Why Is My AC Not Cooling? (17 Real Causes + Fixes, Charts & a Fast Diagnosis Checklist)
If your air conditioner is running but not cooling, you’re not alone—this is one of the most common HVAC problems homeowners face during high-heat weeks. The good news: many causes are easy to diagnose in minutes. The bad news: a few causes can damage your system fast if you keep running it.
What’s on this page
- 1) The 10-Minute “AC Not Cooling” ChecklistStart here
- 2) Decision Tree: Find Your Problem FastVisual
- 3) Charts & Graphs: What “Normal” Looks LikeData
- 4) The 17 Most Common Reasons Your AC Isn’t CoolingDeep dive
- 5) Typical Repair Costs & When to ReplaceMoney
- 6) Phoenix Heat Factors: Why Desert Homes Fail DifferentlyLocal
- 7) Prevent It: Maintenance that Stops “No Cooling” CallsPrevent
- 8) FAQ (Rich Results Ready)Answers
1) The 10-Minute “AC Not Cooling” Checklist (Do This Before You Panic)
This checklist is designed to safely catch the most common “AC running but not cooling” issues. If you’re in Phoenix and it’s 105°F+ outside, these steps can save hours of discomfort and prevent equipment damage.
| Step | What to Check | What It Means |
|---|---|---|
| 1Thermostat mode | Set to COOL (not HEAT), fan to AUTO, temperature set 3–5°F below indoor temp. | Wrong mode/settings can mimic a cooling failure. |
| 2Airflow at vents | Put your hand at a supply vent. Is airflow strong or weak? Check multiple rooms. | Weak airflow points to filter/coil/duct/blower issues. |
| 3Filter condition | Pull the filter. If it’s visibly gray/dusty or bowing inward, replace it. Use correct size/MERV. | Dirty filters reduce airflow and can freeze the coil. |
| 4Outdoor unit | Is the outdoor fan spinning? Is hot air exhausting upward? Clear 24–36" around the condenser. | If heat can’t leave outside, your home won’t cool. |
| 5Ice check | Look for ice on the indoor coil (if visible) or the large copper line near the indoor unit. | Ice = usually low airflow or low refrigerant. Turn system off to thaw. |
| 6Drain/float switch | If your system shuts off randomly, a clogged condensate drain can trigger a safety switch. | Prevents water damage but stops cooling. |
| 7Breaker & disconnect | Check the AC breaker; check the outdoor disconnect (don’t open panels). Reset once only if tripped. | Repeated trips = electrical fault; call a pro. |
| 8Temperature split | Measure return air temp and supply air temp (nearest vent). Normal drop is often ~15–20°F. | Small drop can indicate refrigerant/compressor/airflow issues. |
| 9Doors/attic load | In extreme heat, open doors, poor attic insulation, and sun-exposed glass can overwhelm a system. | Not always a “broken AC”—sometimes it’s heat gain. |
| 10Stop & call | If you smell burning, hear buzzing/clicking, see ice, or the outdoor fan isn’t running—turn it off. | Prevent compressor damage and higher repair bills. |
2) Decision Tree: Find the Likely Cause in 60 Seconds
Use this quick decision tree to narrow down the problem. It’s not a substitute for a professional diagnosis— but it’s extremely effective for identifying the most common failure categories.
Tip: In Phoenix extreme heat, a “working” AC may still struggle if the outdoor coil is dirty or the unit is undersized for current heat gain. That’s why we include desert-specific checks later in this guide.
3) Charts & Graphs: What “Normal” Cooling Looks Like
Most top-ranking “AC not cooling” articles list causes, but they rarely show you what healthy performance looks like. These charts make the problem easier to spot—and help you explain symptoms when you call for service.
Chart A: Normal “Temperature Split” (Return vs Supply)
A common quick test: measure indoor return-air temperature and supply-air temperature at a nearby vent. Many systems often show roughly 15–20°F difference under normal conditions (varies by humidity, airflow, and equipment).
Chart B: Cooling Progress Over the First Hour
If your AC is operating correctly, indoor temperature should trend toward setpoint over time. This illustrative graph shows a healthy vs struggling system during peak heat.
