How to Read a Fuse Box Diagram and the Lid Legend

Here is how to read fuse box diagram charts in one paragraph. Every position in the box has a number. The diagram, printed on the lid or in the owner's manual, pairs each number with two things: the fuse rating in amps and the circuit or component that fuse protects. Hold the lid or the chart the same way round as the box, find the number, and read across. If the lid uses symbols instead of words, the owner's manual's fuse chart is the key that turns each symbol into a component name.

That is the whole method. The rest of this guide covers the parts that trip people up: symbols with no words, rows with no component, "some models" entries, and a lid that does not seem to match the box. The examples below come from Mazda's and Ford's published owner's manuals, read on September 25, 2026.

Where the diagram actually lives

Most cars have more than one fuse box, and each has its own diagram. Mazda's 2021 Mazda3 owner's manual, for example, lists a fuse block in the engine compartment and a second one on the left side of the cabin, each with its own numbered table.

For each box there are usually two versions of the diagram:

• The lid legend. A small printed map on the inside of the cover, or on the box itself. It is right there when you need it, but it is compact, so it often uses abbreviations or pictograms and may leave out the component names.
• The owner's manual chart. The same positions written out as a table, usually with the full component name for each position. This is the version that explains the lid.

If you do not have the manual, most makers publish it free online. Our list of where to get your owner's manual free, by manufacturer shows each maker's source.

How to read fuse box diagram columns, one at a time

The owner's manual chart is a table, and every table reads the same way once you know the columns. Mazda's 2021 Mazda3 manual labels them plainly: No., FUSE RATING and PROTECTED COMPONENT. Here are four rows from that manual's engine compartment table, quoted as printed.

The number is the position in the box. On the Mazda it carries an F prefix. Other makers use plain numbers, letters, or a grid. The same number is usually molded into the box next to the slot, which is how you match paper to plastic.

The rating is the fuse's current rating in amps. It is the figure the replacement must match. It is not a measure of how much power the circuit uses, and it is not a suggestion.

The protected component is what loses power when that fuse blows. It can be a single item (the driver's door lock) or a whole system (engine control).

Read across one row at a time. A common mistake is to read down the rating column and assume the fuse beside the one you want is yours.

Reading the lid legend when it uses symbols

The lid is where people get stuck, because space is tight and many lids print pictograms or short codes rather than words. A typical version of the question: the lid has symbols, not words, so what is fuse 14?

Work it in this order:

1. Get the number from the box, not the lid. Find the slot you care about and read the number molded beside it.
2. Find that number on the lid. Hold the lid the same way round as the box. If the lid has a printed arrow, a "front" marking or a latch outline, line that up first.
3. Translate the symbol with the manual. Find the same number in the owner's manual chart for that box and read the component name.

Mazda's own table shows why step 3 matters. "S-VT" in the rows above is a maker's abbreviation, not a word most owners would recognize. The manual is the only place that pairs an abbreviation like that with the right box and the right position, and the manual's glossary or the dealer can expand it.

If the lid and the manual disagree on a position, trust the manual for your exact model year, then check the section on mismatches below.

What blank rows and "Some Models" mean

A fuse chart is written for every version of a model, not just yours. That creates three kinds of row that look like errors but are not.

A position with no rating and no component. Mazda's 2021 Mazda3 left side table shows positions F1, F2 and F3 with no rating and no component. That usually means the slot is empty on this model. Nothing is missing.

A rating with no component. In the engine compartment table, F6 is listed at 20 A with no component named. The manual is telling you the rating that belongs there without naming the circuit. You still replace like for like at the listed rating. If you need to know what it feeds, that is a dealer or service manual question. Our piece on owner's manual vs service manual explains which document carries the full circuit detail.

An entry marked "Some Models". F2 in the example protects a windshield wiper de-icer on "Some Models". If your car does not have the feature, the slot may be empty or the fuse may be there and feeding nothing you use. Whether your car has it comes down to trim and options. Our guide on how to tell what trim your car is helps you pin that down, since badges are not always reliable.

