When hot weather comes, your air conditioner works hard to keep your home at a comfortable temperature.
But what should you do when you turn your AC on and cold air doesn’t come out of the vents? Or worse yet—nothing happens at all?
In this article, I’ll go over the steps that you should take if your AC isn’t working. I’ll also provide some helpful tips for troubleshooting your AC so you can get your home cool again!
Check the power to your AC
If your home’s air conditioner is not working, the first thing that you need to check is if its power is on.
It may seem obvious, but no power is one of the most common causes of AC problems.
Here’s what you need to check when confirming power to your AC:
- Circuit breaker panel in your home
- Power disconnect at AC condenser
- Power disconnect at the air handler
- Fuses in the power disconnect
First, start by checking the circuit breakers for the AC condenser and air handler. One of the breakers may have tripped or switched off inadvertently.
After confirming that the circuit breakers are good, the next thing to check is the disconnect. The disconnect will have a switch, lever, or pull-out that’s used to turn the power on and off.
Check the disconnect for both the condenser and air handler. Ensure that they are both switched on. If you have a multimeter you can probe the terminals at the disconnect to ensure that it’s getting voltage.
The last thing to check is the fuses. Some disconnects have fuses built into them to protect your home and equipment. A blown fuse usually indicates an issue with your AC.
You can test a fuse to see if it’s blown with a multimeter. Turn your multimeter to the “Ohms” or continuity setting and probe each end of a fuse. There should be a short circuit through the fuse. If there is an open circuit or more than a few Ohms of resistance, then the fuse is blown and needs to be replaced.
Check the condensate overflow switch
After you’ve confirmed that your AC unit is getting power, the next thing to check is the condensate overflow switch.
What does the condensate overflow switch do?
The condensate overflow switch is a normally closed switch that opens up when water reaches a certain level in your air handler.
When the switch floats upward, a connection to your AC opens up and your AC shuts down.
First, you’ll need to check if your AC has excess condensate build-up. If you find excessive amounts of standing water in your AC, then that’s definitely a problem.
How do you fix excessive condensate build-up in your AC?
Condensate build-up is caused by a clogged condensate drain. The condensate drain is a plastic pipe that removes condensate from your AC. If the drain pipe gets clogged, water will start building up in your AC.
Most drain line clogs can be removed by hand. Find the outlet of the drain line and dislodge any debris or clogs that you find.
If that doesn’t work, then you’ll need to take the unclogging a step further. Check out my article below about how to unclog your AC’s condensate drain line:
Check the thermostat
The next thing that you need to check is the thermostat. If the thermostat is not working, then your AC won’t work.
Put your thermostat in cooling mode and turn down the setpoint to get the cooling relay to switch on. On some thermostats, you may hear an audible “click” noise when it calls for cooling.
Every thermostat is different, so keep an eye out for any alarms or unfamiliar icons that may show up on your thermostat.
If you’re using a wired thermostat, it may be worth it to remove the thermostat from your wall and check that the wire connections and terminals are tight. Turn off your AC’s air handler before removing the thermostat from your wall—you don’t want to inadvertently short your thermostat wires together.
If everything looks good at the thermostat, the next thing to check is the control board at the air handler.
Check the control board
The thermostat in your home connects to the control board at the air handler. From the control board, wires run to the contactor in the condensing unit outside.
Here are a few things that you need to check on your control board:
- Check the control board wiring
- Check for blown or bad fuses in the control board
- Check for alarm codes on the control board
I’ll go over each of these below.
Check the control board wiring
The control board interfaces with the thermostat and condensing unit. If certain wires are disconnected, then the contactor won’t pull in.
Specifically, the Y and C terminals on the air handler’s control board are connected to the coil input terminals on the AC contactor.
Here is a control wiring diagram for a residential AC unit:
Check for blown or bad fuses in the control board
While you’re checking out the control board, check to see if the fuse on the board is blown. The 3A fuse is located on the top of the board. Most boards use these types of fuses:
How do you know if a circuit board’s fuse is blown?
When a fuse blows, you’ll see the blown-out filament inside of the fuse. You’ll be able to tell if a fuse is blown if there is a black smudge in the middle of the fuse. If the thin wire inside of a fuse is burned out, then the fuse is blown and needs to be replaced.
Why does a control board need a fuse anyway?
The fuse is installed in the board to protect the board from electrical damage. Shorted wires will blow a control board fuse. If the fuse is bypassed, the short circuit current will damage the control board. That’s why it’s important to replace the fuse if it blows—never bypass the fuse.
What happens if the fuse keeps blowing?
If the fuse keeps blowing, then turn off the air handler unit. Go to the control board and disconnect the thermostat wires from the terminals.
Replace the fuse and turn the air handler back on again. If the new fuse blows again, then the control board is bad and needs to be replaced.
