What's The Difference between 1156/1157/BAU15S?

The BA15S (1156) base has two contact points that are positioned opposite each other, while the BAU15S base has two contact points that are positioned on the side, at a 150-degree angle. Despite having one filament and being of the same diameter, the difference in sockets renders these bulbs non-interchangeable. Attempting to plug a BAU15S with 150-degree contacts into an 1156 socket with 180-degree contacts, or vice versa, will prove unsuccessful.
Furthermore, the 1156 bulb has only one central contact pin at its base, while the 1157 bulb boasts two contact pins. Using an 1156 bulb in an 1157 socket will fail to yield the desired effect, as the bulb base will not make proper contact with the positive terminal. Likewise, the BAU15S cannot serve as a replacement for the 1157 bulb since it also has just one central contact pin and side contacts at a different angle than the 1157 base.

Installation Position

The dual-filament LED 1157 bulb offers the best of both worlds with its high and low brightness options, making it an excellent choice for a variety of applications. As a dual-function bulb, it serves as a turn signal and a daytime running light, making it ideal for front turn signals. Its high-level brightness function acts as a turn signal, while the low-level brightness is perfect for daytime running lights or parking lights. However, some Hyundai/Kia models that have an 1157 dual-filament bulb only offer one wiring circuit, making them solely functional as turn signal lights.

Meanwhile, the LED 1156 and BAU15S bulbs are designated as single-function bulbs as they feature a single filament. Unlike the 1157 bulb, they lack the flexibility to adjust brightness, operating only at one level. As a result, they work best for front or rear turn signals, reverse lights, brake lights, and parking lights, among other applications. Installing an 1157 bulb in the rear light configuration is a common choice for tail lights and brake lights, while the 1156 and BAU15S bulbs work perfectly in similar applications.

How To Pick The Correct LED Bulb?
How do you pick the compatible LED bulbs for your vehicles? Please check and confirm the factory halogen bulb size. Here are some tips for you to choose the correct LED bulb.

1. User manual

You can find the model number of the factory halogen bulbs by checking your vehicle's user manual, then purchase the corresponding LED bulb according to the model number on the user manual.

2. Pull out the factory halogen lights to check

You can take out the factory halogen bulb that needs to be replaced, usually, the halogen bulb socket will show the corresponding model number, you only need to buy the same LED light model according to the factory halogen bulb model.
However, this method is a bit more cumbersome, some vehicles are not convenient to take out the halogen bulb; some vehicle owners have no idea of how to replace the halogen bulb for fear of damage to the bulb.

Conclusion
The difference between 1156, 1157, and BAU15S is mainly in the number of filaments, also the socket is different. You'll need to pick the correct bulb from the three based on your factory halogen bulb.
Have you ever installed these LED bulbs for your turn signals? Feel free to share your experience with us in the comments!

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What's The Difference between 1156/1157/BAU15S?

2. Different Socket