Choosing the right bulb for turn indicators and brake lights is crucial for vehicle safety, compliance, and optimal lighting performance. However, the variety of bulb types—such as T10, T15, 1156, PY21W, P21W, 1157, 3156, 3157, 7440, and 7443—can be confusing, especially when you factor in international naming conventions like BAU15S, BA15S, and BAY15D.
This article by TecUpdater breaks down the differences between these popular automotive bulbs using structured “VS” comparisons to help you identify the correct type for your car.
T10 vs T15 vs W16W – Compact Wedge Bulbs for Small Lighting Zones
- T10 (W5W): A small wedge-type bulb rated around 5W, used for side markers, license plate lights, and interior applications.
- T15 (W16W): Larger and more powerful than T10, usually 16W, and ideal for brighter applications such as reverse lights or indicators.
T10 vs T15: T15 offers higher brightness but may not fit in T10 slots due to its size.
T15 vs W16W: Functionally identical; T15 is the US label, W16W is the European counterpart.
1156 vs PY21W vs P21W – Single-Contact Indicator Bulbs
- 1156 (BA15S / P21W): A single-contact, single-filament bulb for turn indicators and reverse lights.
- PY21W (BAU15S): Similar to 1156 but with offset pins and typically amber-colored for turn signals.
- P21W (BA15S): European designation for 1156; functionally the same but may differ in labeling.
1156 vs PY21W: Not interchangeable; PY21W has angled pins and an amber lens, while 1156 is typically white with straight pins.
1156 vs P21W: Interchangeable in most cases; 1156 is the American term, P21W is the European version.
PY21W vs P21W: PY21W is amber and for indicators, while P21W is white and for reverse or brake use.
1157 vs BAY15D vs P21/5W – Dual-Function Brake and Tail Bulbs
- 1157 (BAY15D / P21/5W): A dual-contact, dual-filament bulb designed for tail and brake light combinations.
- P21/5W vs BAY15D vs 1157: These are all equivalent, used in different regional markets. All support two brightness levels in one bulb.
1156 vs 1157: 1156 is single-contact for one function; 1157 is dual-contact for two functions. They are not interchangeable.
BAY15D vs BA15S: BAY15D supports dual functions; BA15S is for single-function use.
3156 vs 3157 vs P27W vs P27/7W – High-Power T25 Bulbs
- 3156 (P27W / T25): Single-contact bulb used for turn indicators or reverse lighting.
- 3157 (P27/7W / T25): Dual-contact bulb for brake and tail light combinations.
3156 vs 3157: 3156 is single-function; 3157 handles two functions.
P27W vs P27/7W: European equivalents of 3156 and 3157, respectively.
3157 vs 1157: Both are dual-contact, but differ in base design and physical size—3157 is typically used in North American vehicles.
7440 vs 7443 vs W21W vs W21/5W vs WY21W – T20 Bulb Series for Modern Lighting
- 7440 (W21W / WY21W): Single-contact wedge bulb for reverse lights or indicators. WY21W specifically indicates amber color.
- 7443 (W21/5W): Dual-contact bulb used for brake and tail light combinations.
7440 vs 7443: 7440 supports a single function; 7443 supports dual-function operation and is not backward-compatible.
W21W vs WY21W: W21W is white; WY21W is amber but shares the same base.
7443 vs W21/5W: These are equivalent in function, differing only in nomenclature across markets.
Single-Contact vs Double-Contact Bulbs
- Single-contact bulbs (e.g., 1156, PY21W, 7440, 3156) operate one function—either brake, reverse, or indicator.
- Double-contact bulbs (e.g., 1157, 3157, 7443) are designed to perform two functions in the same socket, such as tail and brake light or parking and turn signal.
Single vs Double Contact: Installing the wrong type may lead to malfunction, short circuits, or incorrect brightness levels. Always match the bulb to the correct socket type.
Use Case Summary: Which Bulb for Which Function?
| Application | Common Bulb Types |
|---|---|
| Turn Indicators (White) | 1156, P21W, 7440, 3156 |
| Turn Indicators (Amber) | PY21W, WY21W, 7440 (amber variant) |
| Brake + Tail Combination | 1157, P21/5W, BAY15D, 7443, 3157, W21/5W |
| Reverse Lights | T15, W16W, 3156, 7440 |
| Parking + Indicator Combo | 7443, 3157, 1157 |
| Side Marker / Interior | T10, W5W |
Final Thoughts – T10 vs T15 vs 1156 vs PY21W vs P21W vs 1157 vs 3157 vs 7443
Choosing between T10 vs T15, 1156 vs PY21W vs P21W, or 3157 vs 7443 depends on your vehicle’s socket design, the number of functions needed, and the light output required. For instance:
- T10 vs T15: T15 is brighter and larger; ideal for reverse lights, while T10 is better suited for small interior or marker lights.
- 1156 vs PY21W vs P21W: All are single-function, but PY21W has amber light and offset pins, making it suitable for indicators.
- 1157 vs 3157 vs 7443: All support dual-function applications, but differ in base shape and compatibility.
Before purchasing, always verify your vehicle’s bulb type using the owner’s manual or an online compatibility guide.
About TecUpdater
At TecUpdater, we provide high-performance LED bulb upgrades for all standard automotive sockets—offering longer lifespan, improved brightness, and plug-and-play fitment. Our products are rigorously tested, glare-free, and backed by a 1-year warranty.
For guidance on selecting the correct bulb for your vehicle, visit our catalog or reach out to our expert support team.