Chart C: Most Common “No Cooling” Causes (By Frequency, Field Experience)
This is an illustrative distribution based on common service call patterns: airflow restrictions and dirty outdoor coils are extremely frequent, followed by electrical parts (capacitors/contactors) and refrigerant-related issues.
4) The 17 Most Common Reasons Your AC Isn’t Cooling (And What To Do)
Below is the most complete, homeowner-friendly list of “no cooling” causes—organized the way top-ranking pages do it, but expanded with the details people actually search for: symptoms, why it happens, safe checks, and when it’s time to call a pro.
Cause #1 — Thermostat set wrong (or placed in a “hot spot”)
It sounds obvious, but it’s common: the system is set to FAN ON or HEAT, or the schedule changed. Another sneaky issue: the thermostat sits in a hallway with sun exposure, near a kitchen, or near a return grill that’s pulling hot attic air.
Symptoms: Air blows, but it’s not cold; temperature won’t drop; system runs longer than usual.
Safe check: Set to COOL, FAN AUTO, and lower setpoint 3–5°F. Replace thermostat batteries if applicable.
Cause #2 — Dirty air filter (airflow choke)
A dirty filter is the #1 “simple fix.” Restricted airflow makes your system work harder and can freeze the evaporator coil. In dusty Phoenix conditions, filters can load up fast—especially with pets or construction nearby.
Symptoms: Weak airflow, rooms cool unevenly, ice on lines/coil, higher bills.
Fix: Replace filter with correct size. If your system uses a restrictive filter and airflow suffers, ask about proper MERV selection.
Cause #3 — Frozen evaporator coil (ice blocks cooling)
When airflow is too low or refrigerant charge is off, the indoor coil can drop below freezing and ice over. Once frozen, air can’t pass through the coil—so it “runs” but barely cools.
Symptoms: Ice on the copper suction line, water around indoor unit after thaw, weak airflow.
What to do now: Turn cooling OFF and set fan to ON to thaw (if safe). Replace filter. If it freezes again, call a pro.
Cause #4 — Dirty condenser coil (can’t dump heat outside)
Your outdoor unit’s job is to reject heat. If the coil is packed with dust, cottonwood, or debris, it can’t shed heat— which means the refrigerant stays too hot and the system can’t make cold air indoors.
Symptoms: AC runs constantly, indoor temp barely drops, outdoor air discharge not very hot, unit louder than normal.
Safe check: Clear vegetation and debris. Avoid pressure washing fins. Professional cleaning is best for deep buildup.
Cause #5 — Outdoor fan not spinning (or spinning intermittently)
If the outdoor fan stops, heat stays trapped in the condenser. You may still feel indoor airflow, but it won’t be cold. This often points to a capacitor, motor, or electrical control issue.
Symptoms: Outdoor unit hums, fan not turning, breaker may trip, warm air indoors.
Do not DIY: Capacitors can be dangerous. Turn off and call for service.
Cause #6 — Capacitor or contactor failure (common in extreme heat)
Capacitors help motors start and run. Contactors route power to the compressor and fan. In Phoenix summers, electrical components live a hard life—heat cycling and power surges can shorten their lifespan.
Symptoms: Clicking, buzzing, outdoor unit not starting consistently, short cycling.
Fix: Professional replacement and system check (voltage, amperage, and airflow verification).
Cause #7 — Low refrigerant (often from a leak)
Refrigerant doesn’t “get used up.” If it’s low, there’s usually a leak. Low charge reduces cooling capacity and can freeze the coil. Topping off without leak detection is usually a temporary band-aid.
Symptoms: Warm air, ice on lines, hissing sounds, longer run time, poor temperature split.
What to do: Schedule leak detection and proper repair. Running low charge can damage the compressor.
Cause #8 — Compressor problems (the heart of the system)
The compressor moves refrigerant through the system. If it’s weak, overheating, or failing electrically, you can get airflow with little or no cooling.
Symptoms: Outdoor unit runs but not cooling, breaker trips, loud buzzing, poor performance that worsens over time.
Fix: Professional diagnosis. Depending on age and repair cost, replacement may be the better value.
Cause #9 — Duct leakage (your cold air never arrives)
Duct leaks can dump cooled air into an attic or crawlspace. In Arizona heat, that’s like pouring cold water onto hot pavement. Even a strong system can feel “not cooling” if a major duct is disconnected.