If the new fuse doesn’t blow, then use a jumper wire to engage the fan, and cooling at the control board. If everything works okay when you jump out the cooling and the fan at the control board, then there is an issue with the thermostat or wiring.
Check for alarm codes on the control board
If you have a central AC system, then the control board inside the air handler in your home won’t provide any helpful alarm codes for your AC.
However, some AC condensers have control boards inside of them that may provide an alarm code when your AC is not working.
Alarm codes are typically communicated via a blinking light on the control board or a small LED screen that shows a number.
For instance, your AC control board may provide a high pressure alarm when there is high refrigerant pressure in your AC system.
Every AC is different, so you’ll need to take a look at your owner’s manual to decipher what specific alarm codes mean, and how to address them.
Check the high and low pressure switches
The high and low pressure switches are located in the AC’s condensing unit. Their purpose is to shut off the AC when the refrigerant pressure gets too high or too low.
AC high pressure switch tripping
When the high pressure switch trips, the button on the pressure switch will pop out (usually a little red button). Pressing the button will reset the switch, allowing your AC to function again.
Here are a few things that might cause your AC to have high refrigerant pressure:
- Low airflow through condenser
- Dirty condenser coil
- Broken condenser fan motor
If your high pressure switch is tripping, the first thing to check is for proper airflow through the condenser. Ensure that the condenser fan is operating properly and the condenser coil is clean. Also check that the condenser coil is free of debris and obstructions.
If your condenser coil is dirty, you will need to clean it before your AC runs properly again. Check out my article below for more information about cleaning your AC coil, and the best types of coil cleaners to use:
AC low pressure switch tripping
If the low pressure switch trips, you can’t reset it manually. There is no reset button on the low pressure switch—you need to wait for it to reset automatically on its own.
The most common cause of a low pressure switch tripping is a leak in the refrigerant. A refrigerant leak will cause the system to become improperly charged and this low pressure scenario will cause the low pressure switch to trip.
If your low pressure switch is tripping, there isn’t much that you can do on your own. Call your local HVAC service professional so they can address the issue.
Check the contactor
The contactor is the “switch” that turns on your AC. The thermostat is usually connected to the contactor through the air handler’s control board. When the thermostat calls for cooling, the contactor pulls in to make a connection and turn on your AC condensing unit.
Here are a few things to check if your AC’s contactor isn’t working properly:
- Check the contactor coil’s input voltage
- Check for debris or insects in the contactor
- Check the delay module in the contactor
Check the contactor coil input voltage
The contactor must receive input voltage (usually 24VAC) at its coil in order for it to engage. If there is no voltage at the contactor’s coil, then the contactor won’t pull in.
To test for contactor coil input voltage, first ensure that your thermostat is in cooling mode. Next, turn off power for your AC condensing unit. The contactor coil gets its power from the air handler inside your home, so the power to the condensing unit does not need to be on.
Next, open up the electrical access panel on the side of the condensing unit. Get a multimeter and set it to the “AC Voltage” (VAC) setting. Probe the contactor’s coil input terminals on the sides of the contactor.
If you read somewhere between 20-28 volts, then your contactor coil is getting voltage. If you don’t read any voltage, then there is likely a wiring issue or control board issue at your air handler.
Check for debris or insects in the contactor
A common issue with a contactor is dust, debris, or insects getting trapped inside of it. If there is something trapped in the contactor, then the contactor won’t be able to pull in and turn on your AC.
Caution: make sure the power to your condenser is off before proceeding!
While you’re inside your condensing unit, inspect the contactor for any signs of debris or insects.
On some older contactors, you will be able to see right into the contacts—so you’ll know for certain if there is something stuck inside.
For the newer styles of contactors, you can’t see inside without taking them apart, so there’s not much you can do there.
If you find debris inside your contactor, then dislodge them using a small screwdriver or blow them out using some canned air.
Check the delay module in the contactor
The delay module is a timer that prevents the contactor from short-cycling your condenser unit.
Why is short-cycling such a big deal? Short cycling is when your condensing unit turns on and off, then back on in a short period of time. This is bad for your AC because it causes excess wear and tear on your unit—especially the compressor.
The delay module prevents short cycling by allowing the contactor to turn on only if it has been off for a period of time—usually about 10 minutes.
So if your AC has a contactor with a delay module, you may need to wait a few minutes for the contactor to pull in and your AC to turn on.
How do you know if your AC contactor has a delay module?
Most delay modules are look like a little box with a dial on it. The dial is used to adjust the delay time.
Some contactors have a built-in delay module. In this case, the dial is right on the contactor itself.
Take a look to check what the delay time is on your delay module but don’t adjust it! Adjusting the delay time may cause damage to your AC by allowing it to short cycle. It’s set to a certain time depending on your AC unit—so you’ll know how long you have to wait before your contactor engages.