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This guide sorts AC problems into eight symptom types: no power, warm air, weak airflow, ice buildup, water leaks, strange noises, bad smells, or breaker trips. Three culprits worth ruling out early are a clogged air filter, a failed run capacitor, and a refrigerant leak. Turn the AC off at the thermostat before anything else, check the thermostat and filter first, then call a technician if the problem is not obviously a filter or battery issue. On our 2026 cost guide, repairs run from $150–$400 for a capacitor to $1,200–$3,500 for a compressor.
When your AC stops cooling, the question is rarely "is it broken?" — you already know it is. The real questions are: is this something you can fix in ten minutes, or does it need a technician? Is it safe to keep the system running while you figure it out? And how much is this actually going to cost?
This guide answers all three. It is the single reference you need for every common AC failure mode, written by a homeowner-facing consumer advocate — not an upsell pitch. You will see every symptom, every probable cause, what is safe to DIY, what is not, and realistic 2026 repair pricing so no technician can talk you into a bad deal.
AC systems run on 240 volts and capacitors store a dangerous charge even after power is cut. The safest action a homeowner can take is to set the thermostat to OFF and call a qualified HVAC technician. Do not open the service panel, do not operate the outdoor disconnect, do not open the outdoor cabinet or the air handler access panels, and do not try to flip the breaker to kill the power — the technician will handle the power-down sequence safely on arrival. No diagnosis is worth an injury.
Universal First Step: Turn the System Off
Before you do anything else, set the thermostat to OFF — not just raise the temperature, but completely off. That one action (no electrical work required) stops the compressor from running under abnormal conditions while you wait for the technician. Leave any breakers and the outdoor disconnect alone — the technician handles those on arrival.
Here is why this step is non-negotiable: when an AC keeps running but cannot cool, the compressor is usually operating under abnormal conditions — low refrigerant pressure, poor lubrication, or excess heat. Every additional minute of runtime in that state increases the risk of compressor seizure. Replacing a seized compressor runs $1,200 to $3,500 on our cost guide. A failed capacitor is $150 to $400, and a new air filter costs far less than either. Do not let a cheap problem become an expensive one because you kept the system running while troubleshooting.
The 8 AC Failure Modes, by Symptom
Start with what you can see, hear, or smell. Your symptom tells you which branch to follow.
1. No Power — AC Will Not Turn On at All
The thermostat is blank or the indoor unit shows no signs of life. The one safe homeowner check is the thermostat batteries: if your thermostat uses them, replace them as its manual describes and retry. Everything else in the power chain — breakers at the service panel, the emergency disconnect switch near the indoor air handler, the outdoor disconnect, the low-voltage transformer, the control board — is a technician's job. Do not open the service panel, do not flip breakers trying to find the right one, and do not operate the emergency disconnect. Call a technician. For a focused walkthrough on the blank-thermostat scenario specifically, see why a blank thermostat means your AC won't start.
2. AC Blowing Warm Air (Fan Runs, No Cooling)
The fan is blowing but the air from the vents is room-temperature or warm. Possible causes include a clogged air filter, a failed run capacitor, a frozen evaporator coil, low refrigerant, or a dirty outdoor condenser coil. For a complete symptom-by-symptom walkthrough, see our guide on why your AC may blow warm air. If the system runs continuously but the indoor temperature never drops below 80°F, the diagnosis narrows further — see why an AC running but not cooling below 80° usually points to one of three failures.
3. Weak Airflow From Vents
Air is cold but not moving. Causes: dirty filter, closed or crushed duct, failing blower motor, or a slipping blower belt (older systems). A quick DIY check: replace the filter. If airflow does not improve, ask a technician to check the blower motor, the registers, and the ductwork.
4. AC Freezing Up (Ice on Coils or Refrigerant Lines)
You see visible ice on the copper refrigerant lines or the indoor coil. Two root causes: restricted airflow (dirty filter, blocked return, failed blower) or low refrigerant charge (leak). Set the thermostat to OFF, switch the fan to ON to circulate warm air over the coil, and let the ice melt completely before the technician arrives (this can take several hours) — a frozen system cannot be properly diagnosed. The full step-by-step thaw procedure, plus what to look for when the technician arrives, is in our dedicated guide on AC freezing up in summer.
5. Water Leaking Indoors
Water appears near the indoor air handler, in a ceiling, or down a wall. A clogged condensate drain line is one of the causes ACCA lists in its guide to HVAC condensate, along with a corroded or cracked drain pan, a failed condensate pump, and a dirty air filter that lets the coil freeze and then overflow the pan as it melts. In humid climates like Miami or Tallahassee, the AC pulls more moisture out of the air, so more water runs through that drain line. Set the thermostat to OFF to limit drywall damage, then read our full walkthrough on stopping HVAC water leaks before they cause damage. If the water is dripping from a ceiling vent itself rather than the air handler, see why water drips from a ceiling vent and how to stop it — the diagnosis path is different.
