Reading Hidden Codes on Your Car [Tire Dates, VINs, Fuses]

The small codes on a car sit in five places: the tire sidewalls (DOT date code, size, load and speed rating), the base of the windshield (VIN), the driver's door jamb (tire pressure, paint code, build date), the fuse box lids (fuse map), and under the hood (oil grade, belt and battery labels). Most are molded black on black or printed in 2 to 3 mm characters in a dark corner, so bring a light, hold it low, and photograph each code so you can read it on the screen instead of in the footwell.

Tire DOT codes: how old are your tires

Every tire sold in the United States carries a code on the sidewall that begins with the letters DOT. The full code identifies the plant and tire size, but the part that matters to you is the last four digits, which give the week and year of manufacture. A code ending in 2324 was made in the 23rd week of 2024. One ending in 0819 was made in week 8 of 2019.

Rubber hardens and cracks with age even if the tread looks fine. Many manufacturers suggest an inspection once tires pass five or six years and replacement once they reach ten, regardless of tread. The date code tells you how old your tires are, and it also shows when a "new" tire has been sitting in a warehouse for years. Our article on how to read a tire date code covers older three-digit codes and where to look on each side.

The date code is molded in low relief, often black on black, and is only required on one sidewall. If you cannot find it, check the inner side. A flashlight held at a low angle makes the raised characters cast small shadows; in LoupeLens, 3x zoom with the flashlight dimmed slightly is enough to read them without glare.

Other sidewall markings

• Size, such as 225/45R17, gives width in millimeters, aspect ratio, construction, and rim diameter in inches.
• Load index and speed rating follow the size, for example 94V.
• Max pressure on the sidewall is the tire's limit, not your recommended pressure. See the door jamb section below.
• Tread wear indicators are raised bars in the grooves, 2/32 inch (1.6 mm) high. When the tread is flush with them, the tire is at the legal minimum in most US states.

The VIN

The vehicle identification number is 17 characters long and appears at the base of the windshield on the driver's side, on the door jamb sticker, and on your registration and insurance documents. It never uses the letters I, O, or Q, to avoid confusion with 1 and 0.

On vehicles built for North America, the tenth character encodes the model year and the eleventh the assembly plant; many European makers do not use those positions that way. Parts suppliers and recall lookups key on the VIN, so getting every character right matters. Reading it through the windshield at an angle is a common source of errors; take a photo and read the still image rather than transcribing from the live view.

The door jamb sticker

Open the driver's door and look at the pillar (B-pillar) or the door edge. The stickers here carry more of the numbers you need for maintenance than anywhere else on the car.

• Tire pressure placard. The pressures listed here are what the manufacturer specified for this car. Inflate to these, not to the sidewall maximum.
• Paint code. Usually labeled as paint, color, or with a short alphanumeric code near the bottom. On some makes the code is under the hood or in the trunk instead. You need this code to buy touch-up paint that matches; the color name alone is not enough, since one name can cover several shades.
• Gross vehicle weight rating and axle ratings, which matter if you tow or haul.
• Month and year of manufacture, which is not the same as the model year.

These stickers fade and get scuffed. Photograph them now while they are legible and keep the photo with your other car documents.

Fuse boxes

Most cars have two fuse boxes, one in the engine bay and one in the cabin, often under the dash or behind a kick panel. The lid usually has a diagram that maps each position to a circuit, using abbreviations or icons that are printed small and molded in the same color as the plastic.

The fuses themselves carry their amperage on the top in tiny numerals, and are color coded: for standard blade fuses, 10 A is red, 15 A blue, 20 A yellow, 30 A green. A blown fuse shows a broken or melted strip inside the clear body, which is much easier to confirm at 5x than by squinting.

Always replace a fuse with the same rating. A fuse puller is usually clipped inside the box. If you work under the dash, use the rear camera in LoupeLens with the flashlight on, take a photo of the fuse box label, then sit upright and read the photo instead of squinting in the footwell.

Under the hood

Oil filler cap

The cap is often stamped with the recommended viscosity, such as 0W-20 or 5W-30. This is a quick sanity check when you are standing in the store aisle. The owner's manual remains the authority, since some engines accept more than one grade depending on climate.

Belts and hoses

Belts are printed with a part number along their back. Hoses carry inner diameter and a spec code. Both fade under heat, so photograph them before they become unreadable and you will have the number when a replacement is due.

Battery

The battery label shows the group size, cold cranking amps, and often a date code sticker. Many brands use a letter for the month and a digit for the year, though schemes vary by manufacturer.

Bulbs, wipers, and other small parts

Bulb type is molded into the base of the bulb and sometimes into the housing behind it, in characters a couple of millimeters tall. Wiper blades often have their length stamped on the frame. Reading these directly is more reliable than a store lookup chart, which can list the wrong size for some trim variants.

Quick reference

Code	Where	What it tells you
DOT date (last 4 digits)	Tire sidewall	Week and year of manufacture
VIN	Windshield base, door jamb	Model year, plant, exact identity
Tire pressure placard	Door jamb	Correct inflation for this car
Paint code	Door jamb, hood, or trunk	Exact color for touch-up
Fuse diagram	Fuse box lid	Which fuse protects which circuit
Oil grade	Filler cap	Recommended viscosity

Working in the dark and at awkward angles

Nearly all of these codes are in poorly lit places at awkward angles. Bring your own light, and hold it low so molded characters cast shadows; for printed stickers, light them straight on and dim the light if the label is glossy. Then take a photo and read it sitting comfortably. The photo also gives you the number at the parts counter.

For the general technique, see how to use your phone as a magnifying glass, and for other hard-to-read labels around the house, reading serial and model numbers.