Symptoms: One side of house hot, whistling sounds, dusty rooms, higher bills.
Fix: Duct sealing/repair and static pressure testing.
Cause #10 — Closed/blocked vents or returns
Closing vents can increase static pressure, reducing airflow across the coil and causing freeze-ups. Blocked returns (furniture, rugs) starve the system of air.
Symptoms: Uneven cooling, noisy airflow, coil icing, rooms farthest from unit suffer most.
Fix: Open vents, clear returns, and consider an airflow balance check.
Cause #11 — Dirty indoor coil or blower wheel
Even with a “good” filter, indoor coils and blower wheels can accumulate fine dust over years. That reduces airflow and heat exchange capacity, making the system feel weak.
Symptoms: Gradual loss of cooling over months/years, weak airflow, higher energy use.
Fix: Professional coil/blower cleaning during a tune-up.
Cause #12 — Condensate drain clog / float switch (system shuts off)
Many systems have a safety switch that stops cooling if the drain pan fills—preventing water damage. Homeowners often describe this as “it cools for a bit then stops.”
Symptoms: Cooling stops intermittently, sometimes after humidity spikes, water near unit.
Fix: Drain clearing, pan cleaning, and verifying proper slope and trap configuration.
Cause #13 — Bad thermostat sensor or wiring issue
Loose low-voltage wiring, a failing sensor, or a control board issue can cause erratic cycling. This can look like a cooling problem even when the mechanical components are fine.
Symptoms: Short cycling, random shutdowns, incorrect displayed temperature.
Fix: Professional electrical troubleshooting and control verification.
Cause #14 — Heat pump in heating mode (or reversing valve issue)
If you have a heat pump, a reversing valve or defrost control problem can keep it from switching correctly. You might get lukewarm or hot air when you want cooling.
Symptoms: Air isn’t cool even though “cool” is selected; outdoor unit behavior seems unusual.
Fix: Heat pump-specific diagnosis.
Cause #15 — Oversized unit (short cycles and never dehumidifies)
Bigger isn’t always better. An oversized AC can cool the thermostat quickly, then shut off before removing enough humidity. The house can feel clammy and uncomfortable—“not cooling” from a comfort perspective.
Symptoms: Short run times, hot spots, humidity discomfort, temperature swings.
Fix: Airflow and sizing evaluation (Manual J), potential system adjustments or zoning strategy.
Cause #16 — Undersized unit or extreme heat load
In Phoenix, extreme heat waves can push a properly functioning system to its limit—especially if attic insulation is low, windows face direct sun, or doors open frequently.
Symptoms: AC runs all day, temp holds but won’t reach setpoint, cooling improves at night.
Fix: Reduce heat gain (shade, seals, insulation) and confirm system capacity and airflow.
Cause #17 — Refrigerant metering device issue (TXV/orifice)
The metering device controls refrigerant flow into the indoor coil. If it sticks, clogs, or fails, cooling performance can drop sharply and symptoms can resemble low charge or airflow problems.
Symptoms: Inconsistent cooling, icing, odd pressures, poor split despite decent airflow.
Fix: Professional diagnosis with gauges and superheat/subcool analysis.
If your AC is blowing warm air, freezing up, or the outdoor fan isn’t running, you’ll get faster results with a professional diagnosis.
5) Typical Repair Costs & When Replacement Makes More Sense
Pricing varies by system type, parts availability, and the complexity of diagnosis. The table below shows common repair categories and what homeowners typically expect. The goal is not to “guess your bill,” but to help you make good decisions quickly.
