It is 95 degrees outside. You walk inside expecting relief. Instead, your AC is blowing warm air or barely doing anything at all.
Before you panic and call the first contractor you find, read this first. Most AC cooling problems have a specific cause — and several of them you can fix yourself in under 10 minutes without spending a dollar.
This guide covers the 10 most common reasons your AC is not cooling, what each issue costs to fix, and exactly when you need a professional versus when you can handle it yourself.
Quick Answer
The most common reasons an AC stops cooling are: a dirty or clogged air filter, low refrigerant, a frozen evaporator coil, a tripped circuit breaker, or a failed capacitor. Start with the filter — it causes roughly 25% of all AC service calls and costs nothing to fix.
If your system is over 15 years old and has stopped cooling effectively, replacement may be more cost-effective than repair. Jump to our HVAC repair vs replacement cost guide to run the numbers first.
Top 10 Reasons Your AC Is Not Cooling
1. Dirty or Clogged Air Filter
How common: Extremely common — causes 1 in 4 AC service calls.
What happens: A clogged filter blocks airflow into the system. Without enough air moving over the evaporator coil, the system cannot transfer heat properly. The result is weak, warm, or no airflow from your vents.
How to check: Pull out your filter (usually located at the return air vent or inside the air handler). Hold it up to light. If you cannot see light through it, it is clogged.
Fix: Replace it. A standard MERV 8 filter costs $8 to $20. This takes 2 minutes and costs almost nothing.
Rule: Replace filters every 30 days if you have pets, every 60 days for average households, every 90 days for a single person with no pets.
2. Low Refrigerant (Refrigerant Leak)
How common: Very common, especially in systems over 8 years old.
What happens: Refrigerant is the chemical that actually removes heat from your indoor air. If your system has a leak, refrigerant levels drop and your AC loses its ability to cool. This does not fix itself — refrigerant does not "run out" naturally, so a low level always means there is a leak somewhere.
Signs: AC runs constantly but never reaches the set temperature. Ice forming on the refrigerant lines or evaporator coil. Hissing sound near the outdoor unit.
Fix: This requires a licensed HVAC technician. They will locate the leak, repair it, and recharge the refrigerant. Do not let anyone just "top off" the refrigerant without fixing the leak — it is illegal under EPA regulations and the problem will return within weeks.
Cost: $200 to $1,500 depending on leak location and refrigerant type.
3. Thermostat Set Incorrectly or Malfunctioning
How common: More common than you think — and completely free to fix when it is just a settings issue.
What happens: Before assuming something is broken, check your thermostat. It sounds obvious but a surprising number of service calls are triggered by a thermostat accidentally switched to "heat" or "fan only" mode.
How to check:
Make sure it is set to "cool" not "heat"
Make sure it is set to "auto" not "fan" (fan-only mode blows air without cooling it)
Check that the set temperature is actually lower than the current room temperature
Replace the batteries if the display looks dim
Fix: If settings are correct and the display is unresponsive, the thermostat itself may have failed.
Cost to replace: $150 to $350 installed for a standard programmable thermostat. A smart thermostat (Nest, Ecobee) costs $200 to $400 installed but can reduce energy bills by 10 to 15% annually.
4. Frozen Evaporator Coil
How common: Very common in summer, especially when filters are neglected.
What happens: The evaporator coil inside your air handler needs steady airflow to function. When airflow is restricted — by a dirty filter, blocked vents, or low refrigerant — the coil gets too cold and ice starts forming on it. Once frozen, it cannot absorb heat at all, so your AC blows warm or no air.
Signs: Ice visible on the refrigerant lines coming out of the indoor unit. Water pooling or dripping around the air handler. AC running but no cooling.
What to do immediately: Turn the system off and set it to "fan only" for 2 to 3 hours to let the coil thaw. Do not keep running it in cooling mode — you risk damaging the compressor.
After thawing: Check and replace the filter. If the coil freezes again after that, call a technician — you likely have a refrigerant leak or a blower motor issue.
Cost if professional service is needed: $200 to $800 depending on root cause.
5. Tripped Circuit Breaker or Electrical Issue
How common: Common, especially after power surges or storms.
What happens: Your HVAC system runs on dedicated electrical circuits. A power surge, overloaded circuit, or electrical fault can trip the breaker, cutting power to the outdoor unit, indoor unit, or both.
How to check: Go to your electrical panel and look for any breakers in the middle position (tripped) or fully switched to off. Your HVAC system will typically have two breakers — one for the indoor air handler and one for the outdoor condenser.
Fix: Reset the breaker by switching it fully off, then back on. If it trips again within an hour, do not keep resetting it. A breaker that keeps tripping is a symptom of a deeper electrical problem — a failed capacitor, a grounding issue, or a motor drawing too much current.
