Your central air conditioner has probably been working overtime for years, keeping you cool during hot summer days. But now it seems to be malfunctioning. What should you do?
It’s important to check your AC unit regularly for signs of wear and tear or potential problems before they become serious issues. We have compiled a list of 6 common issues that can help you identify symptoms and take action before things get worse.
Check the Thermostat
This should be the first step of diagnosing lack of cooling issues with your air conditioner.
Is it set to cool mode and set point below current temperature?
This is an easy one, but it gets the best of us. Check to see if the thermostat is in cool mode, and that the desired cooling set point is below current temperature.
Is short-cycle protection mode on?
On Emerson Sensi thermostats, this is referred the A/C Protection setting. This feature of smart thermostats, puts a delay on when the A/C is powered on again after reaching the cooling setpoint. If the air conditioner is powered on and off too frequently, this can cause undue stress on the compressor and start components and lead to premature failure.
If this mode is enabled, wait 15-30 minutes to see if if the unit turns on. You can temporarily turn the setting off, and set the temperature lower to see if the condenser powers on.
Is the thermostat powered on?
Check to see if the thermostat has power. If the thermostat has batteries, change the batteries. If it is powered by the "C" (Common) wire, make sure that the connection is secure and check other connections as well.
Tried all these? Test the thermostat.
The "Y" terminal is what sends the signal to turn on the outside condenser. Internally, the thermostat will send voltage from the "R" wire, sometimes designated as "RC" to the "Y" wire. A quick test is to take the thermostat of the wall, and join the "Y" wire to the "R" wire. If the unit comes on, then you have a faulty thermostat.
Air Flow Issues
Lack of airflow is the #1 reason we see a lack of cooling. Most times, this is due to a dirty air filter, but there can be other causes as well including a dirty and plugged indoor evaporator coil, or dirty blower wheel in the air handling unit. This will happen if the homeowner runs their unit without a filter.
A lesser known, culprit of, however, is closed supply vents are return air registers. We have seen customers who will close supply vents in unused rooms. If this happens to too many vents, the air is unable to move as quickly over the indoor evaporator coil, and evaporator coil to freeze up.
Other things we have seen is some homeowner’s may cover the return air vents with furniture. This can create somewhat of a vacuum effect and prevents the air handler from being able to take in fresh air to push air over the coil.
Fixes
- Change the Filter
- Clean the Blower Wheel and indoor evaporator coil with a nylon brush or compressed air
Bad Capacitor
This is the most common repair we see during cooling season. The capacitor in the outdoor unit has gone bad and the outdoor condenser fan doesn’t spin or compressor doesn’t come on. The capacitor is responsible for inducing a phase shift in the motor’s stator windings and helps the motor turn more efficiently. If this happens, the motor and/or compressor can overheat and eventually fail. Most of the time, the motor or compressor won’t start at all.
On older furnaces, the indoor air handler will have a capacitor as well for the blower motor that pushes air throughout the house. If this fails, it can cause the motor to not start, or fail altogether.
Fixes
- Replace the Capacitor. They typically cost about $20. To have a service technican come out, the cost will range from $150-300
Bad Contactor
A contactor is essentially a high voltage switch that receives a control signal from the thermostat and indoor air handler control board and powers up the compressor and fan motor. If the contactor fails, the outdoor condenser will not turn on, and cool the indoor coil.
Fixes
- Power off the unit via the indoor breaker panel and remove the service disconnect near the condenser and inspect the contactor for signs of pitting and discoloration. Make sure wire connections are tight.
Blown Fuse
If your outdoor unit is having an issue with the motor, compressor capacitor, contactor, or wiring, it can blow a fuse. While sometimes, a simple replacement of the fuse can get you buy, it is usually indicative of a larger issue. Also, a low refrigerant charge can cause this. If there is a leak in the system, you will need to have a service company come perform a leak check, repair it, and recharge the unit with refrigerant.
Fixes
- Power off the unit via the indoor breaker panel and remove the service disconnect near the condenser and inspect the contactor for signs of pitting and discoloration.
- With the unit powered off, check all connections inside the condenser and make sure everything is tight.
- Check the capacitor for bulging, or check that the capacitance of a multimeter, and replace if it is faulty.
- Read amps going to compressor and fan motor, and make sure they are within specification indicated on the compressor and fan motor label. If the compressor is bad, it’s best to just replace the whole unit if not under warranty.
Dirty Condenser Coil
During the allergy season, with cottonwood in the air, it can blanket the outdoor condenser unit, and affect the exchange of heat, and will cause a lack of cooling.
Fixes
An easy and quick fix is to spray the unit down with a hose. Be careful, though, as you don’t want to use high pressure setting and bend the condenser fins. If it is really plugged, you will need to use a specialized coil cleaner to break down the contaminants.
Unit is Overcharged with Refrigerant
This is a common one when an inexperienced technician does not check for air flow issues before adding more refrigerant. It’s all too common for an air conditioning unit to get overcharged with refrigerant and we come across this fairly often.
What happens is when the indoor evaporator coil does not have enough air flow over the coil, it can throw of subcooling and superheat readings on the refrigerant circuit. When the person goes the read the pressures, they think the unit is undercharged, when in fact, the evaporator coil inside is not exchanging heat with the air. This can lead to a frozen evaporator coil.
Fixes
If your unit is overcharged with refrigerant, this is unfortunately something you will not be able to DIY. You will need a technician with proper certifications to recover the refrigerant and ensure the unit has a proper charge.
Get the Repair Parts You Need
If you have managed to find your issue, call us, or browse our air conditioner parts selection to find the repair parts you need. If you need help diagnosing, please contact us, and we will be happy to help!
All in all, we hope this article helps you understand what causes your air conditioner to stop working. We also hope that it gives you some tips on how to diagnose the problem yourself.
Related Articles
- The Nitty-Gritty DIY Air Conditioner Maintenance Checklist
- How to Diagnose, Test, and Replace a Bad A/C Capacitor
About the Author
James Clark is the Ecommerce Manager of TechnicalHotandColdParts.com and HVAC Controls Specialist (certified in Tridium and Carrier i-Vu controls) with a decade of experience at Technical Hot & Cold. His extensive background includes service calls, installations, and providing technical support to parts customers. James has been featured in the Washington Post and continues to share his knowledge, helping homeowners navigate HVAC repairs. In his spare time, he's playing music with his children and spending too much time working on his lawn.