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

The two contact points of the BA15S (1156) base are opposite to each other (parallel 180 degrees), while the two contact points of the BAU15S base are also on the side, but at a 150-degree angle. Both of these bulbs have one filament and function the same in terms of purpose and diameter.
The difference in the sockets also results in them not being interchangeable. You can't plug a BAU15S with 150-degree contacts on both sides of the socket into an 1156 socket with 180-degree contacts, and vice versa.
The 1156 bulb has only one central contact pin at the bottom, while the 1157 bulb has two contact pins. You will find that placing an 1156 bulb in an 1157 socket will not have any effect because the bulb base does not make proper contact with the positive terminal.
Similarly, the BAU15S cannot be used as a replacement for 1157 because it has only one central contact pin at the bottom, and the two contacts on the side of the socket are at a 150-degree angle.

The LED 1157 bulb, featuring dual filaments, offers the convenience of both high and low brightness options. As a result, it functions as a dual-function bulb, suitable for a wide range of applications.
When used as a front turn signal, the 1157 bulb can serve as a turn signal and a daytime running light at the same time. When the front turn signals are activated, the bulb's high-level brightness functions as the turn signal, while the low-level brightness is usually used for daytime running lights or parking lights.
It should be noted that some Hyundai/Kia models, although equipped with an 1157 dual-filament bulb, only have one wiring circuit, which limits them to functioning solely as turn signal lights. For instance, the Hyundai Ioniq SEL has a front turn signal halogen bulb size of 1157, but it lacks daytime running light/parking lights functionality.
When installed in rear lights, the 1157 bulb is commonly used as tail lights and brake lights.
In contrast, the LED 1156 and BAU15S bulbs, with their single filament, are designated as single-function bulbs. Unlike the 1157 bulb, they lack the flexibility to adjust brightness and operate only one level. Consequently, they are unable to function as both a turn signal and a daytime running light simultaneously.
Due to these attributes, the 1156 and BAU15S bulbs are primarily used for front or rear turn signals, reverse lights, brake lights, and parking lights, among other applications.

How To Pick 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.

3. Search on the Lasfit website

You can go to the Lasfit website and search for the correct LED bulb model according to your vehicle make, model, and year, our website has a detailed categorization for you. Here is the link for you: https://www.lasfit.com/

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!