Air conditioners rarely fail without warning. What they do is complain for weeks or months first: an extra noise, a smell, a room that will not cool down, a thermostat that will not keep a setting. The trick is knowing which complaints matter and which ones are the early warning of something much more expensive.
What follows is grouped by the part of the system that is making the complaint, because that is how we approach a call, and each sign comes with the causes we find most often.
What your cooling system is telling you
The air is weak, or it is not cold enough
This is the most common complaint and the one with the widest range of causes. Some are trivial and some are not.
- Low refrigerant, usually from a small leak that has been going for a while
- A clogged air filter starving the evaporator coil
- A dirty evaporator coil, which can happen even with a new filter if the unit has been running a long time without service
- Leaking, disconnected or undersized ductwork, so the cool air never reaches the rooms
- A failing compressor or a blower motor that has lost power
- A thermostat that is not calling for full capacity
New noises from the unit
A system that has been running for years makes a small number of sounds. A new sound is information, not noise to ignore.
- Rattling or knocking usually means a loose panel, a loose motor mount or a failing fan motor
- Buzzing points to an electrical problem, a failing contactor or a failing relay
- Hissing near the lines points to a refrigerant leak
- Gurgling or bubbling from the indoor unit usually means a blocked or slow condensate drain
- Screeching or grinding means a worn bearing or a slipping belt
Odours from the vents
- Musty or damp usually means moisture sitting on the evaporator coil or in the drain pan
- Burning or hot plastic means an electrical fault and should be dealt with the same day
- Dusty usually means dirty filters and dirty ductwork
- Sewer means a cracked condensate drain line, which is unpleasant and can discharge into the house
It cycles too quickly, or it never stops
Short cycling is when the system starts, runs for a few minutes and stops before the house is comfortable. It is hard on the compressor and the starter. The usual causes are an oversized system, low refrigerant, a dirty condenser coil and a failing thermostat. A system that never stops is the opposite problem, and it is usually a clogged filter, an undersized unit, undersized duct runs or a failed cycling switch.
What your furnace or heat pump is telling you
There is no heat at all
On a gas furnace the causes are usually a failed igniter, a gas valve that will not open, a limit switch that has tripped, or a dead blower motor. On a heat pump you are more likely to be looking at a failed reversing valve, a locked outdoor unit, or a thermostat that is not calling for heat.
The heat fades out, or the air comes out cold
- A dirty heat exchanger, which is both a comfort problem and a safety one
- A blocked filter, which starves the airflow across the heat exchanger
- Low refrigerant on a heat pump, which shows up as weak heating and a lot of run time
- A reversing valve that is stuck in the cooling position
- A furnace that is short cycling because the pressure switch is unhappy
The breaker trips, or you smell burning
A tripped breaker usually means a short or a failing capacitor, often a motor that has seized. A burning smell is an electrical fault until proven otherwise. Both of these are a stop sign. Unplug the system if you can do it safely, and do not reset the breaker and walk away.
What the air moving through your house is telling you
One room is always hotter or colder than the rest
Up to 30 percent of heated or cooled air can be lost through leaks in the ductwork, and a room with a collapsed or disconnected run will get nothing at all. Other causes are a register that is blocked or crushed, undersized duct runs, and a zone damper that has stuck in the wrong position.
The thermostat is unresponsive, or the schedule will not hold
Start with the batteries, because a surprising number of dead thermostats are just flat batteries. After that the causes are a faulty thermostat, loose wiring at the thermostat or in the air handler, and a failing control board. A smart thermostat that has lost its network connection can also behave as if it is dead.
There is water where there should not be
Water under the indoor unit is almost always a blocked condensate drain. Water on a ceiling or a wall near a vent usually means the supply line is sweating and the insulation around it has failed, or the drip pan is cracked. Water running down the side of a cabinet in a garage is a different problem again, and it usually comes down to a unit that is not level.
A symptom chart you can check in a minute
| What you notice | Most likely causes | What to do first |
|---|---|---|
| Air is warm or weak at the vents | Low refrigerant, clogged filter, dirty coil, leaking ducts | Check the filter, then call for a proper diagnostic |
| Water under the indoor unit | Blocked condensate drain, cracked pan, failed insulation | Turn the system off and call, a wet floor can become a wet ceiling |
| A new rattling or buzzing noise | Loose motor mount, failing fan motor, electrical fault | Note when it happens and let a technician listen to it |
| Burning smell from the unit | Electrical fault, seized motor | Switch the system off and call the same day |
| Starts and stops every few minutes | Oversized system, low refrigerant, dirty condenser coil | Stop relying on the thermostat setting and get it checked |
| No heat in winter | Igniter, gas valve, limit switch, reversing valve | Do not keep resetting a tripped limit switch |
| Bills keep climbing | Failing parts, refrigerant leak, dirty coils, thermostat fault | Compare the usage with last summer, then book a service |
| Thermostat will not respond | Batteries, faulty thermostat, loose wiring | Replace the batteries first, then call if nothing changes |
Five things you can check safely before you call
- Look at the filter. If it is visibly dirty, replace it and give the system an hour to see whether that changes anything
- Look at the outdoor unit. If the fan is not spinning when the thermostat calls for cooling, that tells the technician a great deal
- Look for standing water under the indoor unit or a wet ceiling, and note where it started
- Check the breaker, but only once, and only if it is safe to reach. Do not keep resetting it
- Write down when the noise, smell or short cycling happens. A pattern is worth more to a technician than a guess
When to stop and call a licensed technician
Some faults are safe to look at, and some are not. These are the ones where you should stop and call a licensed technician.
- Stop if the outdoor unit is not running when the thermostat calls for cooling
- Stop if the breaker trips more than once, or if you smell burning or hot metal anywhere in the system
- Stop if you can hear hissing, or you see oil staining on the refrigerant lines
- Stop if ice has formed on the line set in summer, or if the indoor unit has frozen up
- Stop if there is standing water near the unit, inside a wall cavity or under a ceiling
- Stop if the system has not been serviced for more than a year and it is now the hottest part of the season
One last thing on the money question. Multiply the age of the system by what you spend keeping it going each year. If the total lands close to what replacing it would cost, replacement usually wins. If it is well under, the repair is usually still worth doing. If another contractor has already told you to replace yours, call us for an honest second opinion and free phone guidance on your installation options.
Call (209) 351-6000 and describe the symptom in your own words. We will tell you honestly whether it is a same-day job, a next-day slot, or something you can safely do yourself.
