Most air conditioners give you warning before they quit. The trouble is that the warnings are easy to live with for a few weeks, and a small fault that gets ignored in July turns into a compressor in September. These are the five signs that mean stop adjusting the thermostat and get a licensed tech out, plus what each one usually turns out to be.
1. The air from the vents is not properly cold
Put your hand in a supply vent. On a working system the air should be noticeably colder than the room, cold enough to be uncomfortable to hold your hand in. Air that feels cool but thin, or air that feels like room temperature, means the system is running without doing much.
Change the filter first, because a loaded filter causes exactly this. If a fresh filter does not bring the cold back within a day, the short list is a dirty indoor coil, a failing compressor, or a refrigerant leak. If the air is outright warm rather than weak, start with AC blowing hot air instead, and if the house merely never catches up on hot days, read AC not cooling the house properly.
2. Noises that were not there last summer
A central system makes a low hum outdoors and moving air indoors. Anything else is new information.
- Grinding or a metallic scrape. Motor bearings, or a fan blade catching something. Turn the system off. A blade that contacts the shroud can destroy a coil in seconds.
- A loud buzz at the outdoor unit. Often a contactor chattering or a motor trying to start against a weak capacitor.
- Banging or clanking on startup. Something is loose. Compressor mounts and fan hubs both do this.
- A squeal from the indoor unit. Blower bearings or, on older belt-drive units, the belt.
- Hissing or bubbling near the line set. A refrigerant leak makes this sound.
Noise is worth acting on early because most of these are cheap parts protecting expensive ones.
3. Smells coming out of the vents
- Musty or sour. Almost always biological growth in the drain pan, on the coil, or in the ducts, which stay damp in a house that runs cooling all day. People with asthma or allergies often notice it before anyone else does. The fix is clearing and treating the pan and coil, and sometimes duct cleaning.
- Burning or hot plastic. Switch the system off at the thermostat and at the breaker, and call. This is a motor, a wire or a board overheating.
- A sharp chemical or ether-like smell. Refrigerant. Ventilate the room and call. Handling refrigerant requires EPA Section 608 certification, so this is never a homeowner repair.
- Rotten eggs anywhere near a gas furnace or the return. Leave the house and call your gas company or 911 from outside.
4. Ice on the coil or the copper lines
Ice on an air conditioner in a 95-degree Pasadena, CA afternoon looks wrong because it is. Frost on the insulated copper line at the outdoor unit, or a block of ice on the indoor coil, means the coil is running colder than it should. That comes from restricted airflow, most often a filter or a dirty coil, or from a low refrigerant charge.
Running the system while it is frozen is the part that does damage: liquid refrigerant can return to the compressor, which is not designed for it. Switch cooling off, set the fan to On, and let it thaw for an hour or two. If it freezes again after a filter change, the cause is inside the system.
5. It short cycles, or the breaker keeps tripping
A healthy system runs in long steady cycles, longer on hotter days. A system that starts, runs two or three minutes, stops, and restarts a few minutes later is short cycling. It wears the compressor and the contactor faster than anything else, and it never dehumidifies. The usual causes are a dirty condenser coil pushing pressure up, a weak capacitor, a failing pressure switch, or a system oversized for the house.
A breaker that trips is a separate and firmer signal. Reset it once. If it trips again, leave it off and call. The breaker is doing its job, and repeated resets on a shorted motor or a seizing compressor turn a repair into a replacement.
What you can check yourself
- Replace the filter, and look at what came out.
- Confirm the thermostat is on Cool, fan on Auto, setpoint below room temperature, batteries fresh.
- Kill the power at the outdoor disconnect, clear leaves and debris from around the unit, rinse the fins gently with a garden hose from the outside.
- Check the condensate drain for standing water in the pan or a wet spot where the line exits outside.
- Open every supply register and clear anything sitting against a return grille.
- Note which rooms are worst and at what time of day.
Stop at any panel held on with screws. Pulling the disconnect cuts the high voltage, not everything: the low-voltage control wiring is fed from the indoor unit and can still be live. The sealed compartments also hold a capacitor that keeps its charge with the power off, high-voltage terminals, and refrigerant under pressure.
When to call a licensed tech
Call when any of the five signs above is present after a filter change, and call immediately for a burning smell, ice that returns, a breaker that trips twice, or any gas smell. Also call if the system is simply old and getting expensive to keep running: at that point the useful conversation is repair versus replacement, and it needs someone to look at the actual equipment.
Tell us what you observed. Which sign, when it started, what the filter looked like, what you already tried. An AC repair visit that starts with good information is shorter, and an annual tune-up catches most of this list while it is still a part and not a failure.
Common questions
My house feels clammy even though it is cool. Is that one of the signs?
It can be. An air conditioner dehumidifies while it runs, so a system that satisfies the thermostat in short bursts pulls very little moisture out. That usually points at short cycling or a system oversized for the house. It shows up here in coastal-influenced humid spells rather than in dry heat.
Can I keep running the AC while I wait for the appointment?
If the only symptom is weak cooling, yes. Switch it off and wait if you smell burning, see ice, hear grinding, or the breaker has tripped. Running through those four is what turns a part into a system.
How do I tell a warning sign from normal behavior?
Normal is a steady hum, long run cycles, cold air from every vent, and water dripping outside from the drain line on a humid day. New noises, new smells, ice, water indoors and cycles that keep getting shorter are all changes, and changes are what to act on.
Our diagnostic is $69, told to you before we book and credited toward the work. Call Boost Services at 818-277-5056, 6am to 10pm, seven days, or send us a message.
Written with AI assistance by Boost Services, Heating and Cooling team, and reviewed by our licensed technicians before publishing. Boost Services Heating and Cooling, C-20 #1119892.