6. Strange Noises — Buzzing, Clicking, Grinding, or Squealing
Noise is your system telling you something is mechanically wrong. Buzzing with no compressor start: can point to a failed capacitor or contactor. Clicking rapidly: can point to short-cycling from a refrigerant or pressure switch issue. Grinding: failing bearings or debris inside the blower wheel. Squealing: worn blower belt (older systems) or dry motor bearings. Screaming or hissing: refrigerant leak under pressure. A detailed breakdown of one noise pattern — when the compressor buzzes but the fan will not spin — is in a dedicated deep-dive. If the contactor itself is the source — a steady audible click without the compressor engaging — see why an AC contactor clicks but nothing happens.
7. Bad Smells From Vents
Musty or mildew smell: bacteria or mold in the condensate drain pan or on a damp coil. Rotten egg or sulfur: rare but serious — either a dead animal in the ductwork or, in homes with gas service, a possible gas leak near the return air intake. If you smell gas, leave the building right away: do not flip switches, use phones, or light flames inside, and do not try to find the leak or touch any gas valve. Once you are outside at a safe distance, call 911 or your gas utility’s emergency line. Burning plastic: an electrical component (wiring, motor, control board) may be overheating — set the thermostat to OFF right away, stay away from the equipment, and call a technician. If you see smoke or flames, get everyone out and call 911.
8. Breaker Tripping Repeatedly
The AC breaker trips every time the system starts, or trips randomly during operation. Causes include a failing compressor pulling excessive current, a shorted capacitor, a deteriorated breaker at the end of its service life, or a dirty condenser coil causing the compressor to overheat. Do not reset the breaker yourself. Set the thermostat to OFF, leave the breaker as it is, and call a technician: a breaker that trips is protecting against a fault the technician needs to find. For the fault-by-fault breakdown of why an AC circuit breaker keeps tripping — and what your tech will measure to diagnose it — see the dedicated guide.
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What You Can Safely Check Yourself
These are the only checks we suggest homeowners make. None of them involves the breaker panel, opening any cabinet, or touching the outdoor unit:
- Replace the air filter. Per ENERGY STAR, a dirty filter slows air flow and makes the system work harder. ENERGY STAR advises checking the filter every month and changing it at least every 3 months; hold the old one up to a light, and if it looks dirty, replace it.
- Check the thermostat. Confirm mode is COOL (not HEAT, AUTO, or FAN ONLY), that the setpoint is at least 3 to 5 degrees below the room temperature, and that the batteries are fresh. Some thermostats build in a short compressor-protection delay after a power interruption, so give it several minutes before assuming it is broken.
- Clear debris away from the outdoor unit. The condenser needs open space around it to shed heat. Move leaves, grass clippings, and stored items away from it without touching the unit itself; your installer or the unit’s manual gives the clearance it needs.
- Leave coil cleaning to a technician. The outdoor cabinet contains 240V wiring and a capacitor that can hold a charge after power is cut, and a proper coil clean includes a fin inspection. Ask the technician to check and clean the outdoor coil as part of the spring tune-up — do not attempt it yourself.
- Flush the condensate drain line with white vinegar (prevention). Have your technician show you the drain line’s access point during a tune-up; Trane describes pouring a cup of distilled vinegar down the line to help prevent clogs. If the drain is already clogged or water is overflowing, set the thermostat to OFF and call a technician.
That is the complete safe DIY list. Anything else — breakers, capacitors, contactors, refrigerant, electrical connections, blower motor, coil work, clearing an active drain clog — requires a technician. When in doubt, call.
When to Stop Troubleshooting and Call a Pro
Stop DIY and call a professional when any of the following are true:
- You smell burning plastic or rubber — electrical failure in progress
- You smell gas — leave the building first, then call 911 or your gas utility from outside
- You see ice on the refrigerant lines — likely low charge from a leak
- The breaker has tripped on the AC circuit (leave it off; do not reset it)
- You hear a grinding or screeching noise — mechanical damage is occurring
- Water is actively leaking onto drywall, flooring, or electronics
- The system is older than 10 years and not cooling effectively — age-driven failures compound
- You have already replaced the filter and checked the thermostat and the problem persists
Per our cost guide, the diagnostic fee for a service call typically runs $65 to $150. Some providers credit it toward the repair if you book the work with them, so ask when you call. That is cheap insurance against either misdiagnosing the problem or making it worse.
