It’s 95°F in the shade, the house is climbing past 80°F, and your AC has stopped cooperating. Before you call the first number on Google, know that a sizable share of “broken” air conditioners are fixed by a thermostat setting, a tripped breaker, or a filter that hasn’t been changed since last summer. This guide walks you through the same diagnostic sequence a NATE-certified technician uses—in plain English, with real 2026 repair costs—so you can fix what’s fixable and know the moment to stop and call a pro.
Safety first: Never open the electrical panel on the outdoor condenser or handle the disconnect box unless the power is off at the breaker. Never bypass a float switch or pressure safety. Refrigerant work is legally restricted to EPA Section 608–certified technicians—venting refrigerant is a federal violation and can cause frostbite and blindness.
Before You Start: The 60-Second Power and Settings Check
Roughly 30% of no-cool service calls are resolved in under five minutes. Run this list before you do anything else:
- Thermostat mode — Set to COOL, not AUTO or HEAT, and confirm the fan is set to AUTO (not ON) so you can tell whether the system is actually cycling.
- Setpoint — Lower it 5°F below current room temperature. If the room is 78°F and the thermostat is set to 78°F, nothing will happen.
- Thermostat batteries — Replace AA/AAA cells if the display is blank or flickering, then check the C-wire connection at the base plate.
- Air filter — A filter loaded with dust starves the system of airflow and can freeze the indoor coil within hours.
- Breaker panel — Look for a tripped double-pole breaker (usually a 30-amp or 40-amp for a central AC) in the main panel, plus the outdoor disconnect box near the condenser.
- Float switch — Many systems have a condensate overflow switch that shuts the whole unit down when the drain pan fills.
- Outdoor unit — The fan should be spinning and you should hear the compressor humming. A fan spinning with no compressor hum points to a different diagnosis than total silence.
If those seven items check out and the house is still warming up, move on. For a broader set of symptom-based walkthroughs across furnaces, heat pumps, and mini-splits, our library of HVAC troubleshooting guides is organized by equipment type.
How Your AC Actually Works (and Why That Matters for Diagnosis)
A central air conditioner doesn’t “make cold”—it moves heat. The system has four core parts, and knowing which one is misbehaving narrows your diagnosis fast:
- Evaporator coil (indoors) — Absorbs heat from your home’s air using refrigerant at low pressure.
- Compressor (outdoors) — Pumps refrigerant to high pressure, raising its temperature so it can dump heat outside.
- Condenser coil and fan (outdoors) — Release that heat into the outdoor air.
- Blower and ductwork (indoors) — Distribute the cooled air, typically moving 400 CFM (cubic feet per minute) of air per ton of capacity.
Capacity is measured in BTUs per hour, where 1 ton = 12,000 BTU/h. A rule of thumb for sizing is about 20 BTU per square foot of conditioned space: a 1,500 sq ft home needs roughly 30,000 BTU, or a 2.5-ton unit. Real sizing requires a Manual J load calculation—short-cycling and humidity problems are often the fault of an oversized unit, not a broken one.
Modern equipment is rated in SEER2 (efficiency) and HSPF2 (heat pumps). Federal minimums are 14.3 SEER2 in the North and 15.2 SEER2 in the Southeast and Southwest, and ENERGY STAR–certified systems must exceed those baselines by a meaningful margin. Since January 1, 2025, new residential systems no longer use R-410A; they run on A2L refrigerants such as R-454B or R-32, which matters when you’re pricing a repair on a 10-year-old unit.
Symptom-to-Cause Cheat Sheet
| Symptom | Most likely cause | Check first |
|---|---|---|
| Completely dead, no sound | No power to unit | Thermostat, breaker, float switch, disconnect |
| Outdoor fan runs, no cold air | Compressor not starting | Run capacitor, contactor, compressor |
| Outdoor unit totally silent | Contactor, capacitor, or control board | Thermostat call, 24V signal, breaker |
| Cold air for 10 minutes, then warm | Frozen coil or low refrigerant | Filter, indoor coil ice, refrigerant pressures |
| Weak airflow from vents | Airflow restriction | Filter, dirty blower wheel, crushed duct, closed damper |
| Water pooling near air handler | Clogged condensate drain | Drain line, pan, pump |
| Turns on and off every few minutes | Short cycling | Oversized unit, thermostat placement, low charge |
| Breaker trips repeatedly | Compressor or wiring fault | Stop. Call a technician. |
The 10 Most Common Reasons Your AC Is Not Working
1. A Tripped Breaker or Popped Disconnect Fuse
A compressor that draws locked-rotor amps—often 5 to 7 times normal running current—can trip a breaker on a hot afternoon. Reset it once. If it trips again immediately, stop; repeated trips indicate a shorted compressor or a failing capacitor, and forcing it can turn a $250 repair into a $2,000 one. Also check the small cartridge fuses inside the outdoor disconnect box, which are common failure points homeowners never think to look at.
