The most common reasons an air conditioner is not cooling, from dirty filters to low refrigerant, and how to fix each one.
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The most common reason an air conditioner is not cooling is a dirty or clogged air filter restricting airflow. Check and clean the filter first before troubleshooting anything else — it takes five minutes and solves the problem in a significant share of cases. If the filter is clean and the unit still blows warm or barely cool air, work through the causes below in order from simplest to most complex. See the best air conditioners if the diagnosis leads you to conclude the unit is too old or undersized to fix economically.
An air conditioner that runs but does not cool is usually not broken — it is blocked, misconfigured, or dealing with conditions it was not sized to handle. Most of the causes below are fixable without calling a technician. The exceptions are refrigerant leaks and compressor failures, which require a licensed HVAC professional.
| Symptom | Most Likely Cause | DIY Fix? |
|---|---|---|
| Runs but blows warm air | Dirty filter, frozen coil, or low refrigerant | Filter: yes. Refrigerant: no. |
| Blows very little air | Clogged filter or frozen evaporator coil | Yes |
| Cools briefly then stops | Thermostat issue, capacitor failure, or overheating | Thermostat: yes. Capacitor: licensed tech. |
| Never reaches set temperature | Undersized unit, dirty condenser coil, or refrigerant leak | Coil cleaning: yes. Refrigerant: no. |
| Ice visible on unit or lines | Restricted airflow or refrigerant shortage | Thaw and clean filter: yes. Refrigerant: no. |
| Outdoor unit not running | Capacitor, thermostat wiring, or circuit breaker | Check breaker: yes. Other: licensed tech. |
Below are the 10 most common causes with a clear diagnosis method and fix for each.
A filter blocked with dust reduces airflow over the evaporator coil. Less airflow means less heat transfer, so the air coming out is warmer than it should be. A severely clogged filter can also cause the coil to freeze — see Cause 4 below.
Diagnosis: Remove the filter and hold it up to a light source. If you cannot see light through the mesh, it needs cleaning.
Fix: Clean or replace the filter. Washable filters should be rinsed and fully dried before reinstalling. See the full process in the how to clean an air conditioner filter guide.
Prevention: Clean the filter every two to four weeks during heavy use.
Check that the thermostat is set to “cool” (not “fan only”) and that the set temperature is below the current room temperature. On portable and window units, check that the mode selector is on “cool” and not “fan” or “dry.” This sounds obvious but is a very common cause of service calls.
Diagnosis: Confirm the thermostat mode and set point. On smart thermostats, check the app for any scheduled temperature overrides or vacation modes that may have shifted the settings.
Fix: Set to “cool” mode, lower the set point at least two degrees below the current room temperature, and wait five minutes for the system to respond.
When to call a pro: If the thermostat displays the correct settings but the system does not respond, there may be a wiring fault or a failed thermostat — a technician can diagnose this quickly.
An air conditioner rated for 350 square feet will run continuously and never fully cool a 600-square-foot room on a hot day. This is not a malfunction — the unit is simply not capable of overcoming the heat load of a larger space.
Diagnosis: Measure the room and compare to the unit’s BTU rating. As a rough guide:
| BTU Rating | Approximate Coverage | Notes |
|---|---|---|
| 5,000-6,000 | 150-250 sq ft | Small bedrooms |
| 8,000-10,000 | 300-450 sq ft | Average living room |
| 12,000 | 500-600 sq ft | Add 10% for sunny south-facing rooms |
| 18,000 | 900-1,100 sq ft | Add 20% for high ceilings over 9 feet |
| 24,000+ | 1,200-1,500 sq ft | Open floor plans or poorly insulated homes |
Fix: If the unit is undersized, the only solutions are a larger unit or supplemental cooling for the hottest hours. The air conditioner buying guide covers sizing in detail, including adjustments for sun exposure, ceiling height, and insulation quality.
When to call a pro: For central AC systems, an HVAC technician can perform a Manual J load calculation to determine the correct system size for your home.
