If your ac keeps blowing the 3 amp fuse this article will show you how to find the problem and fix it. I will list out the most common problems first because that is what they typically are. If the simple issues don’t solve the problem then you will need to read the next section and follow the steps.
- Disclosure – Read the disclosure at the bottom of this post.
This first section is going to be how to troubleshoot the reason the ac keeps blowing the 3 amp fuse by using a simple tool.
If you don’t have this tool or don’t have the time to order it then just skip ahead to section number 1.
If you have the luxury of ordering the tool and waiting until it arrives in a couple of days then read this section. You can CLICK HERE to order the short pro tool.
Use a simple tool to troubleshoot 3 amp fuse blowing.
Table of Contents
Here are the steps for troubleshooting with the short pro tool to figure out why your AC keeps blowing the 3 amp fuse.
A. First remove the power to the unit.
The rest of the numbered sections below will be how to troubleshoot your 3 amp fuse blowing by doing some simple steps without any specialized tools. You will need basic hand tools to replace the part when you find it.
B. If the system has a 24v fuse on the transformer’s output, attach the short pros leads to the terminals where the blown fuse was.
C. For plastic-style fuses remove the fuse and simply place one end of the alligator clip on the fuse holder contacts.
D. For inline fuses open the fuse holder and remove the fuse. Place each alligator clip on one of the fuse holder contacts.
E. Place the system thermostat in the OFF mode and Fan in the auto mode.
F. Now power the system up. If the LED light on the Short-Pro tool is lit up then you know you have a short.
G. Finding exactly where the short is located is now just a matter of removing circuits (WIRES) until the LED light goes out. Start with the wiring going to the thermostat. Remove the HOT wire from the board or terminal strip. This is almost always RED. If the LED goes out on the tool then you know the problem is somewhere down that line. You can further isolate it by reconnecting the wire that you just took off and removing connections further down the line, like at the thermostat itself.
The short pro tool is amazing at helping you find the reason the 3 amp fuse is blowing.
But regardless if you have the tool or not you may still want to just do a quick visual of the items I have listed below to see if you can find the problem.
1. One of the low voltage wires may be shorting out causing the 3 amp fuse to blow.
CHECK THE OUTSIDE CONDENSER
This is one of the most common issues that cause the fuse to blow on the inside circuit board. Go outside to the location of your condenser. You will see the brown or tan colored wire coming out of the bottom of the electrical compartment of the condenser.
If the wire is exposed and fully visible you will easily be able to trace it from the condenser all the way to the location it enters the home. Check the wire carefully to ensure the outer coating of the low voltage wire is intact.
Some times it’s very obvious and you can see exposed wires or wire nuts and the insulation will be dry rot and having the copper of the low voltage wire fully exposed.
If the copper wire touches one of the other copper wires it will short out the fuse. If the copper wire is exposed and touches the copper line or the shell of the condenser it will also short out the fuse.
Some times the low voltage wire will come out of the condenser and be hidden under the line-set insulation and or tape.
If this is the case you will need to carefully cut the tape and insulation so you can check the whole wire. I have seen quite often there is a connection under the insulation and when I expose it that is where it is shorting out.
If you find the low voltage wire at the condenser is the reason your 3 amp fuse is blowing here is how to repair it.
When you locate the exact location on the wire what you want to do first is make sure you turn off the power to the inside air handler or furnace. You also want to turn off the power to the outside condenser.
Typically this is very obvious. Normally there is a disconnect switch by the outside condenser and there is a breaker on the air handler or another disconnect by the air handler.
If it’s a furnace it will most likely have a switch just like a light switch.
When you have both the inside and the outside turned off you are ready to make the repair. If you are in doubt about the power being on or off you can always turn off the main breaker to the whole home.
If the wire is just weathered and the copper was exposed and shorted out here is what you do.
Simply cut the outer insulation back with a utility knife and separate each of the individual wires. Wrap tape around those wires individually making sure none of the copper is exposed. after you do that wrap all of the wires with electrical tape to ensure no water will get to the wires.