2. Clogged Air Filter
This is the single cheapest fix in HVAC and the cause of a surprising number of no-cool calls. A 1-inch filter should be replaced every 30 to 90 days; a 4- to 5-inch media filter can go 6 to 12 months. Use MERV 8 to MERV 11 for most homes—MERV 13 and above can choke a system with a marginal blower unless your ductwork was designed for the extra static pressure.
3. Frozen Evaporator Coil
If you see ice on the indoor refrigerant lines or the coil cabinet, airflow or charge is the problem. Turn the system to OFF at the thermostat, set the fan to ON, and let it thaw for 2 to 4 hours with a towel underneath. Running a frozen system can slug liquid refrigerant into the compressor and destroy it. Once thawed, replace the filter and try again. Ice returns? You have a refrigerant leak or a ductwork restriction.
4. Failed Run Capacitor
A weak or bulging capacitor is one of the top three AC failures in the U.S., especially in units over 7 years old. Symptoms include a humming outdoor unit, a compressor that tries to start and gives up, or a condenser fan spinning slowly or not at all. A technician replacement typically runs $150 to $300, including the diagnostic visit. A bulging top or oily residue on the capacitor is a dead giveaway, but testing it requires a multimeter with capacitance function.
5. Dirty Outdoor Condenser Coil
The condenser dumps heat into the air, and a coil matted with cottonwood fluff, grass clippings, and pet hair can’t do it. Rinse gently with a garden hose (never a pressure washer—it bends the fins). Keep 2 feet of clearance on all sides and trim vegetation back from the unit. Dirty coils can add 10% to 25% to your cooling bill and eventually trip the high-pressure switch.
6. Clogged Condensate Drain or Tripped Float Switch
An AC pulls gallons of water out of your air every day. The drain line grows algae and sludge, the pan fills, the float switch trips, and the whole system shuts off as designed. Clearing a drain line with a wet/dry shop vac at the outdoor termination or a condensate drain cleaning attachment typically costs $100 to $250 if you hire it out, and about $0 in materials if you do it yourself. Don’t skip the maintenance flush—water damage to ceilings and air handlers is expensive.
7. Low Refrigerant From a Leak
If your AC blows cool-ish air that never really satisfies the thermostat, has ice on the indoor coil, or hisses at the service valves, you likely have a leak. Here’s the key point: a properly sealed system never “uses up” refrigerant. Topping it off without finding the leak is a temporary patch. Refrigerant charges run roughly $75 to $150 per pound for R-410A in 2026, and a typical residential system holds 6 to 12 pounds—so a full charge with leak detection usually lands $400 to $1,500.
8. Failed Blower Motor or Control Board
If the outdoor unit runs but you feel almost no air at the vents, the indoor blower is the suspect. Blower motors run $300 to $700 installed; ECM variable-speed motors sit at the top of that range. Control boards run $300 to $800 depending on the brand and whether the board is proprietary.
9. Contactor Failure (and Ants)
The contactor is the 24-volt relay that switches 240-volt power to the compressor and fan. Its contacts pit and weld shut over time, and in much of the U.S. a colony of ants nesting between the contacts is a genuine cause of failure. Replacement costs $150 to $350.
10. Duct Leakage and Poor Airflow
The average U.S. home loses 20% to 30% of the air its blower moves to leaks in ductwork, particularly in attics and crawlspaces. Symptoms: rooms at the end of the run never get comfortable, high humidity, and an evaporator coil that ices because of low airflow. Sealing ducts typically runs $400 to $1,200 and often pays for itself in two or three summers.
If Your AC Is Running but Not Cooling
This specific complaint deserves its own walkthrough, because “running” and “cooling” are two different systems. Restricting the diagnosis to the top four causes in order of likelihood—airflow, coil ice, compressor start, and refrigerant charge—resolves most cases. If you’ve replaced the filter, thawed the coil, and confirmed the outdoor unit’s compressor is humming, read our deeper dive on AC not cooling for the pressure and temperature checks a technician uses to isolate the fault.
