How to Know If Your AC Unit Is Broken
Published: May 2, 2026 · By SAVA HVAC & Appliance Repair
Your AC is broken if it's blowing warm air, making unusual noises, short-cycling (turning on and off every few minutes), producing weak airflow, forming ice, or leaking water. Some failures are obvious; others are subtle. Here's how to read the signs accurately so you know whether you need a technician today.
Key Takeaways
- The 7 clearest signs your AC is broken: warm air, weak airflow, unusual noises, ice on the unit, water leaks, short-cycling, and foul odors.
- Supply air should feel 15–20°F cooler than return air — if it doesn't, the system has failed to cool.
- A hissing sound almost always means a refrigerant leak — turn the system off and call a technician; refrigerant is EPA-regulated.
- A clogged filter can reduce airflow by 50% or more and is the most common fixable cause of AC failure before calling for service.
1. AC Blowing Warm or Room-Temperature Air
This is the most obvious sign. Hold your hand near a supply vent while the system is running. The air should feel noticeably cool — typically 15–20°F cooler than the return air (the air coming back through the filter grille). If it feels the same temperature as the room or warmer, something has failed.
Common causes: refrigerant leak, failed compressor, frozen evaporator coil, or a thermostat set incorrectly to HEAT or FAN ONLY.
2. Weak or No Airflow from Vents
Stand at a supply vent when the system is running. If the airflow feels noticeably weaker than it used to, or if some rooms get air and others don't, this points to a blower motor failure, a frozen coil blocking airflow, a collapsed duct, or a severely clogged air filter.
Check your filter first — a completely clogged filter can reduce airflow by 50% or more and is a common cause of system shutdown.
3. Strange Noises
A properly functioning AC runs with a consistent hum. Any new or unusual sound is a warning sign:
- Hissing or bubbling: Refrigerant leak. Turn the system off and call a technician. Refrigerant is a regulated substance under EPA Section 608.
- Grinding or scraping: Metal-on-metal contact in the blower motor or compressor. Run time = damage. Turn off the system immediately.
- Loud banging or clanking: A loose or broken component inside the unit — possibly a broken blade, loose mount, or failed connecting rod.
- Squealing: A worn belt (older systems) or a bearing failing in the blower motor.
- Clicking that doesn't stop: Relay or control board issue. Intermittent clicking at startup is normal; continuous clicking is not.
4. Ice Forming on the Unit
Ice on the evaporator coil (inside) or refrigerant lines (the copper pipes connecting indoor and outdoor units) signals that the coil has dropped below freezing. This happens when airflow is severely restricted — usually a clogged filter or blocked vents — or when refrigerant levels are low.
What to do: Switch the thermostat to FAN ONLY (not OFF — the fan helps thaw). Do not run in COOL mode while the coil is frozen. After 1–2 hours of fan-only operation, check if the ice has melted. If the system refreezes after restarting, call a technician — you likely have a refrigerant leak or airflow problem that needs professional repair.
5. Water Leaking Around the Indoor Unit
The indoor air handler has a condensate drain that removes moisture collected from the air. When this drain clogs — with algae, mold, or debris — water backs up and overflows. You'll see water pooling around the base of the unit or dripping from the ceiling in systems installed in attics.
A clogged drain line is typically a straightforward fix. Left unaddressed, the water overflow can cause significant water damage to ceilings, walls, and flooring — and promote mold growth.
6. Short-Cycling (Turning On and Off Every Few Minutes)
A properly functioning AC runs in cycles of 15–20 minutes during normal conditions. If your system turns on for 2–5 minutes, shuts off, and repeats, it's short-cycling. This is hard on the compressor — one of the most expensive components in the system — and wastes significant energy.
Common causes: an oversized system, a refrigerant overcharge or undercharge, a dirty evaporator coil, or a failing thermostat sensor.
7. Foul or Musty Odors from Vents
A musty smell from vents points to mold or mildew in the evaporator coil, drain pan, or ductwork — a common issue in humid Chicago summers. A burning smell indicates an electrical issue — an overheating motor, a failing capacitor, or wiring that's arcing. If you smell burning, turn off the system and call a technician immediately.
According to ASHRAE Standard 62.2, indoor air quality is directly affected by HVAC system condition — mold growth in evaporator coils and drain pans is a documented source of indoor air contamination. ASHRAE recommends regular inspection and cleaning of these components as part of routine HVAC maintenance.
The U.S. Department of Energy estimates that approximately 87% of U.S. homes use air conditioning — making AC maintenance one of the most impactful actions homeowners can take for both comfort and indoor air quality. A system showing two or more of the warning signs above warrants a professional diagnostic visit before the problem compounds.
Frequently Asked Questions
How do I know if my AC unit is broken?
Key signs: blowing warm air, weak airflow, unusual noises (hissing, grinding, banging), ice on the indoor unit or refrigerant lines, water leaking from the air handler, short-cycling, or foul odors from vents. Any of these warrants a professional diagnosis.
Why is my AC running but not cooling?
Running but not cooling usually means a refrigerant leak, frozen evaporator coil, failed compressor, or a dirty condenser coil that can't release heat. Each requires professional diagnosis — these aren't DIY repairs.
What does a hissing sound from my AC mean?
Hissing indicates a refrigerant leak — refrigerant escaping under pressure. Turn the system off and call a technician. Refrigerant handling requires EPA certification and is not a DIY repair.
Why is there ice on my AC?
Ice forms when the evaporator coil temperature drops below freezing — usually due to a clogged filter, blocked vents, or low refrigerant. Switch to FAN ONLY to thaw. Do not run in cooling mode while frozen — it can destroy the compressor.
Should I turn off my AC if it's broken?
Yes — if you hear grinding, smell burning, or see ice, turn the system off. Running a failing AC can turn a minor repair into a major one. If you have a frozen coil, switch to FAN ONLY (not fully off) to thaw before the technician arrives.
Why is water leaking from my AC unit?
Water leaking from the indoor air handler almost always means the condensate drain line is clogged with algae or debris. Water backs up in the drain pan and overflows. This is typically a straightforward fix but must be addressed quickly — left unresolved, it can cause significant water damage to ceilings, walls, and flooring, and promote mold growth.
What does short-cycling mean and why is it bad?
Short-cycling means the system turns on and off every 2–5 minutes instead of running normal 15–20 minute cycles. It stresses the compressor — the most expensive component in the system — wastes energy, and signals a fault: oversizing, refrigerant imbalance, a dirty coil, or a thermostat problem. Have it diagnosed before it leads to compressor failure.
How much does it cost to diagnose a broken AC in Chicago?
SAVA charges a $79 diagnostic fee to inspect and identify the specific cause of the failure. That fee is credited toward the repair cost if you proceed. You'll receive a written estimate before any work begins — no surprise charges.
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