If airflow is restricted (dirty filter, blocked vents) or refrigerant is low, the evaporator coil can drop below freezing and ice over. A frozen coil cannot absorb heat from air, so the unit blows warm or very little air.
Diagnosis: Turn the unit off and check for ice on the coil (visible through the filter slot) or on the suction line (the larger insulated copper pipe). You may also notice water dripping from the unit as ice melts.
Fix: Turn the unit to fan-only mode (or just off) for two to three hours to let ice melt completely. While waiting, clean the filter. After thawing, restart in cool mode and monitor. If the coil refreezes within a few hours despite a clean filter, the cause is likely low refrigerant, which requires a technician.
Prevention: Keep the filter clean. Ensure supply vents are open and not blocked by furniture or curtains.
The outdoor condenser coil (or the rear of a window unit) releases heat outside. If it is coated with dirt, leaves, or debris, heat cannot escape and the system loses efficiency significantly — in extreme cases reducing cooling output by 30 percent or more.
Diagnosis: Inspect the outdoor coil fins. Heavy coating of dirt, cottonwood fluff (common in early summer), or matted leaves is clearly visible.
Fix: For window units, unplug the unit, remove it from the window, and gently spray the rear coil with a garden hose from the inside out. For outdoor central AC condenser units, rinse the coil fins with a hose on a gentle setting, spraying from inside the unit outward through the fins. Never spray directly into the electrical components. Avoid pressure washers, which bend the fins.
Prevention: Clean outdoor coils once per season (spring, before the cooling season begins). Keep landscaping trimmed back at least 18 inches from the unit on all sides.
For central AC systems, check that supply vents in the problem room are fully open and not blocked by furniture, rugs, or curtains. Also check the return air vent (the large grille that pulls air in) — blocking the return starves the system of air to cool.
Diagnosis: Walk through the home and hold your hand near each supply vent to feel for airflow. A vent that blows weakly while adjacent rooms blow strongly suggests a partially closed damper or a blockage in that duct run.
Fix: Open all supply vents fully. Move furniture blocking return grilles. If one zone consistently has weak airflow despite open vents, the duct run to that area may have a disconnection or obstruction — a duct inspection is warranted.
Note: Closing supply vents in unused rooms does not improve efficiency in central systems — it increases static pressure and can stress the blower motor. Leave all vents open.
Refrigerant does not get consumed — if the system is low on refrigerant, there is a leak somewhere. Refrigerant shortage is one of the most common causes of a system that runs continuously but cannot reach the set temperature.
Diagnosis: Signs include ice on the evaporator coil despite a clean filter, hissing or bubbling sounds near the refrigerant lines, and the unit running constantly without reaching the set temperature. You may also notice the indoor unit blowing air that is cool but not cold.
Fix: This is not a DIY fix. Refrigerant handling is regulated and requires a certified technician to locate the leak, repair it, and recharge the system. A system that needs repeated recharges without a confirmed leak repair is wasting money — the refrigerant will continue to escape.
When to replace vs. repair: A system that keeps losing refrigerant and needs repeated recharges is usually nearing the end of its useful life. See the best energy-efficient air conditioners for modern replacements with better sealed systems and lower refrigerant leak risk.
Capacitors are inexpensive components that help start the compressor and fan motors. When a capacitor fails, the motor it serves may hum but not start, or may start slowly and draw excessive current. A failed outdoor fan motor means the condenser cannot release heat, causing the system to run warm or trip a high-pressure cutoff switch.
Diagnosis: If you hear the outdoor unit humming but the fan blade is not spinning, or the outdoor unit does not come on at all while the indoor unit runs, a capacitor failure is the most likely cause.
Fix: Capacitor replacement is a relatively affordable repair — often $100 to $300 including labor — compared to a compressor replacement. Symptoms include the outdoor unit running but no cold air, or the unit starting, running briefly, then shutting off. Fan motor replacement is more expensive but less common.
Safety note: Capacitors store high-voltage electrical charge even when the system is off. Do not attempt to handle or replace a capacitor without proper electrical training and discharge tools.
