Blown Fuse: Short Circuit or Overload?

A blown fuse is a clue. Before you fit a new one, look at how the link broke: a small, clean break and a burnt, blackened one point to two very different problems, and only one of them is fixed by a new fuse.

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How it works

  1. Pull it and lookHold the fuse up to the light and look through the casing at the link.
  2. Read the breakA clean gap usually means over current. Burn marks or debris mean a dead short.
  3. Fix the cause firstThen fit a new fuse of the same rating. Never a bigger one.

Read the break

Hold the fuse up to the light and look through the casing at the link, the thin metal strip that runs between the blades. It breaks in one of two ways, and each one points to a different problem.

A blown Mini fuse with a broken link next to a good fuse with its link intact
Left: blown. The link inside has broken. Right: good. The link runs unbroken from blade to blade.
Good

Link intact

The fuse isn't the problem. Some fuses crack so finely you can't see it, though, so if the circuit is still dead, check the fuse with a test light or multimeter before you move on.

Over current

A small, clean break

A neat gap in the link, with a clean, clear casing, generally means over current, not a short. The circuit drew more than the fuse is rated for, for long enough to melt the link at its narrowest point.

Common causes: an accessory added to a circuit that was already near its limit, a motor that's wearing out and drawing more (blower, wipers, a window or seat motor), or a fuse that's too small for the job.

Dead short

Burnt link, burn marks or debris

When the link is burnt away and there are black burn marks or bits of metal on the inside of the casing, that's a dead short: a wire or part touching ground. Short circuits are explosive in nature, and the fuse shows it.

Common causes: a wire chafed through where it passes bare metal, a wire pinched under a panel, seat or trim screw, water in a connector, or a failed part.

What to do when a fuse blows

The same four steps whatever the circuit.

1. Switch off and pull it

Turn the key off and pull the fuse with a fuse puller. The fuse box lid or your owner's manual shows which fuse runs what.

2. Read the break

A clean gap or a burnt, blackened link? That tells you whether you're looking for too much load or a short.

3. Fix the cause

For an overload, take load off the circuit or repair the failing part. For a short, find where the wire touches ground before you go any further.

4. Fit the same rating

Use the same type and amp rating. Never fit a bigger fuse to stop it blowing: the fuse protects the wire, and a bigger one lets it overheat.

Before you fit a new fuse

Intermittent shorts

A clean break isn't always over current

Sometimes an intermittent short, like a wire that only touches ground over bumps, when a door opens or when the steering turns, leaves a clean break too. More often than not, though, short circuits are explosive and blow the link apart.

If a new fuse blows straight away, or every time something moves, treat it as a short.

Test light

Check a fuse without pulling it

Most blade fuses have two small test points on top. With the circuit switched on, touch a test light to each one. If it lights on both, the fuse is good. If it lights on one side only, the fuse is blown.

Our advice

Look at the blades too

Corroded or discoloured blades, or melted plastic around them, point to a poor connection heating up in the fuse box. Clean or replace the terminal, not just the fuse.

Safety

Never fit a bigger fuse

A fuse is sized to protect the wire behind it. Fit a bigger one and the wire becomes the weakest link, which is how wiring fires start. If the right fuse keeps blowing, the circuit has a problem to find.

Free tool

Not sure which fuse you have?

Our fuse identifier shows every common car fuse to scale, with its other names, size and colour code, and the fuse tap that fits it. You can even hold yours against the screen at true size.

How to identify car fusesShop fuses

Common questions

Why does my car fuse keep blowing?

Either the circuit is drawing more than the fuse is rated for, or there's a short. Look at the old fuse: a clean break points to too much load, and a burnt link with black marks points to a short. Never fix it with a bigger fuse.

What does a burnt or black fuse mean?

A dead short. When a wire or part touches ground, the current spikes and the link burns away violently, leaving black marks or metal debris inside the casing. Find the short before you fit a new fuse.

What does a clean break in a fuse mean?

Generally over current, not a short. The circuit drew more than the fuse is rated for, long enough to melt the link. An intermittent short can sometimes leave a clean break too, so if a new fuse blows straight away, treat it as a short.

Can a blown fuse look normal?

Yes. A fine crack in the link can be hard to see. Test it: with the circuit on, a test light should light on both test points on top of the fuse. If it only lights on one side, the fuse is blown.

Can I put in a bigger fuse?

No. The fuse protects the wire. A bigger fuse lets the wire overheat instead of the fuse blowing. Always use the same type and amp rating.

How do I know which replacement fuse to buy?

Match the type and the number stamped on top. Not sure of the type? Our fuse identifier shows every common car fuse at true size.

Not sure what you're looking at?

Text us a clear photo of the blown fuse and tell us what it powers. We'll tell you what we see.

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