Cost: Breaker reset is free. If there is an underlying electrical issue, expect $150 to $650 for professional diagnosis and repair.
6. Failed Capacitor
How common: One of the most common single-component failures, particularly in hot climates like Texas, Arizona, and Florida.
What happens: The capacitor is a small cylindrical component inside the outdoor unit that gives the compressor and fan motors the jolt of electricity they need to start up. When it fails, the motors cannot start. You may hear a humming sound from the outdoor unit but see the fan not spinning, or the system may simply not respond at all.
Signs: Outdoor unit humming but not running. AC starts briefly then shuts off. System struggles to start in hot weather.
Fix: A technician swap. Capacitors cost $10 to $30 as a part but require a professional to replace safely due to the stored electrical charge inside — they can cause serious injury if handled without proper training.
Cost total: $150 to $350 for parts and labor.
7. Blocked or Closed Vents
How common: Very common in homes where furniture has been rearranged or vents have been manually closed.
What happens: Many homeowners close vents in unused rooms thinking it saves energy. It does not — it actually creates pressure imbalances in the duct system that force the system to work harder and can cause the evaporator coil to freeze. A single blocked return air vent can significantly reduce the entire system's cooling capacity.
How to check: Walk through every room and make sure all supply vents (blowing air out) and return vents (pulling air in) are fully open and unobstructed. Check that no furniture, rugs, or curtains are blocking vents.
Fix: Open all vents. Free.
If the problem persists after opening all vents, you may have duct leakage — see issue number 8 below.
8. Leaking or Damaged Ductwork
How common: Extremely common in homes over 15 years old. The US Department of Energy estimates the average American home loses 20 to 30% of conditioned air through duct leaks.
What happens: If your ducts have holes, disconnected joints, or poor insulation, cooled air leaks into your attic, walls, or crawlspace before it ever reaches your rooms. The result is a system that runs constantly, drives up your energy bill, and never quite gets the house comfortable.
Signs: Some rooms are always significantly warmer than others. Energy bills are high despite normal usage. Excessive dust near vents.
Fix: A professional duct inspection and sealing. Techniques include mastic sealant for small leaks, metal tape (not standard duct tape which fails over time), and aeroseal duct sealing for whole-system treatment.
Cost: $300 to $1,000 for standard sealing. Full duct replacement runs $1,500 to $5,000 depending on home size.
9. Compressor Failure
How common: Less common than the issues above but far more serious when it happens.
What happens: The compressor is the core of your air conditioning system. When it fails, the system simply cannot cool at all. Compressors fail due to age, refrigerant issues, electrical problems, or overheating. A failed compressor is the HVAC repair that most often triggers the replacement vs repair conversation.
Signs: Outdoor unit is completely silent when the system should be running. System blows air but with zero cooling effect. Loud clunking or grinding from the outdoor unit before it stopped working.
Cost to repair: $800 to $2,500 for compressor replacement alone.
Important: If your system is over 10 years old and the compressor has failed, get a full replacement quote before authorizing a compressor repair. In many cases, a new system is only $2,000 to $4,000 more than a compressor replacement on an aging unit — and comes with a full warranty, higher efficiency, and modern refrigerant. Read our full breakdown in the repair or replace HVAC system 2026 cost analysis.
10. Wrong Size HVAC System
How common: More common than most homeowners realize estimated to affect 50% of US homes according to industry data.
What happens: An oversized AC unit cools the air so fast it shuts off before it has time to remove humidity. The result is a home that feels cold but clammy and uncomfortable. An undersized unit runs constantly but never reaches the set temperature because it simply lacks the capacity to cool the space.
Signs of oversized system: Short cycling (turns on and off every few minutes). High indoor humidity even when the temperature is technically reached. Rooms feel cold but air feels damp and heavy.
Signs of undersized system: System runs 24/7 in summer without reaching set temperature. Utility bills are extremely high.
Fix: Neither problem has a cheap fix. The only real solution is proper system sizing using a Manual J load calculation performed by a licensed HVAC contractor. If you are replacing your system, insist on this calculation before accepting any equipment quote. A contractor who recommends a system size based only on square footage without measuring windows, insulation, ceiling height, and orientation is not doing their job.
Cost implication: Replacing a wrongly sized system means full replacement cost ($5,000 to $12,500). See our HVAC installation and repair costs guide across the USA for what to expect by city and system type.
