Updated October 2026. Maintenance and replacement guidance comes from ENERGY STAR’s heating and cooling pages, read on October 26, 2026. We’ve added what a failing AC means when you buy or sell a home.
The six warning signs of a failing AC are weak cooling, odd noises, poor airflow, bad smells, short cycling and rising power bills. ENERGY STAR says to consider replacing a central air conditioner that’s more than 10 years old, and notes that airflow problems alone can cut efficiency by up to 15%. Change or check the filter first; it’s the cheapest fix.
Jump to: ⏱️ Warning signs · 📍 Running but not cooling · 🪜 What you can check · 💲 Repair or replace · 🏠 Buying or selling · ❓ FAQ
Key takeaways
- ENERGY STAR says to inspect, clean or change the filter once a month in cooling season. A dirty filter raises costs and can damage the system.
- Book a pro check-up every spring, before contractors get busy.
- Consider replacing an AC over 10 years old. A correctly installed ENERGY STAR unit can save up to 20% on heating and cooling costs.
- A failing AC shows up in home inspections, so fix it or price it in before you list.
What are the warning signs of a failing AC?
Most problems give you a hint before the system quits. Here are the six we’d watch for, and what usually causes each one.
| Warning sign | Common causes | Who should fix it |
|---|---|---|
| Weak cooling, warm air | Dirty filter, dirty coils, low refrigerant, failing compressor | You (filter); a pro for the rest |
| Banging, squealing or grinding | Loose parts, worn motor or fan bearings | A pro |
| Weak or uneven airflow | Clogged filter, blocked or leaky ducts, failing blower | You (filter, vents); a pro for ducts and blower |
| Musty or burning smell | Mold in the unit or ducts; electrical fault | A pro, right away for a burning smell |
| Short cycling (on, off, on) | Thermostat problems, refrigerant issues, wrong-size unit | You (thermostat settings); a pro for the rest |
| Power bills climbing | Dirty filter or coils, low refrigerant, aging system | You (filter); a pro for a tune-up |
Weak cooling
If the house won’t get down to the temperature you set, start with the filter. Then look at the outdoor unit. Leaves, grass and dirt on the coils make the system work harder. ENERGY STAR notes that dirty coils reduce the system’s ability to cool and make it run longer, which costs more and shortens its life. Low refrigerant also hurts cooling, and only a licensed technician should check or add it.
When the cause isn’t obvious, calling an AC repair service can tell you whether it’s low refrigerant, blocked airflow, a failing compressor or another fault.
Strange noises
A healthy AC hums. Banging or clanking can mean loose parts. Squealing or grinding often points to a worn motor or fan. Turn the system off if the noise is loud and new, and call a technician before a small part breaks a big one.
Poor airflow
Weak air from the vents usually means something is in the way. Check that vents aren’t blocked by furniture or rugs. A clogged filter is the next suspect. Leaky or crushed ducts and a failing blower motor need a pro. Good airflow matters: ENERGY STAR says airflow problems can reduce a system’s efficiency by up to 15 percent. Regular maintenance keeps most of these in check.
Poor airflow in one room or a home office? Close fewer vents elsewhere, keep the door open when you can and ask a technician to check the duct to that room.
Bad smells
A musty smell often means mold or mildew in the unit, the drain pan or the ducts. A burning smell can mean an electrical problem. Shut the system off and call a pro right away for a burning smell.
Short cycling
An AC that turns on and off every few minutes is short cycling. Check the thermostat first: its batteries, its settings and whether it sits in direct sun. If those are fine, the cause may be refrigerant, a frozen coil or a unit that’s too big for the house.
High energy bills
Compare this summer’s bills with the same months last year. A big jump with no change in habits often means the system is working harder than it should. Dirty filters and coils are common causes. So is age.
Why is your AC running constantly but not cooling?
In hot, humid places like Florida, an AC may run for long stretches on the hottest afternoons even when it’s healthy. If it runs all day and the house still won’t cool down, something’s wrong. Check these in order:
- Is the filter clean?
- Is the outdoor unit running, and are its coils clear of debris?
- Is ice forming on the indoor coil or refrigerant line? If so, turn the cooling off and run the fan only, then call a pro.
- Is water backing up near the indoor unit? ENERGY STAR says a plugged condensate drain can cause water damage and affect humidity.
If all of those look fine, call a technician. Low refrigerant or a failing compressor are common causes, and neither is a do-it-yourself fix.
How can you spot wear and tear on your AC unit yourself?
