If your multimeter suddenly stops reading current or shows “OL” (open loop) on the amperage setting, the most likely cause is a blown internal fuse. You can confirm this in under two minutes with a visual check or a simple continuity test. Most digital multimeters contain two internal fuses, and figuring out which one failed is straightforward once you know what to look for.
Why Multimeter Fuses Blow
The fuses inside your multimeter exist to protect the meter (and you) from excess current. The most common way to blow one is accidentally leaving the test leads in the amperage jack while trying to measure voltage, or attempting to measure current in a circuit that exceeds the fuse’s rating. It happens to beginners and experienced electricians alike. The meter won’t give you an error message that says “fuse blown.” It just stops responding on certain settings, which is why people end up searching for answers.
Symptoms of a Blown Fuse
A blown multimeter fuse produces very specific behavior. Your meter will still power on and the display will work normally, but certain functions will stop giving readings. If the fuse for the milliamp/microamp jack is blown, you’ll get no reading (or “OL”) when you try to measure small currents. If the fuse for the 10A jack is blown, the high-current amperage function won’t work. Voltage and resistance measurements typically still function fine, because those functions don’t route through the same fuses.
If your multimeter is completely dead and won’t turn on at all, the problem is almost certainly a dead battery, not a blown fuse.
Visual Inspection
Start by opening the back of your multimeter. Most meters use small Phillips screws to hold the back panel in place. Remove the battery cover and back panel to expose the circuit board, where you’ll find one or two cylindrical fuses seated in clips or holders.
If your multimeter uses glass fuses, a blown fuse is often obvious: the glass tube will be discolored (darkened or cloudy) and the thin metal filament inside will be visibly broken or melted. A good glass fuse has a clean, intact wire running from one metal cap to the other.
Ceramic fuses are harder to diagnose visually because the opaque body hides the filament. Ceramic fuse casings range from light beige to dark gray, and a blown one looks identical to a good one from the outside. For ceramic fuses, you’ll need to test with a second meter or use the self-test method below.
Testing With a Second Multimeter
This is the most reliable method. Remove the suspect fuse from your multimeter and grab a second meter (borrow one if you need to). Set the second multimeter to continuity mode, which is usually marked with a small speaker or sound-wave icon on the dial. Touch the two probes together first to confirm you hear a beep, which tells you the second meter is working correctly.
Then place one probe on each metal end cap of the fuse. Press firmly to get solid contact. If the fuse is good, you’ll hear a continuous beep. If you hear nothing, the fuse is blown.
You can also use resistance mode (ohms) instead of continuity. A good fuse reads 0 ohms or very close to it. A blown fuse reads “OL” or infinity, meaning there’s no electrical path through it.
Self-Testing Without a Second Meter
If you only own one multimeter, you can still test the fuse, but you’ll need to remove it from the meter first. Set your multimeter to continuity or resistance mode, then touch the probes to the two ends of the removed fuse just as described above. This works because the continuity and resistance functions route through the meter’s internal circuitry differently than the amperage function. They don’t depend on the same fuse. So even if your amp fuse is blown, the ohms/continuity setting still works and can test the removed fuse.
If both fuses are blown, this method won’t help you confirm the second one. You’d need to replace the first fuse, then test the second.
Which Fuse Is Which
Most digital multimeters have two fuses. One protects the milliamp/microamp current input (commonly rated at 200mA to 500mA). The other protects the high-current 10A input and is typically a larger, beefier fuse rated at 10A or higher. The fuses are usually different physical sizes, making them easy to tell apart. Check the markings printed near each fuse holder on the circuit board, or consult your meter’s manual to confirm which is which.
Choosing the Right Replacement Fuse
This is where people make costly mistakes. You need to match four things exactly when replacing a multimeter fuse:
- Amperage rating: The number printed on the fuse’s end cap (like 0.5A or 10A). This determines how much current the fuse can handle before it blows.
- Voltage rating: Also printed on the fuse or listed in the manual. Multimeter fuses often carry higher voltage ratings than household fuses, sometimes 600V or more.
- Physical size: Common sizes include 5x20mm and 6.3x32mm. The wrong size simply won’t fit the holder.
- Blow type: Fast-blow fuses cut the circuit the instant current exceeds the rating. Slow-blow fuses tolerate brief surges before tripping. Your manual specifies which type your meter requires.
Never substitute a higher-amperage fuse or a standard household fuse. Multimeter fuses, particularly in safety-rated meters, have high interrupt ratings (10,000 amps or more in Fluke meters, for example). These specialized fuses are designed to safely interrupt a fault from a high-energy circuit. A cheap generic fuse may not interrupt cleanly during a serious fault, which creates a real safety hazard. Replacement fuses from your meter’s manufacturer typically cost between $5 and $15, and that’s a worthwhile investment.
The amperage, voltage, and blow type are usually printed directly on the fuse body or on a label near the fuse holder inside the meter. If the markings have worn off, your owner’s manual or the manufacturer’s website will list the exact specifications.