Table of Contents
1. Connector Base Design of P21W
The 1156 also called P21W, the P21W bulb has a BA15S base, and the P21W base design is 180°, which features two symmetrical pins. This design allows for easy installation, as the bulb can be inserted into the socket in either orientation.
2. Connector Base Design of PY21W
The 1156 also called PY21W, the PY21W bulb has a BAU15S base, which features offset pins. The pins are not aligned symmetrically; instead, they are positioned at an angle (typically 150° apart). This offset design ensures that the bulb can only be installed in one specific orientation. This is critical for turn signals and other signaling lights, where proper directional installation is important for safety.
3. 1156 Bulbs Application
P21W (BA15S): The symmetrical pin design allows the bulb to be used in various applications where the direction of light or signal doesn’t matter, such as brake lights, tail lights, or reverse lights.
PY21W (BAU15S): The offset pins ensure that the amber-colored PY21W bulb is installed correctly in turn signal or indicator light sockets, preventing improper installation. This ensures the light is properly aligned for optimal performance, especially in signaling applications requiring amber light.
4. The P21W and PY21W Compatibility
Due to the different pin alignments, PY21W and P21W bulbs are not interchangeable in the same sockets. A socket designed for a PY21W bulb will not accept a P21W bulb and vice versa.
In Conclusion, the key difference between the PY21W and P21W bulbs lies in their connector base design. The P21W has symmetrical pins (BA15S), while the PY21W has offset pins (BAU15S). This difference in base design ensures that each bulb is installed correctly in its intended application, making them non-interchangeable. Always check the specifications of your vehicle to ensure you are using the correct bulb type.
You can check the fitment with your vehicles here:
Color Selection Reference
| Front Side Marker | Amber |
| Rear Turn Signal | Amber/Red |
| Front Turn Signal | Amber/Red/Switchback |
| Brake Light | Red |
| Rear Side Marker | Red |
| Tail Light | Red |
| High Mount Stop Light | Red |
| Back Up Light | White |
| Dome Light | White |
| License Plate | White |
| Parking Light | White |
| Luggage Compartment | White |
| Courtesy Light | White |
| Glove Box Light | White |
| Interior Door | White |
| Map Light | White |
| Reading Light | White |
| Stepwell Light | White |
| Trunk Light | White |
| Vanity Mirror Light | White |
