Here are the main reasons why an air conditioner compressor isn’t running.
Table of Contents
Let’s get right to troubleshooting why your compressor motor isn’t running.
I run across a lot of people who call the whole outdoor unit the compressor. So let’s make sure we are talking specifically about the compressor and not the whole condenser.
*Disclaimer – Please read at the bottom of the page.
The whole outdoor unit outside is named a condenser. shown in the picture here.
The motor at the bottom of the condenser is called the compressor motor. So when someone says their compressor isn’t working they should specifically be referring to the motor on the bottom of the condenser.
Most of the time even when the compressor motor doesn’t come on the condenser fan motor will still come on. I have gone on a lot of service calls and the homeowner will say their compressor isn’t working. What they actually mean is the condenser fan motor isn’t working.
So if it’s just the condenser fan motor that isn’t rotating and needs to be replaced and you don’t need to troubleshoot the compressor motor.
If you have done your due diligence and know it’s the compressor motor that isn’t coming on then keep reading this and I will go over what I most commonly find that prevents a condenser compressor from coming on.
Compressor Won’t Start Reason #1
Most often the reason your compressor motor won’t come on is because of a weak or totally bad capacitor.
Some compressors have a dual-run capacitor and some have a single-run capacitor. If the capacitor is weak it may not provide enough starting capability to get the compressor going.
The picture below is what a typical home condenser capacitor will look like. This one pictured is a dual-run capacitor that assists the (condenser compressor) and (fan motor) to come on.
Quite often you can determine a capacitor is bad by just looking at it. Take a look at the picture below and you can see the swollen top part of the capacitor. Sometimes they will swell on the sides or the bottom also.
The easy way to test the capacitor is if you already have a multimeter that has the capability of testing a capacitor in the Capacitance C setting which usually also has a UF symbol.
If you don’t have a meter that can do that then I suggest you buy a cheap one if this is all you are going to use it for.
There are multiple ways to check a capacitor with a meter and you can click here if you want more information on how to do that.
Compressor Won’t Start Reason # 2
The next reason a compressor may not be coming on is because of a bad Contactor. Power comes into the contactor from the AC disconnect.
Using a multimeter to check voltage the contactor should have approximately 230v – 240v if it’s a residential condensing unit.
That 240v should be on the bottom main wire connections and the top two-wire connections. The 240v is checked by putting the ends of the multimeter leads on the terminals.
If you have voltage coming into the contactor but not on the top terminals then there is a problem.
The contactor may be bad but first, we need to verify a few things before we can conclude the contactor is bad.
Is the contactor engaged (pulled-in) in order to transfer the power to the top of the contactor? If it’s not pulling in then it may not be receiving 24 volts from the low voltage wire. If this is the case you can read section # 3
Compressor Won’t Start Reason #3
Maybe the reason your compressor isn’t coming on is that the Contactor isn’t getting 24 volts. If the Contactor isn’t getting 24 volts to the side terminals then the contactor will not pull in.
If the Contactor doesn’t pull in it will not send the 240 volts to the top of the contactor which sends power to the fan motor and the compressor.
The easiest way to know if the Contactor is getting 24 volts and engaging is if the condenser fan motor comes on. Just because the compressor won’t come on doesn’t mean the fan motor won’t come on.
If the contactor pulls in and is getting 24 volts then the fan will come on and spin. So that’s an easy way to check if the contactor is getting 24 volts.
Reasons Why An Air Conditioner Compressor Isn’t Running Reason #4
The thermostat isn’t functioning correctly can contribute to the reasons Why An Air Conditioner Compressor Isn’t Running.
Sometimes the thermostat just needs new batteries.
But sometimes the thermostat just isn’t functioning properly and needs to be replaced.
An easy way to test if the thermostat is the reason the system isn’t working is to remove the faceplate on the thermostat and jump the wires with an insulated piece of copper wire or other conductive metal.
The thermostat wires are low voltage at 24-27 volts and even though that won’t shock a person dramatically it is still best to be as safe as possible when working with any live circuit. Make sure to take all the proper safety precautions before attempting to test the system while having power.
The thermostat has various color-coded wires. The red wire indicates the wire bringing in the low voltage power. I use a wire that has alligator clamps on the end and connects one to the red screw terminal and the green screw terminal. This should make the inside blower motor come on.
After that, I will leave the clamp on the red and move the other clamp to the yellow screw terminal and check to see if the compressor outside comes on. Typically I create a quiet surrounding so I can hear it kick on without having to walk outside and look.
You can also have someone outside looking at the condenser to listen to if the compressor comes on and if the condenser fan motor kicks on.
I have come across this a lot and is definitely one of the reasons why an Air Conditioner Compressor isn’t running.
If the inside blower motor comes on after you connect the red and green wire together then you know the inside blower motor works and the board is sending the correct signal.
If the outside condenser compressor and fan motor come on after touching the red and yellow wire together then you know your outside condenser is at least responsive and operational.
