The 3157 and 3156 are two widely used bulbs for turn signals, brake lights, and taillights. Have you ever learned about these two types of lights when you were picking out LED turn signals for your vehicle? Is there still some confusion about the difference between these two? Are you not sure how to pick the right bulb? This article will introduce you to 3157 and 3156 and hopefully help you out!What's The Difference Between 3157 And 3156?The main difference between 3156 and 3157 bulbs lies in the number of filaments. The 3156 bulb has only one filament, while the 3157 bulb has two filaments.To determine whether it is LED 3157 or LED 3156, the main difference lies in the wiring. The 3156 bulb has only one wire, while the 3157 bulb has two wires.Installation PositionAs for the installation position, due to the dual filaments of the LED 3157 bulb, it has both high and low brightness, making it a dual-function bulb. When used as a front turn signal, the 3157 bulb can simultaneously function as a turn signal and daytime running light (DRL). When you activate the front turn signal, the high brightness of the bulb serves as the turn signal. In contrast, low brightness is usually used for DRL or parking lights. Additionally, the 3157 bulbs can also be used as side marker lights on some trucks.When installed in the rear lights, the 3157 bulb is typically used as a tail light and brake light. Some American trucks, like F-150 trucks, may have a combination of tail, brake, and rear turn signal functions using a single 3157 bulb.On the other hand, the LED 3156 bulb has only one filament, making it a single-function bulb. It has only one brightness level and cannot switch between high and low brightness like the 3157 bulb. It cannot be used simultaneously as a turn signal and DRL.Due to its attributes, the 3156 bulb is primarily used for front or rear turn signals, reverse lights, and parking lights.Are 3157 And 3156 Interchangeable?When replacing or upgrading the lighting system, ensuring compatibility between the existing socket, wiring, and bulb is crucial. The base of the 3157 and 3156 bulbs is the same, so the 3157 bulb can be installed in the base of a 3156 bulb, and vice versa. But will they work?When 3157 is installed in the 3156 socket, it can light up, but since the 3156 socket is designed for a 3156 bulb with only one filament, even if the 3157 bulb is installed, it will only have one brightness level and cannot switch between high and low brightness.On the other hand, installing a 3156 bulb in a 3157 socket will not work. The reason is that the 3156 bulb is designed for single-filament function, while the 3157 socket is designed for dual-filament bulbs. Due to the different wiring, the 3156 bulb cannot function properly in a socket designed for dual-filament bulbs like the 3157.How To Pick The Correct LED Bulb?How do you pick the correct LED bulb model? It depends on the size of your factory halogen bulbs. Here are some tips to assist you in choosing the appropriate LED bulb.1. Review the user manualYou can consult your vehicle's user manual, which typically provides details about the bulb size of the factory halogen bulbs. Once you have determined the bulb size, you can purchase the corresponding LED bulb based on the information in the manual.2. Inspect the factory halogen lightsYou can remove the factory halogen bulb that requires replacement. Usually, the halogen bulb socket will show the corresponding model number. By identifying the model number, you can purchase an LED light with the same model to ensure compatibility.However, this method is a bit more cumbersome, some vehicles are not convenient to take out the halogen bulb. additionally, individuals with limited experience in bulb modification might be hesitant to undertake this task, fearing potential damage.3. Search on the Lasfit websiteAnother useful resource is the Lasfit website, where you can search for the correct LED bulb model specific to your vehicle's make, model, and year. Our website provides detailed categorization, making it easier for you to locate the appropriate LED bulb. Using the seach bar to sort out the year, made ,and trim so that you can easily find your matching lights.ConclusionThe difference between 3157 and 3156 is mainly in the number of filaments.When choosing between LED 3157 and LED 3156 bulbs, it depends on the factory bulb size of your vehicle. If your factory halogen bulb type is 3156, then your LED bulb size should also be 3156. Similarly, if your factory halogen bulb size is 3157, then you should choose a 3157 LED bulb.Have you ever installed LED 3156 or 3157 for your turn signals? Feel free to share your experience with us in the comments!