The Most Common Reasons Your Air Conditioner Isn’t Working.

Here is a list of the most common reasons your air conditioner isn’t working. Imagine it’s 5 PM when you get home from work and a wall of heat hits your face when you open your front door.

That’s when you need to know about the various reasons your HVAC system isn’t coming on. Ask any service technician who’s been running service calls for a number of years and they will tell you the issues they run into are repetitive.

Here are some of the most common reasons that you will find are preventing your air conditioner from coming on and or cooling properly.

The Airflow Is Restricted.

Your air conditioner or heater will not function correctly if the airflow is blocked or restricted. Some of the ways your airflow can become restricted are simple fixes. Many HVAC systems have air filters in a few different locations depending on what part of the country you live in.

Some air filters are low on the wall in the hallway and some are high on the ceiling. Those types of filters are considered the primary frontline air filters.

Some residential HVAC systems are in the hallway closet and have a filter at the base lower part of the unit. This type of filter is also called a primary air filter if it’s the first filter the home’s air comes into contact with.

dirty evaporator coils
dirty evaporator coils

If your primary air filter is extremely clogged it will prevent the airflow and will make the system not cool. Sometimes the dirty filter will cause the indoor blower motor to burn up. This happens because the blower motor has to struggle and may overheat trying to pull the air across a restricted filter.

Having a dirty filter is definitely a Common Reasons Your Air Conditioner Isn’t Working.

Some HVAC systems have more than one filter.

Some systems have a primary filter like the one I described above and other systems also have a secondary filter. Many times the secondary filter is a 4″ or 5″ filter and it will definitely restrict the air if it’s dirty.

Check all of the filters you have to ensure they are clean and aren’t restricting the airflow.

Another way the airflow is restricted is if the evaporator coil is clogged with dust and or other biological growth.

The evaporator coil needs to be free of impacted dust, mold, or other debris to ensure proper operation.

The refrigerant is low.

If the refrigerant (FREON) is low your system may still be coming on but not cooling the home very well. Some systems have R-22 refrigerant and others use R-410A refrigerant.

The refrigerant is the vehicle that absorbs the heat in your home and carries it out. If your refrigerant is low that’s one of the common reasons your air conditioner isn’t working.

A refrigerant leak may be anywhere from the condenser outside all the way to the evaporator coil inside. A service technician will use a tool that will beep when refrigerant comes in contact with the probe.

Typically when an HVAC system is still working and the copper line outside is freezing up that may be an indication the system is low on refrigerant.

The Fuse is Bad.

If one of the fuses on the outside or the inside disconnect is bad or blown then the system will not come on. Many of the times I find a bad fuse it’s because the fuse is doing its job and letting you know you have a bigger problem.

Reasons Why An Air Conditioner Compressor Isn't Running
Reasons Why An Air Conditioner Compressor Isn’t Running

However, sometimes the fuse is just bad and needs to be replaced.

The fuse is installed as a safety device to protect your system and shut off power if an unsafe condition occurs.

The capacitor is weak or bad.

The capacitor is a part of the system that’s tested during routine maintenance. Some systems have a capacitor on the inside and also have one on the outside.

common reason an ac wont come on
A bad dual run capacitor

The capacitor on the inside is on the blower motor and assists the blower motor. The other capacitor is on the condenser outside and is used to assist the condenser fan motor and the compressor.

Single run capacitor
Single run capacitor

You may find your system has a combination of single-run capacitors and dual-run capacitors.

common reasons hvac wont come on
Dual Run Capacitor

A single run capacitor will start and operate a single motor such as a condenser fan motor or a blower motor.

A dual-run capacitor will be in the condenser and will assist the condenser fan motor and the compressor motor. If you look at the picture above you can see the top of the capacitor is swollen which indicates it’s bad.

Professional HVAC technicians use multimeters to test capacitors. The bottom line is a capacitor is a Common Reason Your Air Conditioner Isn’t Working.

The contactor is bad.

Depending on what type of system you have there may be more than 1 contactor on the HVAC system. When you have natural gas as your heat source then you will most likely only have a contactor on the condenser outside.

Common Reasons Your Air Conditioner Isn't Working

Because the contactor isn’t transferring power from one side to the other then the equipment will not receive the proper voltage.

When you look closely at the contactor above you will see the contact tips are burned and damaged. That contactor had to be changed so the condenser would come on.

The contactors also have a coil that comes on when low voltage 24V is sent to the coil. The coil can go bad for a multitude of reasons.

Outside Air Conditioner Condenser Unit Not Turning Off
common HVAC parts

If you have electric heat on the air handler then you may have a bad contactor preventing components from coming on. Because contactors are (wear and tear) and will typically not last the whole life of the system it’s one of the Common Reasons Your Air Conditioner Isn’t Working and may need to be changed.