Fuses, relays and spares in the same box

Not everything in a fuse box is a fuse, and not every fuse is the same shape.

• Relays are usually larger plug-in blocks. They are electrically operated switches, not fuses, and the diagram gives them their own positions or their own list. Read their labels the same way, but do not treat a relay position as a fuse rating.
• Spare fuses may sit in marked slots with nothing wired to them. The diagram usually labels them as spares.
• A fuse puller may live in the box. Mazda's 2021 CX-9 manual says to use the fuse puller provided on the fuse block in the engine compartment.
• Different fuse formats. A single vehicle can use more than one fuse size. Ford's 2022 Motorhome chassis owner's manual, for example, lists eight fuse types, including Mini, Micro 2, Micro 3, Maxi and several cartridge styles.

When the diagram shows a much larger position, it is often a high-current fuse or a relay. Those are not always owner-serviceable, which the last section covers.

When the lid does not match your car

Sometimes the positions on the lid do not line up with what you see, or the rating on the lid differs from the fuse in the slot. Four reasons account for most of it.

1. The chart is for a different model year or build. A chart from a different year, or even from before a mid-year change, can be wrong for your car. Our explainer on model year vs build date covers why.
2. You are reading the wrong box's chart. With two or three boxes, it is easy to hold the engine compartment chart over the cabin box.
3. Someone fitted the wrong fuse. A previous owner may have put a different rating in the slot. The chart is the reference, not the fuse you find there.
4. The lid itself was swapped. Lids are separate parts and can be replaced with one from a different vehicle.

In every case the fix is the same: read the owner's manual chart for your exact vehicle, and if that still does not settle it, ask a dealer parts or service department to confirm from the VIN.

Before you pull anything

Reading the diagram is safe. Acting on it needs a few rules from the maker's own instructions.

• Switch off first. Mazda's 2021 CX-9 manual says to make sure the ignition is switched off, and other switches are off, before opening the fuse panel cover.
• Match the rating exactly. The same manual says to insert a new fuse "of the same amperage rating", and Ford's owner's manual warns that "using a fuse with a higher amperage rating can cause severe wire damage and could start a fire." You can read that warning on Ford's online owner's manual page for changing a fuse (read September 25, 2026).
• Leave the main fuse alone. Mazda's manual says not to replace the MAIN fuse yourself and to have a dealer do it, because it is a high-current fuse and incorrect replacement could cause an electrical shock or a short circuit resulting in a fire.
• A fuse that blows twice is a fault, not a fuse problem. Mazda's 2021 Mazda3 manual says that if the same fuse blows again, avoid using that system and consult a dealer as soon as possible.

You can see how Mazda lays out the full tables on the 2021 Mazda3 owner's manual fuse panel page (read September 25, 2026).

Frequently asked questions

Where is the fuse box diagram in my car?

Usually in two places: printed inside the fuse box lid, and in the owner's manual's fuse section. Many cars have more than one box, commonly one under the hood and one inside the cabin, and each has its own diagram.

What do the numbers on a fuse box diagram mean?

Each number is a position in the box. The diagram pairs it with the fuse rating in amps and the component that fuse protects. The same number is usually molded into the box beside the slot.

Why does my fuse box diagram show a rating but no component?

The manual is giving you the correct rating for that position without naming the circuit. Mazda's 2021 Mazda3 manual does this for engine compartment position F6, listed at 20 A. Replace like for like and ask a dealer if you need to know what it feeds.

What does "Some Models" mean on a fuse chart?

The chart covers every version of the model. A "Some Models" entry only applies if your car has that feature, which depends on trim and options.

The lid and the manual disagree. Which is right?

Use the owner's manual chart for your exact model year and box. If they still disagree, a dealer can confirm the correct layout from your VIN.