| Issue Category | What’s Usually Involved | When Replacement Might Be Smarter |
|---|---|---|
| Airflow / filter / basic maintenance | Filter change, outdoor coil cleaning, airflow checks, drain clearing | Usually repair/maintain (best ROI) |
| Electrical parts (capacitor, contactor) | Part replacement + verifying amperage/voltage and system operation | If repeated failures occur, check underlying causes (overheating, failing motor) |
| Blower motor / fan motor | Motor replacement, airflow verification, static pressure check | Older units where multiple major parts are failing |
| Refrigerant leak / low charge | Leak detection, repair, evacuation, recharge, performance testing | If coil is leaking on an older system or refrigerant type is costly/obsolete |
| Compressor or major sealed-system work | Complex repairs, potential system downtime, higher labor + parts | If system is older and repair cost approaches replacement value |
6) Phoenix Desert Factors: Why “Not Cooling” Happens More Here
Phoenix is hard on air conditioners. Not just because it’s hot—because systems run long hours under intense sun, outdoor coils load up with dust, and attic temps can be brutal. Here are the desert-specific reasons your AC may feel weak:
Outdoor coil dust & restricted airflow
Fine dust can coat condenser fins and reduce heat rejection. Even partial blockage can reduce capacity during peak hours. Keeping clearance around the unit matters more in the desert than many homeowners realize.
Attic heat gain
When attic temperatures soar, duct losses increase and supply air warms before it reaches rooms—especially with leaky or poorly insulated ducts. That can look like “no cooling,” even if the system is producing cold air at the coil.
Sun exposure & glass load
West-facing windows, large sliding doors, and poor shading can add massive afternoon heat gain. A system that feels “fine” at 10AM may struggle after 3PM.
7) Prevent It: Maintenance That Stops “AC Not Cooling” Emergencies
Most “no cooling” calls are preventable with a few habit changes. Here’s a maintenance plan tuned for Phoenix conditions:
Monthly (Peak Season)
- Check filter and replace as needed (many homes need more frequent changes in summer).
- Clear the condenser area (keep 24–36 inches of open space around the outdoor unit).
- Listen for changes (new buzzing, clicking, or rattling is an early warning).
Seasonal (Spring + Mid-Summer)
- Professional tune-up with coil inspection, electrical testing, and airflow verification.
- Drain line check and cleaning to prevent float switch shutdowns.
- Thermostat calibration and comfort settings review.
A tune-up catches failing capacitors, dirty coils, airflow restrictions, and drain issues before you lose cooling.
8) FAQ: AC Running But Not Cooling
These answers are written to match real search queries (and formatted to be friendly for rich results).
Why is my AC running but not cooling the house?
The most common causes are restricted airflow (dirty filter, blocked returns, frozen coil), dirty outdoor condenser coils (can’t dump heat), electrical part failures (capacitor/contactor), or low refrigerant due to a leak. Start with thermostat settings and filter, then check for ice and outdoor fan operation.
Why is my AC blowing warm air?
Warm air usually points to heat not being removed effectively: the outdoor unit may not be running properly, coils may be dirty, refrigerant charge may be low, or the compressor may be struggling. If the outdoor fan isn’t spinning or you see ice, turn the system off and call a pro.
Can a dirty filter really stop cooling?
Yes. A clogged filter reduces airflow, which reduces heat transfer at the indoor coil and can cause freezing. Once the coil freezes, airflow drops further and cooling can stop almost entirely.
What temperature should my AC blow out?
The more reliable check is the temperature split between return air and supply air, often around 15–20°F under typical conditions. Exact vent temperature varies with humidity, airflow, and equipment type.
Should I turn my AC off if it isn’t cooling?
If you see ice, smell burning, hear loud buzzing/clicking, or the outdoor fan isn’t running, yes—turn it off. Continuing to run can damage components (especially the compressor) and increase repair cost.
Why does my AC cool at night but not during the day?
That pattern often suggests a heat load or heat rejection problem: intense daytime sun/attic heat, dirty outdoor coils, restricted airflow, or an undersized system. A technician can confirm whether the system is healthy and the home needs heat-gain improvements.
When should I call an HVAC professional?
Call if your system is blowing warm air after basic checks, repeatedly freezes, trips breakers, short-cycles, or if the outdoor fan/compressor won’t run. Refrigerant work and high-voltage diagnostics should be handled by a licensed technician.
Get Your Cooling Back Fast
If your AC isn’t cooling, the fastest path is a clean diagnosis: airflow, heat rejection, electrical health, and refrigerant performance. Cold Stinger Heating & Air Conditioning serves Phoenix homeowners with repair-first honesty and clear options.
Disclaimer: This guide is educational and may not cover every possible system configuration. For safety, do not open electrical panels or handle refrigerant.