How to Tell if Your AC Fuse Is Blown [All Types]

Did your AC unit suddenly stop working and now it seems like it doesn’t have any power at all?

It could be due to a blown fuse.

AC units blow their fuse when a component goes bad – such as a capacitor, fan motor, or compressor.

In other cases, bad wiring can also cause an AC fuse to blow.

In this article, I’ll show you how to tell if your AC fuse is blown.

I’ll also go over 3 different types of fuses – so you know how to tell if any type of AC fuse is blown.

How to Tell if Your AC Unit’s Fuse Is Blown

Cartridge fuses are used in most disconnect switches for AC condenser units. If your condenser unit stops running, then one or both of the cartridge fuses inside the disconnect could be blown.

To check your AC unit’s fuse, follow these steps:

  1. Turn off the circuit breaker that goes to your AC.
  2. Open the cover to the AC unit disconnect switch.
  3. Pull the handle in the disconnect to remove the fuses.
  4. Use your multimeter’s resistance or continuity setting to test the fuses.
Location of cartridge fuses in an AC condenser disconnect
The fuses in an AC condenser disconnect are hidden behind the handle
A good fuse will have a very low resistance rating, around 0 or 1 ohms. A blown fuse will show ‘OL’, which stands for ‘Open Line”

How to Tell if Your AC Control Board Fuse Is Blown

Control boards in ACs usually use a blade fuse or, in some cases a glass cartridge fuse.

Fortunately, both of these types of fuses have visual indicators to tell whether they are blown.

How to Tell if an AC Control Board Blade Fuse Is Blown

Blade fuses are the types of fuses that most people are familiar with since they’re also used in automobiles.

A blade fuse consists of a plastic case with two metal prongs sticking out that plug into your control board.

If a blade fuse is blown, you can look at the center of the plastic casing on the fuse to check on the melting element:

  • If there is a black smudge in the center of the plastic case, then the fuse is blown.
  • If the element is not connected in the center, then the fuse is blown.
Visual comparison of good versus blown blade fuse for HVAC system control board
Here is a comparison of a good versus bad blade fuse for an AC control board. The element for the good fuse remains intact. The element for the blown fuse is burned away.

How to Tell if an AC Control Board Glass Cartridge Fuse Is Blown

Glass cartridge fuses aren’t as common as blade fuses in control boards. But they are used in some control boards, so I thought I’d include them here.

Glass cartridge fuses have a transparent glass tube that the melting element goes through. The melting element in a glass cartridge fuse is visible, so you can see whether the fuse is good or blown.

Visual comparison of good versus blown glass cartridge fuse for HVAC system control board
In a glass cartridge fuse, you can clearly see if the element is melted through the glass tube.

What to Do When Your AC Fuse Is Blown

If your AC’s fuse blows, the first thing that you need to do is figure out what caused the fuse to blow.

Fuses don’t just blow for no reason – there’s always a cause.

In fact, a fuse is a safety device – if the fuse doesn’t blow when a component goes bad, then your AC unit will burn out or catch fire.

Whatever you do – Do not just replace a bad fuse with a new fuse without fixing the AC unit first.

What you need to do is pinpoint the cause of the blown fuse.

Here are a few reasons why an AC might blow its fuse:

I’ll go over these causes below.

AC Fuse Blown Because of Bad Capacitor

The capacitor in your AC unit is used to provide an initial “jolt” of electricity to start the electric motors in your AC unit.

If your AC unit’s capacitor is bad, the compressor or condenser fan will draw too much electricity and blow a fuse inside the disconnect.

If your AC unit’s fuse is blown, it could have a bad capacitor that needs to be swapped out.

If you have a multimeter, you can test your AC capacitor to see if it’s bad.

AC Fuse Blown Because of Bad Condenser Fan Motor

A bad AC condenser fan motor is another cause of a blown fuse in an AC unit.

A condenser fan motor that’s burned out or seized up will overdraw amps and burn out its fuse.

If you suspect that your AC condenser fan motor is bad, then you can test the motor to see if it’s working properly.

AC Fuse Blown Because of Short Circuit in Wiring

A short circuit in your AC’s wiring will blow fuses instantly.

A short circuit happens when an exposed piece of wire touches another wire or a piece of metal in your AC unit.

Short circuits can also happen if a component in your AC unit is miswired.

AC Fuse Blown Because of a Bad Compressor

A bad AC compressor will almost always cause your AC unit’s fuses to blow.

The compressor is the “heart” of your AC unit and is the primary driver of the refrigeration process that cools your home.

If your compressor’s windings short circuit or burn out, then your AC compressor won’t turn on and will blow fuses instead.

A locked-up compressor eventually happens after years of wear and tear, and will also cause fuses to blow.

If the compressor in your AC goes bad, in most cases you should just get a whole new AC unit.

Hi, my name is Trey Lewis and I’m the founder and chief editor at HVAC Training Shop. My goal for this website is to help homeowners troubleshoot and maintain their home’s HVAC systems. Whether it’s changing an air filter, troubleshooting a blower motor, or just buying a new humidifier, I want to make sure that you’re covered.

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