When to Call a Professional
Handle these yourself:
Replacing the air filter
Resetting the circuit breaker (once only)
Checking and correcting thermostat settings
Opening blocked vents
Thawing a frozen coil (turn to fan mode, wait 3 hours)
Call a licensed HVAC technician for:
Any refrigerant issue (legally required)
Compressor problems
Electrical issues beyond a simple breaker reset
A coil that keeps refreezing after you have replaced the filter
Any issue you cannot diagnose with certainty within 15 minutes
A professional diagnostic visit typically costs $75 to $150. Many contractors waive or credit this fee toward any repair they perform on the same visit — always ask upfront.
For help finding a qualified contractor and knowing what questions to ask, read our guide on how to choose a reliable home contractor.
Repair Cost Overview
Here is a quick reference for what the most common AC repairs cost in 2026:
Problem | DIY Possible? | Average Repair Cost |
|---|---|---|
Dirty air filter | Yes | $8 – $20 (filter only) |
Thermostat replacement | Sometimes | $150 – $400 |
Capacitor replacement | No | $150 – $350 |
Refrigerant recharge + leak repair | No | $200 – $1,500 |
Frozen coil (filter was the cause) | Yes | $0 |
Frozen coil (refrigerant leak) | No | $300 – $900 |
Electrical repair | No | $150 – $650 |
Duct sealing | No | $300 – $1,000 |
Compressor replacement | No | $800 – $2,500 |
Full system replacement | No | $5,000 – $12,500 |
For city-specific pricing in Dallas, Phoenix, Houston, New York, and more, use our HVAC repair vs replacement cost guide with city-by-city comparison.
How to Prevent These Issues
The majority of the 10 problems above are preventable with basic annual maintenance. The homeowners who never deal with surprise AC failures are not lucky they are consistent.
Follow this simple prevention routine:
Every month: check and replace the air filter if needed. This single habit prevents frozen coils, airflow problems, and compressor strain all at once.
Every spring (before cooling season): schedule a professional tune-up. A technician will check refrigerant levels, clean the coils, test the capacitor, inspect electrical connections, and verify the system is ready for summer demand. Annual tune-ups cost $75 to $200 and routinely catch $500 to $2,000 problems before they become emergencies.
Every year: clean the area around your outdoor unit. Remove leaves, cut back any plants or bushes within 2 feet, and gently rinse the condenser fins with a garden hose.
For a full maintenance schedule and a comparison of service plan options, read our guide on HVAC maintenance plans vs one-time tune-ups.
If you are in Dallas, our ultimate Dallas TX home maintenance checklist 2026 covers HVAC maintenance alongside every other major home system on a seasonal schedule.
Find Local HVAC Repair Experts Near You
You now know exactly what is wrong and roughly what it should cost. The next step is finding a qualified technician who will not overcharge you.
Use our local HVAC contractor directory to find licensed, reviewed professionals in your city. We cover 1,500+ cities across all 50 states search by city, service type, and budget to find the right contractor without the guesswork.
Do not call the first number you find on Google. Get at least two quotes, verify the contractor's license, and ask whether the diagnostic fee is credited toward the repair. Those three steps alone can save you $200 to $600 on a typical service call.
Frequently Asked Questions
Q: Why is my AC running but not cooling the house? A: The most likely causes are a clogged air filter, low refrigerant, a frozen evaporator coil, or a failed capacitor. Start by replacing the filter and checking that all vents are open. If the problem persists, call a technician running a compromised system can damage the compressor, which is the most expensive component to replace.
Q: How do I know if my AC is low on refrigerant? A: Signs include the system running constantly without reaching the set temperature, ice forming on the refrigerant lines or indoor unit, a hissing sound near the outdoor unit, and higher-than-normal electricity bills. Low refrigerant always means a leak topping it off without fixing the leak is a temporary and illegal solution.
Q: Can I recharge my AC refrigerant myself? A: No. Handling refrigerants requires EPA Section 608 certification. It is illegal for uncertified individuals to purchase or handle refrigerants used in home HVAC systems. Always hire a licensed technician for any refrigerant work.
Q: Why does my AC turn on and off every few minutes? A: This is called short cycling and it is almost always caused by an oversized system, a clogged filter restricting airflow, low refrigerant, or a failing thermostat. Short cycling puts extreme stress on the compressor and dramatically shortens system life. Do not ignore it.
Q: How much does it cost to fix an AC that is not cooling? A: It depends entirely on the cause. A dirty filter costs nothing to fix. A capacitor repair runs $150 to $350. A refrigerant leak repair costs $200 to $1,500. Compressor failure runs $800 to $2,500. If the system is old and the repair is major, a full replacement at $5,000 to $12,500 may be the smarter long-term investment.
Still weighing repair against replacement? Read our complete HVAC repair vs replacement 2026 cost guide with city-by-city pricing across 8 major US markets then use our free HVAC cost calculator to get an instant estimate for your specific location.
