How do you tell if a fuse is blown?
The radio's dead. Or the 12-volt socket, or the wipers, or the dome light. One thing, all the way off, and everything else in the car is fine. That pattern is a fuse until proven otherwise. Here's how to tell if a fuse is blown in about ninety seconds: look first, test second, and only then start pricing out switches.
Applies to
- Process
- Automotive 12 V electrical diagnosis
- Materials
- Blade fuses (mini, low-profile mini, standard ATO/ATC, maxi) and older glass tube fuses
- Setup
- Fuse puller, phone flashlight, multimeter or 12 V test light
- When
- First step when one circuit goes completely dead
- Not for
- Hybrid and EV high-voltage circuits (orange cables) — that's shop work
Why it matters
A fuse is the cheapest part in the circuit and it's built to be the part that fails. It's a deliberately skinny strip of metal that melts before the wire behind it gets hot enough to burn. Checking it first saves a lot of people from replacing a switch, then a motor, then the fuse anyway.
The tell is a complete failure. A fuse doesn't make a window slow or a radio crackly. It's either passing current or it isn't. So when something is totally dead, and especially when two unrelated things died at the same moment (say the radio and the power socket), they probably share a fuse.
The blown ones aren't always obvious, though.
A hairline break in the link reads as intact until you get light behind it. A low-profile mini barely shows you the link at all. That's why the test step exists.
What good looks like
- Good: one continuous metal strip, with light visible around it
- Good: clean plastic body, no smoke film, no browning on the blades
- Good: continuity beeps across the two blades, or reads close to 0 Ω
- Good: a test light glows on both test points with the key on
- Blown: a gap in the strip, usually at the narrow middle section
- Blown: a dark or silvery smear inside the body, which means the link vaporized on a hard short
How to apply it
- Turn the ignition off before you pull or push in any fuse.
- Find the right fuse box. Most cars have one inside (under the dash, behind a kick panel, or on the end of the dash with the door open) and a second under the hood near the battery.
- Find the fuse. The diagram is usually on the back of the lid or in the owner's manual. Match the circuit name to the slot.
- Pull it with the plastic puller that's often clipped inside the lid. Pliers crack blade fuses and slip on minis.
- Backlight it with a phone flashlight and look at the link through the side of the body.
- If you can't call it, test it (below) instead of guessing.
- Replace like for like: same type, same amp rating, read off the number on top. Never a higher one.
- Turn the key on and check the circuit works. If the new fuse blows right away, stop. There's a short.
Test it on your setup
- Multimeter, fuse out: set it to continuity (or the lowest ohms range) and touch one probe to each blade. A beep or a reading near 0 Ω is good. No beep, or OL on the screen, is blown.
- Test light, fuse in: clip the ground lead to bare metal or the battery negative, turn the key to on, and touch the probe to the two small metal test points on top of the fuse. Light on both sides is a good fuse. Light on one side only is a blown one.
- Light on neither side doesn't mean the fuse is bad. It means that slot isn't live in that key position, or its feed is dead upstream. Prove the light on a fuse you know works, then try another key position.
Exceptions & tradeoffs
A fuse that blows again the second you turn the key is telling you something. Don't keep feeding it fuses. Something is touching ground that shouldn't be (a pinched wire, a wet connector, a dying motor) and the fuse is doing its job.
Mind you, a fuse that blows once and then holds for months was usually a one-off: a coin in the power socket, a cheap charger, a wiper frozen to the glass on a cold morning.
Never put in a higher-rated fuse to "stop it blowing". The fuse protects wire sized for that rating. Go up and the next weak point is the wiring harness, behind the dash, where you can't see it smoke.
Hybrids and EVs run a separate high-voltage system on orange cables with its own fuses. Everything here is the ordinary 12 V fuse box only.
Common mistakes
- Judging a fuse by eye in a dim footwellGet light behind it, or put a meter on it
- Fitting a 20 A where a 10 A wasMatch the original rating exactly — the wiring is sized for it
- Checking the inside box when the circuit lives under the hoodCheck both boxes; the lid diagrams say which circuits are where
- Trusting the colour on a faded or off-brand fuseRead the number stamped on top; colour is only a hint
- Calling a fuse bad because the test light stayed dark on both sidesDark on both sides means no power to the slot. Prove the circuit is live first
Frequently asked questions
- What does a blown fuse look like?The metal strip inside has a gap in it, usually at the narrow middle section. A fuse that took a hard short can also have a dark or silvery smear inside the plastic, or browned blades. A good fuse shows one continuous strip with light around it.
- How do you tell if a car fuse is blown without pulling it?Use a 12 V test light or a multimeter on the two metal test points on top of the fuse, with the key on. Power on both points means the fuse is good. Power on one side only means it's blown. No power on either side means that circuit isn't live right now.
- Can you test a fuse with a multimeter?Yes. Pull the fuse, set the meter to continuity or the lowest ohms range, and touch a probe to each blade. A beep or a reading near 0 Ω means it's good. No beep, or OL, means it's blown.
- What do the fuse colours mean?On standard and mini blade fuses the colour marks the amp rating: 5 A tan, 7.5 A brown, 10 A red, 15 A blue, 20 A yellow, 25 A clear, 30 A green. Maxi fuses reuse some colours for different ratings, so always read the number stamped on top.
- Can I drive with a blown fuse?It depends on the circuit. A dead radio or power socket won't stop you. A blown fuse for the headlights, brake lights, wipers, or anything engine-related is a reason to fix it before you drive.
- Why does my new fuse keep blowing?Something in that circuit is drawing too much current or shorting to ground: a pinched or chafed wire, a wet connector, a failing motor, or an accessory pulling more than the circuit was built for. A bigger fuse won't fix it. Finding the short will.
Inspecting the fuse link and confirming with a continuity check — Test A Fuse?! Nooo Waaay! ↗ at 0:17–0:40.
Credit to The No Way Guy for the clip that prompted this one. The look-then-test method is the standard fuse check; the expanded steps, test-light method, colour guide and FAQ are Start Here's editorial notes, not a quotation from the creator.