What AC Repairs Cost in 2026
| Repair | Typical Installed Cost (USD) |
|---|---|
| Service call / diagnostic fee | $75 – $150 (often credited toward repairs) |
| Air filter (DIY, 1-inch) | $10 – $30 |
| Run capacitor | $150 – $300 |
| Contactor | $150 – $350 |
| Condenser fan motor | $250 – $600 |
| Blower motor | $300 – $700 |
| Thermostat replacement | $150 – $500 |
| Condensate drain clearing | $100 – $250 |
| Refrigerant recharge (R-410A) | $400 – $1,500 |
| Evaporator coil replacement | $600 – $2,000 |
| Compressor replacement | $1,200 – $2,500 |
| Duct sealing | $400 – $1,200 |
| Full system replacement (3-ton, installed) | $5,000 – $12,000 |
The average homeowner repair bill for a no-cool call is around $250 to $600. Anything quoted above $1,000 deserves a second opinion, and a compressor quote should always be compared against the price of a new system—there’s no scenario where a $2,000 compressor on a 16-year-old R-410A unit makes financial sense.
Repair or Replace? Use the $5,000 Rule
Multiply the unit’s age by the repair cost. If the result exceeds $5,000, replacement generally wins. Example: a 12-year-old condenser needing a $600 repair gives you $7,200—past the threshold. Also lean toward replacement if the system is 12 to 15 years old, uses phased-out R-410A, has already had a leak repair, or is rated below today’s SEER2 minimums. High-efficiency replacements and qualifying heat pumps may be eligible for federal 25C tax credits (capped at $600 for central AC, up to $2,000 for heat pumps through 2032), plus utility rebates worth several hundred dollars.
When to Stop Troubleshooting and Call a Pro
Put the multimeter down and call a licensed technician if any of these apply:
- The breaker trips more than once, or you smell burning plastic.
- You see refrigerant oil at the service valves, or you hear hissing.
- The indoor coil is frozen again after a proper thaw and filter change.
- You see sparks, scorch marks, or a melted wire nut anywhere.
- The outdoor unit hums and trips a breaker within seconds (compressor or capacitor).
- You’re not comfortable working inside a 240-volt electrical panel—no article, including this one, replaces training.
For a vetted, licensed technician in your area, start with our AC repair resource page to see what a proper diagnostic visit should include, so you can spot upsells and know what you’re paying for.
15 Minutes of Maintenance That Prevents Most Breakdowns
- Monthly (in cooling season): Check or replace the filter; confirm the drain line is flowing.
- Spring: Rinse the condenser coil, clear debris within 2 feet, tighten the outdoor disconnect cover.
- Spring: Test the float switch by lifting it—the system should shut off immediately.
- Summer: Hose down the outdoor coil once a month; trim weeds and grass away from the pad.
- Annually: Book a professional tune-up ($150 to $350) that includes a refrigerant pressure check, capacitor and contactor testing, amp draw readings, and a coil inspection. Manufacturer warranties frequently require documented annual maintenance to stay valid.
Frequently Asked Questions
Why is my AC running but not cooling the house? The most common culprits are a clogged air filter, a frozen evaporator coil, a failed run capacitor keeping the compressor from starting, or low refrigerant from a leak. Check the filter and confirm the outdoor compressor is actually running—not just the fan—then inspect the indoor coil for ice. If the coil is frozen, shut the system off and thaw it completely before troubleshooting further.
How much does it cost to fix an AC that isn’t working? Most residential repairs land between $150 and $600, with a typical bill of $250 to $400. A run capacitor costs $150 to $300, a blower motor $300 to $700, a condenser fan motor $250 to $600, and a compressor $1,200 to $2,500. The diagnostic visit itself runs $75 to $150 and is often credited toward the repair.
Can I fix my AC myself, or do I need a technician? You can safely handle roughly a third of no-cool calls: thermostat settings and batteries, a tripped breaker, a clogged filter, a clogged condensate drain, and a dirty condenser coil. Anything involving refrigerant, capacitors, contactors, compressors, or 240-volt wiring requires an EPA Section 608–certified technician and, in most states, a licensed HVAC contractor. Attempting that work voids warranties and is a genuine electrocution risk.
The Bottom Line
An AC that stops working is usually a power, airflow, or starting-component problem—and the first three things to check cost nothing: the thermostat setting, the filter, and the breaker. Work through the checklist in order, fix what’s safely fixable, and treat refrigerant, capacitors, compressors, and 240-volt circuits as firm stop signs. Budget $150 to $600 for a routine repair, expect to spend $5,000 to $12,000 when a system past 12 years old needs replacing, and keep up with annual maintenance to protect both your warranty and your comfort. Handle the five-minute checks yourself, call a NATE-certified tech for everything else, and you’ll never overpay for a problem you could have diagnosed in your own hallway.