The suction line (the larger, insulated copper pipe running from the outdoor unit to the indoor coil) is covered in foam insulation to prevent heat gain before the refrigerant reaches the indoor coil. If this insulation is cracked, missing, or has gaps, the refrigerant absorbs outdoor heat before it can do its job inside, reducing cooling capacity.
Diagnosis: Visually inspect the suction line along its full length from the outdoor unit to where it enters the home. Look for sections where the black foam is split, crumbled, or absent.
Fix: Replace damaged sections with foam pipe insulation from a hardware store — a simple DIY fix. Choose foam rated for refrigerant lines (at least 3/4-inch wall thickness). Secure seams with UV-resistant tape designed for outdoor pipe insulation.
Prevention: Check the line insulation each spring. UV degradation typically causes cracking after five to ten years of outdoor exposure.
Air conditioners lose efficiency over years of use as compressors wear, coils accumulate scale, and sealed systems develop micro-leaks. A unit over 12 to 15 years old that struggles to cool despite a clean filter and clear coils may simply be at the end of its efficient life.
Diagnosis: Check the nameplate for the manufacture date (or look up the serial number on the manufacturer’s website). Units more than 12 years old using R-22 refrigerant are particularly expensive to maintain due to refrigerant scarcity.
Fix: Replacing an aging unit with a modern high-efficiency model often cuts cooling electricity bills by 20 to 40 percent. Read how to reduce air conditioner electricity costs for a comparison of efficiency gains from newer models, and see how much electricity an air conditioner uses to model the annual savings from replacement.
Replacement options: For replacement options, the best air conditioners ranking covers current top performers in every category.
Call a licensed HVAC technician when you have confirmed or suspected: refrigerant leak, compressor failure, capacitor or fan motor failure, ductwork disconnection or damage, or an electrical fault. All of these involve either refrigerant handling (requires EPA certification), high-voltage electrical work (capacitors, motors, control boards), or physical access to sealed components that require professional tools to service safely.
Most cooling problems can be resolved by working through the list above before calling a technician. If you have ruled out every DIY cause and the system still does not cool, a refrigerant or compressor issue is likely, and a licensed HVAC technician is the next step.
For portable units that develop cooling problems, the why is my portable AC leaking water guide covers the additional water-management issues specific to portable units, which can also appear alongside or cause cooling problems.
The most common causes are a dirty air filter, a frozen evaporator coil, a thermostat set to "fan only," an undersized unit for the space, or a refrigerant leak. Start by cleaning the filter and checking the thermostat mode before assuming a mechanical failure.
Warm air from a running AC usually points to restricted airflow (dirty filter or blocked vents), a frozen evaporator coil, low refrigerant, or dirty condenser coils. Check the filter first. If the coil is frozen, turn the unit to fan-only for a few hours to thaw it, then investigate the underlying cause.
No. Refrigerant handling is federally regulated in the United States and requires an EPA Section 608 certified technician. Attempting to add refrigerant without repairing the leak is also just a temporary fix -- the system will eventually lose refrigerant again and need a proper leak repair.
Turn the AC off and look for ice buildup on the copper lines or on the front of the indoor unit. You may also notice water dripping from the unit as the ice melts, or the supply air vents blowing very little or no air despite the fan running. Thaw the coil by running the unit in fan-only mode for two to three hours.
Uneven cooling in a central AC system is usually caused by closed or blocked supply vents in problem rooms, a disconnected duct run, an imbalanced duct system, or a room with significantly more heat load (south-facing windows, poor insulation). Open all vents fully and check that return air grilles are unobstructed. Persistent imbalance may need a duct balance adjustment by an HVAC technician.
A common rule of thumb is the 5,000 rule: multiply the unit's age in years by the estimated repair cost. If the result exceeds $5,000, replacement is usually more economical than repair. For units using R-22 refrigerant, factor in the high cost of that refrigerant when estimating repair costs. A unit over 12-15 years old with compressor failure is almost always worth replacing rather than repairing.
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