You don’t need tools for most of it. Once a month in cooling season, we’d do a quick walk-around:
- Check the filter, and clean or change it if it’s dirty.
- Look at the outdoor unit for bent fins, rust, oil spots or plants growing into it.
- Make sure the condensate drain line isn’t clogged and the drain pan isn’t full.
- Listen for new noises when the system starts and stops.
- Note how long it takes to cool the house on a hot day.
A pro check-up covers the rest. ENERGY STAR’s checklist includes tightening electrical connections, lubricating moving parts, checking the refrigerant level, cleaning the coils and adjusting the blower. It suggests checking the cooling system in the spring, before contractors get busy.
When you hire, ENERGY STAR suggests checking the contractor’s license and insurance in your state, asking for referrals and calling references. Have your system’s model and maintenance history ready.
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Buy a home →Should you repair or replace your AC?
ENERGY STAR says it may be time to call a contractor about replacement when:
- Your air conditioner or heat pump is more than 10 years old.
- It needs frequent repairs and your energy bills keep rising.
- Some rooms are too hot or too cold, or the home has humidity or dust problems.
- The system is noisy, which can point to undersized ducts or an indoor coil problem.
It also says a correctly installed ENERGY STAR unit can save up to 20 percent on heating and cooling costs. If your system is near the 10-year mark and a big repair comes up, get a replacement quote too before you decide.
Our guides to common AC problems you shouldn’t ignore and signs your AC system may need repair go deeper on each fault.
What does a faulty AC mean when you buy or sell a home?
A lot, in warm states. Home inspectors check the cooling system, and an old or failing unit often turns into a repair request or a price cut. For sellers, the choice is to fix it, offer a credit, or price the home with the issue in mind. If your state’s disclosure form asks about the cooling system, answer honestly about what you know.
We cover the decision in should you get AC installed before selling your home, and how a newer system can help in how an efficient AC system can raise your home’s comfort and value. For other fixes worth doing first, see important house repairs you should make.
When the house is ready, a flat fee MLS listing puts it on the MLS from $99, with $0 due to Beycome at closing. Sellers in Florida and Texas can close with Beycome Title, from $99 in Florida (prepaid at checkout) and $299 per side in Texas.
Buying? Ask the seller for the system’s age and service records, and read the inspection report closely. Our guide to inspecting appliances when buying a house includes the AC. With the Beycome buyer program there’s no upfront fee on homes listed by other agents. Beycome keeps 1% of the price, with a $1,499 minimum, and credits the rest of the buyer-agent commission at closing, with lender approval. The credit can’t apply where no buyer-agent commission is offered, like most homes sold by owner, or on homes listed by Beycome sellers.
Frequently asked questions
What are the warning signs of a failing AC?
Weak cooling, loud or new noises, weak airflow, musty or burning smells, short cycling and rising power bills. Check the filter first, then call a technician.
How can I spot wear and tear on my AC unit?
Look for a dirty filter, bent or dirty coil fins, rust or oil on the outdoor unit, a clogged drain line and new noises. A spring check-up by a pro catches the rest.
Why is my AC running but not cooling the house?
Common causes are a dirty filter, dirty outdoor coils, a frozen indoor coil, low refrigerant or a failing compressor. If ice is forming, switch to fan only and call a pro.
How often should I change my AC filter?
ENERGY STAR says to inspect, clean or change it once a month. Change it whenever it looks dirty.
How old is too old for an air conditioner?
ENERGY STAR suggests considering a replacement once a central air conditioner or heat pump is more than 10 years old, especially if repairs and bills keep rising.
Should I fix my AC before selling my house?
Often, yes, in warm states, since inspectors check it and buyers notice. If you don’t fix it, disclose what you know and price the home or offer a credit to match.
Reviewed by Beycome Brokerage Realty LLC, a licensed real estate brokerage. Licenses are listed at beycome.com/faq/how-to/are-you-a-licensed-broker/.
Maintenance, efficiency and replacement figures are from ENERGY STAR’s maintenance checklist, “When is it time to replace?” and contractor hiring tips, read October 26, 2026. Have refrigerant, electrical and gas work done by a licensed technician.
Beycome prices are as of October 2026. “$0 at closing” means no Beycome fee at closing; other closing costs still apply. Beycome Title: Florida, Texas, Michigan, Ohio, Illinois and Indiana. Buyer program: no upfront fee; Beycome keeps 1% of the price with a $1,499 minimum and credits the rest, subject to lender approval and state rules; not on homes listed by Beycome sellers or where no buyer-agent commission is offered. This is general information, not professional HVAC advice.
Sources
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