Make sure to insert another 3 amp fuse in the board.
Don’t put any of the covers back on the air handler yet. You still need to test the system.
Turn all of the power back on and verify that the fuse didn’t blow right away. If the fuse didn’t blow then turn the thermostat in the cooling mode first and then the heating mode.
Verify that everything is functional and then you can put the covers back on.
That should have solved the reason the 3 amp fuse was blowing on the ac system or in this case to be more specific the 3 amp fuse was blowing on the air handler circuit board.
You may also find the low voltage wire damaged in an attic or in a basement.
The point is to check the low voltage wire in places that are easy to see or that may be prone to physical damage.
I have seen rodents, squirrels, and items people put in the attic to be the reason the wire is damaged.
The key point to remember when doing this is to look closely at as much of the wire as possible. It shouldn’t take more than 3-10 minutes and you may have the 3 amp blown fuse problem solved.
You can do a visual with no special tools and it’s very quick. Moving on to the next possible reason your AC keeps blowing the 3 amp fuse.
2. The low voltage wires are connected wrong and the 3 amp fuse is blowing
If the system was working and all of the sudden just stopped working you don’t need to worry about reading section 2.
However, if you changed the thermostat of any other part inside the air handler or on the condenser you will need to verify all of the wires are in the correct position and the color codes are correct.
This would only apply if you were messing around with the small low voltage wires. This would not apply if you were messing with the 120v or 240v wires feeding the equipment.
Hopefully, you took some pictures with your phone before removing the wires and can just backtrack and look at your picture to connect everything correctly.
3. A bad thermostat with a direct short is causing the 3 amp fuse to blow
I have seen the back of the circuit board on a digital thermostat have a direct short in them. When this happens it’s easy to verify if this is the cause.
I know you may be thinking well if the back of the thermostat is burned then the whole thing would not work. However, I have seen thermostats blow the fuse on specific settings and when the thermostat is sending a specific function to start and stop the equipment. This is rare but it does happen.
First, you need to ask yourself is my thermostat is digital. If the answer is yes then proceed.
grab the faceplate of the thermostat and pull it off the wall. Looking at the back of the soldered circuitry make sure there are no burn marks. Also, make sure you don’t smell anything like a burned smell.
Do a really good visual and just make sure everything looks good. If the thermostat has batteries it should still be lit up and appear to be working correctly.
4. The contactor is bad which is causing the 3 amp fuse to blow
Sometimes the contactor is burned up and bad. Typically you can do a quick visual to ensure the contactor is in good condition and fully functional.
You will have to take off the access cover to the electrical components on the condenser. When you have it off shine a flashlight at the coil pack on the back of the contactor. The coil pack should be shiny copper and have no burn marks on it.
If you see any burn marks or smell any type of electrical burn smell then the contactor is most likely the reason the ac keeps blowing the 3 amp fuse. Depending on where you live in the country some supply stores will sell you the contactor.
But because of liability reasons, you may have to order it online.
5. The board is bad and the 3 amp fuse keeps blowing.
You may have a bad circuit board in your air handler or furnace. The way I test if the circuit board is bad is I remove all of the wires from the circuit board except the 24 volts feeding the board power.
With a fresh fuse in or a tester turn the power on. if the board blows the fuse right away then the board is bad.
Typically you will see some burn marks on the front or the back of the board. You may also smell an electrical burning type of smell.
If this is the case you will have to get your model and serial number off the equipment and order the exact board your specific unit needs.
If you order anything through the link on my page I will make a small affiliate commission. It’s not much at all but helps me have the time to continue providing this content.
I hope this post and or video has helped you find and solve your problem. If this post has helped you please take a minute out of your busy schedule and leave me a comment. Thanks.
DISCLOSURE – WARNING! This post is 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 exit this website.
Income & Affiliate Disclaimer. If you purchase a product or service from one of the links on this page you should assume I earn an affiliate commission. Meaning I earn money because you purchase through one of my links. You should also assume that by clicking on my links you may receive email communications from me.