Miniature Bulb Cross-Reference

What it is

Miniature lamps for vehicle signal, marker and interior lighting are sold by trade numbers such as 1156, 1157, 194 and 3157. The number defines the bulb's base, glass shape, filament count and electrical rating. Bulbs with different numbers can share a base but differ in brightness, current or voltage. Truck and bus systems at 24 V use different bulbs from 12 V systems even when the base is the same.

How to identify

Read the number on the base or glass. If it is worn, identify the base: bayonet bulbs have pins on the side of a metal base; wedge bulbs have a flat glass or plastic base with wires folded over it. Count the contacts on the bottom: one contact means single filament; two means dual filament or a double-contact single filament.

Chart

Key points

• 1157 has offset bayonet pins so it seats only one way. Forcing it the other way reverses the filaments.
• 3157 and 7443 look alike but use different bases and are not a direct swap.
• An LED replacement draws much less current. Flashers may "hyperflash" and some vehicle systems report a bulb-out fault. Load resistors or LED-compatible flashers fix this.
• LED inserts in a housing designed for incandescent bulbs may not meet FMVSS 108 output or color. A sealed DOT-marked LED lamp assembly is the compliant upgrade.
• Some LED bulbs are polarity sensitive. If one does not light, try turning it around in the socket.

Common mistakes

• Using a 12 V bulb in a 24 V system, which burns it out at once.
• Using a single-filament 1156 in a 1157 socket, losing the tail or stop function.
• Fitting a colored LED of the wrong color behind a lens, which changes the emitted color.

Governing standards: No single standard for trade numbers; bulb manufacturer data governs. FMVSS 108 for lamps on vehicles.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.