With this information, it’s safe to say that if the system is operational by bypassing the thermostat then the thermostat needs to be replaced. This is the reason the compressor isn’t coming on.
If you are attempting to do this procedure it’s important to NOT allow the red wire in the thermostat to touch other wires than the GREEN OR YELLOW. If the wire or conductor you use comes into contact with some other wire or is grounded out it will blow the small car-style fuse on the inside board.
If this were to happen you will need to ensure the red wire isn’t touching anything and turn off the power to the inside unit and replace the small fuse.
Here is a picture of the fuse and the fuse on the board.
If you determine the outside compressor motor and fan motor come on it’s a safe bet that you just need a new thermostat.
Compressor Won’t Start Reason # 5
The low voltage wire is broken or shorting out. Typically the wire is either brown or grey and will have a cut or exposed copper near the outside condenser.
This happens a lot because the wire isn’t encased in a conduit and is prone to physical damage and the elements such as sun and rain.
Sometimes you can’t see the wire because it is wrapped in tape or inside of the conduit.
I have seen squirrels, mice, rats, dogs, and lawn equipment damage the wire and it prevents the compressor from coming on. The damage is usually done by the condenser. However, I have seen the damage up by the soffit before the wire goes into the attic and I have also seen the wire damaged in the attic because of mice.
Compressor Won’t Start Reason # 6
The control board on the inside air handler or furnace isn’t sending a signal to the condenser. The board may be sending a signal (Voltage) to other parts such as the inside blower motor but not to the condenser outside.
This is often hard to detect and most technicians have checked several other parts of the system before they can or will determine the control board is bad. Sometimes with the power off you can inspect the back and front of the board to look for burn marks.
If the control board is bad then you will need to get the brand, model, and serial number off the unit and order the board online.
Compressor Won’t Start Reason # 7
The compressor wires coming off the capacitor and the contactor which are going to the compressor are cut or burned and shorted out. If this happens it prevents the proper voltage from reaching the plug on the compressor.
The wires going to the compressor are easy to see. If you look inside the condenser then you will see the wires going to the compressor plug.
With the power off completely to the condenser, you will need to inspect those wires all the way to the compressor plug. I take the plug off by inserting my screwdriver and popping it off. I inspect the connections to ensure they don’t have any burning.
If the wires aren’t severed or burned then you can assume this is not the reason why your air conditioner compressor isn’t running.
Compressor Won’t Start Reason # 8
The high-pressure shut-off switch may be preventing the compressor from starting. This can happen if there is too much refrigerant in the system. Another possible way the high-pressure switch is engaged is if the metering device is restricted. The metering device will be a TXV Thermal Expansion Valve or a piston.
When the metering device is restricted it will cause the pressures to go really high and shut off the compressor.
This is something that is hard to detect and typically takes an experienced technician to diagnose.
This isn’t extremely common, but when this does happen the homeowner most likely knew the system wasn’t running very well for some time.
Compressor Won’t Start Reason # 9
The low-pressure shut-off switch isn’t allowing the compressor to start. The low-pressure switch shuts the compressor off when the pressure gets low. This is typically just because the system is low on refrigerant.
To resolve this the system will need to have refrigerant put in.
If this is the case the homeowner may have saw the copper tubing freeze up or may have noticed the system just wasn’t keeping up with heat and not cooling the home very well.
The system may have a leak. The leak could be anywhere but I find them most often in the evaporator coil.
Sometimes the leak is very slow and takes a year or more to leak out. Regardless the homeowner will have a decision to make.
Reasons Why An Air Conditioner Compressor Isn’t Running Reason # 10
The compressor may have overheated and isn’t starting because of the internal temperature switch. If you feel a lot of heat coming off the inside of the system then the compressor may be overheated.
If this is the case obviously something is wrong and it needs to be addressed before running the system again for a prolonged period of time. I wanted to go over the procedure to get it cooled down just so technicians can determine if the compressor will start back up.
An internal temperature switch will open preventing power to the compressor coils.
When the compressor overheats to that extent then water from a garden hose needs to run over the top of the compressor for about 15 minutes to properly cool it down.
When the compressor cools down to a safe temperature the internal switch will close and when power is restored to the system it will come back on.
Under normal operation, the compressor will be very warm but it won’t be so hot that it’s radiating heat off of it. If you sense a lot of heat coming out of the top of the condenser then you can turn the thermostat off and just wait a few hours.
Hopefully one of the 10 reasons I listed helped you troubleshoot the reason the compressor will not come on. Please leave a comment below.
* WARNING! This post and information on this website are for trained and certified technicians only. It is for educational purposes only. Under no circumstances should anyone who is not qualified or have the required permits or licensing attempt to use any information in this post to try and repair an HVAC or electrical system on their own. The opinions and methods I use are my opinions and I give no guarantee of any type of outcome. In other words, if you are not a trained and certified technician you should not attempt any repairs on your own. You should hire a licensed professional to do the work.
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