What a Technician Does on Arrival
Knowing what the technician will actually check helps you recognize a thorough diagnosis from a hurried upsell:
- Verify power and thermostat signals. Multimeter readings at the disconnect, contactor, and thermostat-to-air-handler wiring.
- Inspect the air filter and return airflow. If the filter is the problem, a good tech will tell you instead of upselling.
- Test the run capacitor. A capacitance meter reads the microfarad value — if the reading falls outside the tolerance printed on the capacitor’s label, it gets replaced ($150 to $400 including labor, per our cost guide).
- Measure refrigerant pressures. Gauges on the suction and liquid lines read high-side and low-side pressures. Abnormal readings point to a leak, a blockage, or a sealed-system issue.
- Inspect the evaporator coil and drain pan. Ice, dirt, algae, or corrosion.
- Check the outdoor condenser. Coil cleanliness, fan operation, contactor condition, compressor amperage draw.
- Measure supply and return temperatures. The difference between the two is compared against what the system should deliver under the day’s indoor and outdoor conditions.
A tech who skips multiple steps and jumps straight to a big-ticket recommendation ("you need a new compressor") without showing you the readings is waving a red flag. Ask to see the measurements.
What AC Repairs Cost in 2026
Before a technician quotes you, here is the honest 2026 reality check. These figures come straight from our complete HVAC Cost Guide — the single source of truth for pricing on this site, so there is no drift between articles:
- Diagnostic / service call fee: $65–$150 (some providers credit it toward the repair; ask when you call)
- Minor AC repair — capacitor, contactor, thermostat, drain line: $75–$500
- Refrigerant recharge (R-410A): $150–$600 per recharge
- Blower motor replacement: $300–$1,000
- Major AC repair — evaporator coil or compressor replacement: $600–$3,500
- Emergency / after-hours surcharge: $100–$300 added to the base price
- Full AC replacement (3-ton system): $3,200–$7,000 installed
Ranges from our 2026 cost guide (Cool Call Pro editorial estimates). Actual costs vary by region, provider, and system age.
If you are actively evaluating a quote right now, go straight to our 2026 HVAC cost guide page. It breaks down every repair line and explains what drives the price up or down. For replacements that exceed repair-budget, see HVAC financing options and the repair-versus-replace decision framework.
Climate Matters: How Your Region Changes the Diagnosis
The same AC symptom can have different likely causes depending on where you live. A technician in Phoenix sees different failures than one in Fayetteville or San Antonio, and that changes how you should prioritize your diagnostics.
Hot-humid climates (Miami, Tallahassee, Houston, New Orleans, the Gulf Coast): The AC pulls a lot of moisture out of the air, so the condensate drain line carries more water and has more chances to clog. DOE’s federal purchasing guidance assumes an average service life of 18 years for a residential central AC in hot-humid Southeast states, the shortest of its three regions (a cost-modelling assumption, not a promise for any one unit). If you live in a coastal or Gulf state, read our guide to a broken AC in July before the heat arrives.
Hot-dry climates (Phoenix, Las Vegas, Albuquerque, the Southwest): Systems run long hours at high outdoor temperatures, and dust can build up on the outdoor coil — both good reasons to have a technician test the capacitor and clean the coil before summer. DOE’s guidance assumes a 25-year average central-AC service life in hot-dry Southwest states. The spring AC tune-up checklist covers what that visit should include.
Mixed-humid climates (Charlotte, Nashville, Atlanta, the Southeast): Homes here need both cooling and heating, so maintenance should cover both sides. ENERGY STAR recommends checking the cooling system in the spring and the heating system in the fall.
Browse local service providers in Texas, Florida, or explore all service areas to find independent technicians serving your area.
How to Prevent AC Failures
ENERGY STAR recommends annual pre-season check-ups by a contractor: the cooling system in the spring and the heating system in the fall. The short list:
- Check the filter monthly and change it at least every 3 months, per ENERGY STAR, or sooner if it looks dirty.
- Schedule an annual spring tune-up. ENERGY STAR’s checklist for that visit includes tightening electrical connections, checking the refrigerant level, and inspecting the condensate drain. A seasonal tune-up runs $80–$200 on our cost guide.
- Keep the outdoor unit clear. Move grass clippings, leaves, and stored items away from it without touching the unit.
- Flush the drain line with vinegar. A cup of distilled white vinegar down the line helps prevent clogs (Trane). Ask your technician to show you the access point and how often your system needs it.
