The 1156 is a single-filament bulb (BA15s) used for one lighting circuit, while the 1157 is a dual-filament bulb (BA15d) that carries two circuits for separate tail and brake/turn lighting. They are not always interchangeable in a given socket or wiring harness.
Overview of the two bulb types
In automotive lighting, the number and base type encode how the bulb is wired and what it can illuminate. The 1156 and 1157 look similar at a glance, but their electrical contacts and filament arrangements are different, which affects compatibility and function.
Key differences at a glance
Below is a quick comparison to help you spot the essential distinctions.
- Filaments: 1156 has a single filament; 1157 has two filaments (dual-filament).
- Circuits: 1156 supports one lighting circuit; 1157 supports two (commonly tail light for the dim filament and brake/turn for the bright filament).
- Base and contacts: 1156 typically uses a BA15s base with a single bottom contact; 1157 uses a BA15d base with two bottom contacts to feed two filaments.
- Wattage: 1156 is usually around 27 watts for the single filament; 1157 is commonly about 27W for the bright filament and around 7W for the dim tail filament (values can vary by manufacturer).
- Interchangeability: In many fixtures, you cannot swap an 1156 for an 1157 (or vice versa) without changing sockets or rewiring due to the different base and circuit requirements.
Choosing the correct bulb type ensures proper illumination, avoids fuse or wiring issues, and maintains consistent signaling for other drivers.
Compatibility and installation tips
Use these guidance points to verify compatibility and install safely.
- Check the socket type: verify whether your fixture uses a BA15s (single contact) or BA15d (dual contact) base, and whether the circuit is meant for a single or dual filament bulb.
- Match the vehicle specification: consult the owner’s manual or the existing bulb markings to confirm the correct part number and wattage.
- Consider the function: if you need separate brake/turn and tail illumination, you’ll typically want an 1157; for a single-function tail or signal, an 1156 may be appropriate.
- Safety first: unplug the battery and let bulbs cool before replacing; handle bulbs by the base, not the glass, to avoid oil contamination.
If in doubt, replace with the exact OEM specification or install both bulbs on the same side using the same type to maintain balanced lighting.
Summary
The 1156 is a single-filament, single-circuit bulb (BA15s) commonly used for one lighting function, while the 1157 is a dual-filament, two-circuit bulb (BA15d) designed for separate tail and brake/turn lighting. They differ in base/contact configuration and wattage, and are not always interchangeable in fixtures. Always refer to your vehicle’s manual or the existing bulb markings to choose the correct type.