it freezes again, the cause is likely a refrigerant leak that needs professional repair.
Why does my AC run all day but not cool below 78 degrees?
This can indicate low refrigerant, a dirty evaporator or condenser coil, a failing compressor, or an undersized system. It can also indicate that the outdoor temperature is simply above the system's design capacity — most residential systems are sized to maintain about a 20-degree differential from outdoor to indoor temperature. If it's 102°F outside, keeping the house at 78°F is within spec; 72°F may not be achievable.
How much does it cost to fix an AC that isn't cooling?
Depends entirely on the cause. A dirty filter costs $5 to $25 to fix yourself. A capacitor replacement runs $150 to $400. A refrigerant leak repair runs $200 to $1,500 depending on severity. A compressor failure — worst case — runs $1,500 to $3,500. The diagnostic fee alone ($75 to $200) tells you which category you're in.