Common reasons why, and what to do
There could be many reasons why warm air is blowing from your vents when you turn on your air conditioning, but an often overlooked – and potentially serious reason – is if the outside unit of your AC (the condenser) isn’t turning on. The condenser works alongside the indoor unit to cool the air in your home by dispersing the heat that’s removed from the air inside the home by the inside unit.
Both units need to be running efficiently to produce cool air. So, even if the inside unit turns on, the AC won’t work unless the outdoor condenser unit is working. There are two ways to find out if the outside unit is running or not:
- See if the condenser is making any sounds. Some units are designed to have quiet operations, but there should still be a low hum. Take a look at the fan blades — if they're not spinning and there is no sound, that's an obvious sign your outdoor unit may not be working. If your fan blades are spinning but the air feels cooler, like standing in front of a fan, the compressor might have issues—such as a blocked indoor coil preventing proper airflow.
- Check if the indoor air handler is releasing warm air. Find a vent and place your hand in front to feel the air coming out. If it is blowing hot air, the outdoor unit may not be working properly. The warm air is only passing through the AC system instead of going through the cooling cycle.
In addition to having a lack of cool air, another sign you could have a faulty outdoor AC unit is when you hear unusual noises.
Reasons Why Your Outdoor AC Unit Won’t Turn On
Here are some common reasons why the condenser unit isn’t working and what you can do to fix them.
- Tripped Breaker or Blown Fuse
If the AC’s breaker has tripped or its fuse has blown, your air conditioner will not turn on. If you’re comfortable, go to your electrical panel and reset the breaker. If the breaker immediately re-trips, you’ve got an overloaded circuit or other electrical hazards that need fast, professional repair for your safety. For any electrical issues, it’s always best to reach out to a professional to ensure safety.
- Faulty Thermostat
Your thermostat communicates with the indoor and outdoor units. If it’s not set correctly, the AC might not turn on. Make sure there is a functional battery and check that it’s not broken and is properly mounted to its base and thermostat wires. Ensure that the thermostat is switched to “cool” and set at least 3-5 degrees below room temperature. Wait a few minutes to see whether these minor adjustments clear up the issue.
- Capacitor Failure
Capacitors help start the compressor and store energy to ensure the fan blades always spin. They can sustain damage from high temperatures and power surges. A bad capacitor won’t be able to power the fan. If you notice a buzzing or humming noise coming from the outside AC unit, the capacitor may have failed and the AC condenser fan is trying to spin without power from the capacitor. Capacitors can age and wear over time—in which case, it should be replaced by a professional HVAC technician.
- Fan Motor Failure
The motor keeps your whole AC system going. The fan in the condenser unit is linked to a motor, which allows it to rotate. So, a malfunctioning fan motor due to worn bearings or wear and tear over time can cause the fan not to work and the outside AC unit not to run. If your motor has worn out, no matter what switches you adjust, the AC won’t turn on. The outside part of your AC system may pick up all kinds of dirt, including sticks and leaves, which can get caught in its components. When this happens, the fan won’t spin correctly. Ask an expert to inspect the motor and let you know if it can be repaired. If not, it’s time to replace your air conditioner.
- Frozen Coils
When your air conditioner freezes, it’s typically the indoor evaporator coil that’s affected, often due to airflow issues or a refrigerant charge problem. The outdoor condensing coil would only freeze if it’s a heat pump operating in defrost mode. If you notice your AC isn’t turning on or is failing to cool properly, turn off the system and allow the coils to thaw completely. A dirty, clogged air filter is a common cause of frozen indoor coils, so be sure to replace your filter every three months to maintain proper airflow. Additionally, have a professional HVAC technician inspect the system for issues like a blocked condensate line or a malfunctioning fan.
- Tripped Float Switch
A tripped float switch in the evaporator drain pan means that the water level in the drain pan of your evaporator coil has risen too high, causing a safety mechanism (the float switch) to activate and shut off the system, usually because of a clogged condensate drain line that prevents water from draining properly. When the float switch trips, the AC unit will usually shut off to prevent further water buildup. Ensure the float switch is working and not stuck. If the switch is faulty, it may need to be replaced. Regularly clean and inspect the condensate drain line and evaporator coil to prevent future issues with a tripped float switch.
- Low Refrigerant Levels
Refrigerant is a chemical compound found in aluminum or copper coils that circulates through your system while it's running. As warm air moves over the coils, the refrigerant absorbs the heat and cools it down. If there's a refrigerant leak in the coils, the system will have trouble keeping up, the indoor coil may freeze, and eventually the low-pressure switch will prevent the system from operating to protect the compressor from damage. Other signs include ice buildup when in indoor unit cooling mode, warm air coming from the vents and unusual noises. If you suspect a refrigerant leak, contact a professional HVAC technician to inspect and repair the leak. Attempting to handle refrigerant yourself can be dangerous and may result in further damage to your system. (Professionals are required to have an EPA certification to handle refrigerant and homeowners should not do so without proper certifications and training.)
When to Seek Professional Help
Licensed professionals have the equipment and skills necessary to diagnose and repair complex problems. They can provide a more thorough and accurate diagnosis of any issues and offer targeted solutions. You should seek professional help for your outdoor AC unit if it's not turning on when basic troubleshooting steps mentioned earlier fail to resolve the issue, especially if you suspect a problem with the compressor, refrigerant leaks, significant electrical concerns, strange odors or damage to the outdoor unit itself. It's best to call an HVAC professional if you can't identify the problem, fix it with simple checks or the AC components are not easily accessible.
Preventative Maintenance
To prevent your outdoor AC unit from not turning on, you should regularly check the thermostat settings, frequently change your filters, clean the condenser coils and surrounding area of debris, inspect the condenser fan for proper function and consider having a professional HVAC technician perform a yearly maintenance tune-up to inspect components like the compressor and refrigerant levels.
If you have any questions about why your outdoor AC unit isn’t turning on or if you want to schedule your yearly tune-up, your local Lennox Dealer can help.
This has been reviewed by Nick Orth, Director of Outdoor Cooling, for accuracy. Nick brings over 20 years of expertise in the HVAC space.
