Quick answer
Common signs your AC needs repair include warm air from the vents, a house that will not cool down after sunset, ice on the refrigerant lines, short cycling, water around the indoor unit, new noises and rising bills. The DOE ties many of these to low refrigerant, worn controls, leaky ducts or dirty filters; replacing a clogged filter alone can cut energy use 5 to 15 percent.
Updated · By the AC Phoenix Pros editorial team · How we research
This is an independent homeowner guide. We are not a contractor and do not perform HVAC work. We explain warning signs and how to hire, and if you email us a request we reply with next steps and, where a vetted licensed local contractor is available, an introduction.
Nine warning signs and what usually causes them
| Sign | Common cause | How urgent in Phoenix |
|---|---|---|
| 1. Air from vents is warm or barely cool | Low refrigerant, a failed capacitor or compressor, dirty coil | High in summer |
| 2. House will not cool down at night | Undersized or undercharged system, dirty filter or coil | Medium to high |
| 3. Ice on the copper lines or indoor coil | Low refrigerant or restricted airflow | High: turn the system off and call |
| 4. Short cycling (turning on and off rapidly) | Worn compressor and fan controls, thermostat problems | Medium |
| 5. Water around the indoor unit | Clogged condensate drain | Medium: water damage risk |
| 6. New buzzing, grinding or squealing | Electrical parts, motors or bearings | Medium to high |
| 7. Bills rising without a change in use | Dirty filter, leaking ducts, aging equipment | Low to medium |
| 8. Some rooms much hotter than others | Leaking, clogged or constricted ducts | Low to medium |
| 9. Humidity or musty smell indoors | Drainage problems, oversized or failing system | Low to medium |
The causes come from the U.S. Department of Energy's list of common air conditioner problems, which includes low or leaking refrigerant, compressor and fan controls "worn out from having your system turn off and on too frequently," leaking or constricted ducts, clogged filters and clogged drains (DOE Home Cooling 101). ENERGY STAR adds rising bills, frequent repairs, uneven temperatures, humidity problems and noise as signs a system may be nearing replacement (ENERGY STAR).
Quick checks before you call anyone
- Thermostat: confirm it is set to cool, set below the room temperature, and reading correctly. The DOE notes it may simply need new batteries.
- Filter: a clogged filter restricts airflow and reduces cooling. The DOE says routinely replacing or cleaning filters can lower an air conditioner's energy use by 5% to 15% (DOE Home Cooling 101).
- Outdoor unit: clear debris and leaves from the fan and condenser.
- Condensate drain: if water is pooling, the drain may be clogged.
If these do not fix it, stop there. Refrigerant and electrical repairs are for a licensed technician.
Why ice on the lines needs attention
Ice usually means low refrigerant or poor airflow. Continuing to run a frozen system risks further damage, and the compressor is the most expensive part to replace. Switch the system off and arrange a diagnosis. If refrigerant is low, there is a leak: the EPA advises asking the technician to locate and repair it instead of "topping it off" (EPA).
When a warning sign becomes an emergency
In Phoenix, a cooling failure can be a health risk. Maricopa County counted 430 heat-related deaths in 2025; 111 happened indoors, and in 72% of indoor cases where an AC unit was present, it was not working (Maricopa County 2025 Heat-Related Deaths Report). If someone in the home is struggling with the heat, call 911. If the house is getting dangerously hot but no one is ill, move to a cooler place while you arrange a repair. Our emergency AC guide covers what to do next.
Timing your repair
Most of these signs are easier and cheaper to deal with before peak season. A spring visit is covered in our tune-up cost guide. If the diagnosis is a major part on an older system, read repair or replace before approving the work.
Check the contractor
Before anyone works on your system, look up the company in the Arizona ROC contractor search and confirm an air conditioning classification such as R-39. A written quote should name the failed part. Our AC repair guide explains common repairs and their costs.
Sending a request
Email dispatch@acphoenixpros.com with your ZIP code, the system's age, which of the signs above you are seeing, and photos of the indoor and outdoor units. The contact page explains what happens next. We are not an emergency service.
Frequently asked questions
Why does my AC run all day but the house stays warm?
Common causes include a dirty filter or coil restricting airflow, low refrigerant from a leak, or leaking ducts. The DOE notes that duct leakage can account for about 30 percent of a cooling system's energy use. Check the filter first. If a clean filter does not help, have a licensed technician check the refrigerant charge and airflow.
Is short cycling bad for my air conditioner?
Yes. Short cycling means the system turns on and off too frequently. The DOE says compressor and fan controls can wear out from frequent cycling, and the cause may be a thermostat issue, an oversized system or an electrical fault. It is worth a diagnosis before it turns into a failed compressor.
What should I do if water is leaking from my indoor AC unit?
Turn the system off if water is spreading, and check whether the condensate drain is clogged. The DOE suggests occasionally passing a stiff wire through the unit's drain channels, and notes that a clogged drain can keep the unit from removing humidity and can stain walls or carpet. If it keeps happening, have it inspected.
Sources
Independent homeowner guide operated by SWKM GmbH. Not a licensed contractor. We do not perform HVAC work. More about this site.