- Ask about surge protection. A whole-house surge protector is installed at the main electrical panel by a professional and is meant to protect electronics, including HVAC control boards, from voltage surges.
- Monitor your electric bill. A jump in cooling-season usage with no change in rates or weather can point to a problem such as a dirty coil or a refrigerant leak, which is worth having checked early.
For the full year-round maintenance playbook — seasonal tasks, filter schedules, inspection checklists — see our 12-month HVAC maintenance checklist.
Related Guides for Specific Symptoms
This guide is the overview. Each symptom below has its own dedicated article with expanded diagnostic steps and symptom-specific detail:
AC Blowing Warm Air but Running
The fan runs but the air is warm: the possible causes, from filter to refrigerant, and what the technician checks.
AC Running but Not Cooling Below 80°F
When the system runs nonstop but the indoor temperature never drops.
AC Freezing Up in Summer
The safe thaw steps and what puts ice on the coils in mid-July.
AC Circuit Breaker Keeps Tripping
The faults behind repeat trips, and why the breaker is a technician call.
Thermostat Blank, AC Won't Start
The focused walkthrough for the no-power scenario.
AC Contactor Clicking but Nothing Happens
When the contactor pulls in but the compressor never engages.
AC Compressor Buzzing but Fan Not Spinning
The buzz-with-no-fan symptom and why it often points to the capacitor.
AC Capacitor Bulging or Leaking?
A 60-second, hands-off safety check before you call.
HVAC Leaking Water Inside the House
What to do when water shows up indoors, and when it is a technician call.
Water Dripping From a Ceiling Vent
A different diagnosis path than a leak at the air handler.
Why Refrigerant Leaks Cause AC Ice
The science behind the frost line and why a leak only gets more expensive.
AC Broke in July? Emergency Options and Costs
What to do when the AC fails in peak heat.
Spring AC Tune-Up Checklist
The preventive work to book before summer.
Trusted Industry Sources
The guidance in this article is consistent with published recommendations from:
- U.S. Department of Energy — Purchasing Energy-Efficient Central Air Conditioners
- EPA Section 608 Refrigerant Handling Regulations
- ENERGY STAR — Clean Heating & Cooling Products
- ENERGY STAR — Maintenance Checklist
- EPA — Refrigerant Sales Restriction
- AHRI — Residential Central Air Conditioner Efficiency Standards
- ACCA — The Ultimate Guide to HVAC Condensate
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Call Now — (844) 582-1795Disclosure: We are a referral service and may receive compensation for qualified calls. Calls may be routed to an independent provider network and may be recorded. Pricing and availability vary by provider and location.
Frequently Asked Questions
Turn the system OFF at the thermostat before doing anything else. Running an AC that cannot cool forces the compressor to work without proper refrigerant pressure or airflow, which can cause it to overheat and seize. Replacing a seized compressor runs $1,200 to $3,500 on our cost guide. After turning it off, check the thermostat mode and batteries and the air filter; if those are fine, call a technician.
Signs of low refrigerant include ice forming on the copper refrigerant lines near the indoor air handler, a hissing or bubbling sound, the system running constantly without reaching the set temperature, and higher-than-normal electric bills. Refrigerant is not consumed — if the level is low, there is a leak. You cannot add refrigerant yourself: under EPA’s refrigerant sales restriction, only EPA-certified technicians may buy refrigerant, with limited exceptions, and refrigerant work falls under EPA Section 608.
Possible causes include a dirty air filter (ENERGY STAR notes a dirty filter slows air flow and makes the system work harder), a failed run capacitor preventing the compressor from engaging, a frozen evaporator coil, low refrigerant from a leak, or a dirty outdoor condenser coil that cannot shed heat. Start by checking the filter and thermostat settings. Outside, look (without touching) to see whether the condenser fan is spinning and whether debris is piled against the unit, and tell the technician what you see.
On our 2026 cost guide, a diagnostic service call runs $65 to $150. Capacitor replacement is $150 to $400, contactor replacement $100 to $300, and an R-410A refrigerant recharge $150 to $600. Blower motor replacement is $300 to $1,000 and evaporator coil replacement $600 to $2,000. Compressor replacement runs $1,200 to $3,500. Night, weekend, and holiday calls add $100 to $300 on top of the repair.
You can safely replace the air filter, check and correct thermostat settings, move loose debris away from (not touching) the outdoor unit, and flush the condensate drain line with white vinegar as prevention. Everything else — electrical components, refrigerant handling, sealed-system repairs, blower motor replacement, coil cleaning, and anything inside the outdoor cabinet or the service panel — is a technician task. Capacitors store a dangerous electrical charge even with the power cut, and refrigerant work is federally regulated under EPA Section 608.
