When your air conditioner stops working, it may seem like there are infinite reasons why. However, a blown fuse is a common cause and can be simple to assess. How can you tell if your AC fuse is blown? Locate the fuse behind the unit’s access panel and perform a visual inspection or multimeter testing. Learn how to check AC fuse to troubleshoot and solve the problem.

How to Check Fuse When Your AC Won’t Work

Like many fuses, an AC fuse is a safety device that blows if the unit is in danger of receiving too much power. The fuse absorbs this change in power and blows to prevent more serious damage to the unit. We’ll detail how to check AC fuse and what happens when it blows but, first, here’s where to find it in your unit.

Where is the AC Fuse Located?

Where is the AC fuse on most HVAC systems? They’re typically in the unit’s control board. This board can often be found behind an access panel on the unit’s lower compartment. While some panels must be unscrewed to access the control board, others have clips that are lifted to remove the panel. The fuse is usually inserted directly in the control board and labeled with an amp, or voltage, rating.

What Happens When the AC Fuse is Blown?

On some units, when the AC fuse is blown the blower will blow continuously, regardless of your home’s temperature and thermostat setting. However, on most AC units, all function will cease when the fuse is blown. This is because all the thermostat’s low-level voltage controls are processed through the fuse. These commands can’t be processed if the fuse blows and the unit will stop running.

How to Test a Fuse With a Multimeter

A fuse conducts current from one side to the other via a metal bridge. When there is too much current, this bridge collapses. A fuse is “blown” when the collapsed metal bridge is unable to conduct current. 

How to Check AC Fuse

In addition to other measurements, a digital multimeter can gauge if there is continuity between the two connection points of the fuse’s metal bridge. When measuring for continuity, the multimeter’s digital display will show a radio wave symbol. 

Here’s how to check AC fuse with a digital multimeter:

  1. Make sure the multimeter is operating correctly by touching the two probes together. If the multimeter makes an audible sound and shows an initial reading, it’s in proper working order.
  2. Place the probes on the fuse’s specific connection points. Where you place the probes depends on the type of fuse your unit uses.
  3. An audible sound and a digital reading similar to the initial one indicates that the fuse’s connection points have continuity and the fuse is operational. What if there is no sound when the probes are applied? If there’s no sound, no reading or a reading of “OL,” on the multimeter’s display the fuse is blown and requires replacement.

How to Test a Fuse Without a Multimeter

You can also test a fuse by performing a visual inspection. If a black flash mark is visible inside the fuse or its window appears obscured, it’s likely blown. Wondering how to check AC fuse if you can’t see inside it and don’t have a multimeter? In this case, we suggest removing the fuse and replacing it with an exact model. Take care to provide an exact match to avoid overdrawing the circuit or blowing the fuse again.

While an AC fuse can blow if it malfunctions, a blown fuse can also indicate a larger issue with your unit. Even if you know how to check AC fuse, you’ll likely need a professional air conditioning repair if it blows again. In this case, contact Dependable Heating & Air to handle any HVAC repair.