The Breaker is Tripped.

The circuit breaker might be tripped and the unit isn’t turning on. If you have a 240v condenser and a 240v air handler then those will be 2-pole breakers.

The Most Common Reasons Your Air Conditioner Isn't Working
2-pole breaker

The 2-pole breaker above will be in the electrical panel box on your home. The breaker will shut off when it senses a component is overheating or has grounded out causing a direct short.

The Most Common Reasons Your Air Conditioner Isn't Working
single pole breaker

If you have a 120v furnace or another type of 120v unit then that will be a single-pole breaker. The picture directly above is a single-pole breaker and is what it will look like in your home electrical panel.

The breaker is there to protect your home and equipment when something goes wrong and needs to be shut down.

Sometimes it’s just a bad breaker that simply needs to be replaced. From my experience in the field most of the time the breaker was good and was simply shutting off because it was doing its job.

The thermostat is bad.

The thermostat may be bad and if this is the case it will be a simple repair. Unfortunately, when I was in the field servicing systems I didn’t find very many bad thermostats were the problem.

One of the easiest ways to test if it’s only the thermostat is to jump out the wires in a thermostat. The thermostat is similar to a switch that controls a room light. When a thermostat is bad it isn’t sending voltage to the correct HVAC component and therefore it won’t start.

The Most Common Reasons Your Air Conditioner Isn't Working
Typical residential thermostat

When the faceplate of the thermostat is removed the wires will be exposed. Service professionals will use a conductive wire with a magnet or clip on the end to clip onto the t-stat screws and transfer power from one wire to the other.

The red wire is normally the wire that carries the 24v low voltage into the t-stat. When a wire is introduced to connect the red wire to the green wire the indoor blower motor will come on.

If the ac system isn’t variable speed then the red, yellow, and green wires can be connected. When those wires have connected to the condenser, the blower motor will come on.

When professionals do this test it lets them know if all the system works then it must just be the thermostat. The bottom line is to perform a test to determine if the thermostat is bad or if some other component is bad.

The blower motor is bad.

The blower motor may be bad. When this is the case the condenser outside may come on but the inside blower motor isn’t coming on or circulating air.

One simple way to test if this may be the reason your whole system isn’t cooling or heating is to turn on the fan at the thermostat.

The Most Common Reasons Your Air Conditioner Isn't Working

The thermostat has a switch that says fan on or fan auto. Turn that switch to fan on. If the fan comes on and is blowing as normal then you know it’s working as normal. If the fan motor doesn’t come on then you know it may be the blower motor.

It may also be the circuit board but this is just a simple test to try and isolate where to start the troubleshooting.

The condenser fan motor is bad.

The condenser fan motor may be bad. This is also a very common reason the system won’t come on. This issue is fairly easy to determine.

Turn the system on and if the inside seems to start normally and is blowing air then go outside and check if the condenser fan is turning.

How To Change A Goodman Air Conditioner Condenser Motor

Assuming the other components are transferring power correctly this may isolate the problem to just being the condenser fan motor.

reasons why the air conditioner breaker will trip

The above pictures show what the condenser fan motor and blades look like when they are removed from the units.

Doing this procedure if anything else will assist you in communicating with the service technician about what needs to be repaired.

The thermostat wire is bad.

In some circumstances, the low voltage thermostat wire is damaged and may have shorted out. I have seen pets damage the low voltage wire by the condenser.

I have seen rodents and squirrels chew through the low voltage thermostat wire. I’ve seen low voltage t-stat wires damaged in attics, under houses, and at the condenser location outside.

If the low voltage wires shorted out it may have blown the fuse on one or more circuit boards. The most common location the fuse would have blown would be on the inside circuit board.

Look at the fuse at the top of this post to see what the fuse looks like. They will normally be a 3 or 5-amp fuse to protect the circuit board.

Holes in your ducts.

Having holes in your HVAC system ducts won’t prevent your system from coming on but will definitely hinder the performance. When the system is in the attic and the ducts have a hole the system will suck in hot attic air.

The ducts or plenums may have collapsed or become disconnected. If this is the case the entire system will still come on but without proper airflow. The system will not cool or heat the home without proper airflow.

ducts causing an  hvac system to not run

This duct has a hole in it because another duct became separated.

In conclusion

Some of the most common reasons your air conditioner isn’t working maybe an easy fix. Sometimes the reason a part is bad is due to other more serious problems.

I’m a big believer in having professional routine maintenance on the system.

From my many years working on HVAC systems, it seems like the systems checked by trained eyes are the ones that last the longest without major parts breaking.

This post will help you isolate where the problems may be and allow you to talk to your HVAC professional with more confidence.

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