There is no single figure for a compressor alone. For the whole system, DOE’s federal purchasing guidance assumes an average service life of 18 to 25 years for a residential central air conditioner (18 in hot-humid Southeast states, 25 in hot-dry Southwest states, 24 in northern states); these are cost-modelling assumptions, not a promise of how long any one unit will last. Compressor warranty terms vary by brand and model, so check your unit’s warranty. ENERGY STAR suggests talking to a contractor about replacement once an AC is more than 10 years old.
Yes — stop running the system and call a technician. Grinding, squealing, banging, or screeching noises indicate mechanical failure that will only get worse if the system keeps running. Continuing to run through these symptoms can turn a smaller repair, such as a $150 to $400 capacitor, into a $1,200 to $3,500 compressor replacement (see our cost guide).
A frozen AC coil means ice has formed on the indoor evaporator coil, blocking airflow and heat exchange. The two root causes are restricted airflow (dirty filter, blocked return, failed blower motor) or low refrigerant charge (leak). Turn the thermostat to OFF, switch the fan to ON to circulate warm air over the coil, and wait until the ice has fully melted, which can take several hours. Do not chip ice off the coil.
A musty smell usually means bacteria or mold on a damp evaporator coil or in the condensate drain pan. A rotten-egg smell can indicate a dead animal in the ductwork or, in a home with gas service, a gas leak: leave the building right away, do not flip switches or use phones inside, and call 911 or your gas utility from outside. A burning-plastic smell can mean an electrical component is overheating — set the thermostat to OFF and call a technician.
A breaker that trips repeatedly when the AC starts indicates the system is drawing too much current. Possible causes include a failing compressor pulling excessive amperage, a shorted capacitor or wiring, a dirty condenser coil causing the compressor to overheat, or a worn breaker. Do not reset the breaker yourself: set the thermostat to OFF and call a technician. A breaker that trips is protecting against a fault the technician needs to find.
Many jurisdictions require a mechanical permit for AC replacement because the work involves refrigerant, electrical connections, and equipment that affects building safety. Requirements and fees vary, so check with your city or county building department, and ask your contractor whether they will pull the permit and whether it is included in the quote.
ENERGY STAR advises checking your air filter every month, especially in summer and winter, changing it if it looks dirty after a month, and changing it at least every 3 months. For thick 4-inch or 5-inch media filters, follow the filter maker’s replacement interval.
If your AC runs nonstop but the temperature never reaches the setpoint, the system cannot keep up with the heat load. Causes include low refrigerant, a dirty coil, a failing compressor, or an undersized system. In extreme heat, continuous running is normal if the indoor temperature still tracks close to setpoint.
A common rule of thumb (not an industry standard) is the “$5,000 rule”: multiply the system’s age in years by the repair quote, and if the result is more than $5,000, get a replacement quote before approving the repair. For example, a 12-year-old AC with a $1,200 compressor quote gives 12 × 1,200 = 14,400. Use it alongside ENERGY STAR’s guidance to consider replacement once an AC is more than 10 years old, and see our repair-or-replace framework.
SEER (Seasonal Energy Efficiency Ratio) measures cooling output per unit of electricity over a season. Since January 1, 2023, equipment is rated as SEER2 under a newer DOE test that gives a lower number for the same unit. AHRI lists the federal minimums as 13.4 SEER2 (14 SEER) for split-system central ACs in the North and 14.3 SEER2 (15 SEER) for split-system ACs under 45,000 Btu/h in the Southeast and Southwest. Whether a higher-SEER2 unit pays off depends on your electricity price and cooling hours: divide the extra installed cost by the annual dollar savings. Our SEER payback guide walks through the math.
ACCA’s condensate guide lists four typical causes of condensate overflow: a corroded or cracked drain pan, a clogged or dislodged drain line, a failed condensate pump, and a dirty air filter that lets the evaporator coil freeze and then overflow the pan when it melts. Set the thermostat to OFF to limit water damage and call a technician.
Running an AC during a thunderstorm is generally safe, but a nearby lightning strike can damage the capacitor, contactor, or control board. If you are in a storm path, consider setting the AC to OFF at the thermostat until the storm passes. A whole-house surge protector at the main panel is one form of protection to ask a technician or electrician about.
Per our cost guide, night, weekend, and holiday calls add $100 to $300 on top of the repair cost, and a diagnostic fee typically runs $65 to $150. If the problem can wait until regular hours without damage, waiting avoids that surcharge. For a system actively leaking, making burning smells, or in extreme heat with vulnerable occupants, call now.
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