1156 vs 1157 bulbs
The bayonet bulbs in turn signals, brake lights and reversing lights
1156 and 1157 are the North American numbers for the bayonet bulbs used in turn signals, brake lights and reversing lights. A 1156 has one filament on a BA15s base; a 1157 has two filaments, for tail and brake or parking and turn, on a BAY15d base with offset pins. Their European counterparts are the P21W and P21/5W.
Push in, twist, and the bulb locks: bayonet bulbs have lit turn signals and brake lights for generations. The difference between a 1156 and a 1157 comes down to one extra filament and one pin moved up the base, and getting it wrong means a brake light that stays dark or a bulb that will not lock in.
What is the difference between 1156 and 1157?
A 1156 has one filament and one contact on its base, and lights at one brightness, for turn signals or reversing lights. A 1157 has two filaments and two contacts: a dim one for the tail or parking light and a bright one for the brake light or turn signal. The bases look alike, but their pins differ.
On a 1156, the BA15s base, the two locating pins sit opposite each other at the same height, so the bulb fits either way round. On a 1157, the BAY15d base, one pin sits higher than the other, so it fits only one way and each filament meets the right contact. The bottom of the base is the quickest check: one contact means a 1156, two mean a 1157.
| Bulb | Base | Filaments | Rating | Typical use |
|---|---|---|---|---|
| 1156 | BA15s | 1 | 2.1 A, about 27 W | Turn signals, reversing lights |
| P21W | BA15s | 1 | 21 W, 460 lm | Turn signals, reversing and rear fog lights |
| PY21W | BAU15s | 1 | 21 W, 280 lm, amber | Amber turn signals |
| 1157 | BAY15d | 2 | 2.1/0.59 A, about 27/8 W | Brake and tail, parking and turn |
| P21/5W | BAY15d | 2 | 21/5 W, 440/45 lm | Brake and tail lights |
| P21/4W | BAZ15d | 2 | 21/4 W | Brake and tail lights on some European cars |
Can you use a 1157 in a 1156 socket?
No. A 1157's pins sit at different heights, so it will not lock into a 1156 socket, and a 1156 lacks the second contact a 1157 socket needs. Within each type, US and European bulbs interchange: 1156 with P21W, and 1157 with P21/5W, although the US bulbs draw a little more power.
The P21/4W is the trap. It looks like a P21/5W but uses a BAZ15d base, whose pins are offset differently, so it fits only sockets made for it. If a new bulb will not lock, compare the pin positions with the old one before forcing anything.
Amber bulbs and the BAU15s base
Turn signals behind clear lenses use amber bulbs. In Europe that is the PY21W, on a BAU15s base whose pins are not opposite each other, so a clear P21W cannot be fitted by mistake. US amber versions carry an A or NA suffix, such as 1156NA, and use the normal base.
The amber color must come from the bulb, so a clear bulb in an amber-bulb lamp is not just dimmer but the wrong color for a turn signal. The same applies to LED replacements: an amber lamp with a clear lens needs LEDs that emit amber.
Choosing LED 1156 and 1157 bulbs
Match the base, then check four things. The LED must produce the right color, red or amber, if the lens is clear. Dual-filament replacements must keep the tail light clearly dimmer than the brake light. Turn signals may hyperflash. And the bulb must emit sideways and forward as the filament did, or the lamp looks patchy.
Some LED 1157 bulbs make too little difference between their tail and brake modes, and drivers behind rely on that step to see you braking. Choose bulbs whose maker states both outputs, and check from behind the car that the brake light is clearly brighter. LED turn signals usually need a fix for hyperflash; see CANbus LED bulbs and hyperflash.
In UNECE countries, the rules list an approved LED substitute only for the amber PY21W among these bulbs, valid in the vehicles its maker names; there is no approved LED P21W or P21/5W.
Frequently asked questions
Can I put a 1157 bulb in a 1156 socket?
No. Its pins are offset, so it will not lock, and the socket has only one contact. Use a 1156 or its European equivalent, the P21W.
How do I tell a 1156 from a 1157?
Look at the bottom of the base: a 1156 has one contact, a 1157 two. The 1157's side pins also sit at different heights.
Is a P21W the same as a 1156?
They share the BA15s base and interchange. The P21W is rated 21 W at 12 V; the 1156 draws about 27 W at 12.8 V, so it is a little brighter and hotter.
Why are my LED brake lights no brighter than the tail lights?
Some LED 1157 bulbs barely change output between their two modes. Choose bulbs whose maker states both outputs, and check from behind the car that braking makes a clear difference.
Sources and further reading
- IEC 60061-1, Lamp caps and holders together with gauges for the control of interchangeability and safety: Lamp caps (BA15s, BAY15d, BAU15s and BAZ15d caps).
- UN Regulation No. 37, uniform provisions for filament light sources in approved vehicle lamps, with category sheets in the Consolidated Resolution R.E.5 (P21W, P21/5W, PY21W and P21/4W).
- UN Regulation No. 128, LED light sources, and R.E.5 Group 4, LED substitute light source categories (PY21W/LED).
- Manufacturers' miniature lamp data for the 1156 and 1157, rated at 12.8 V.
Image credits
- P21/5W bulb: btr, CC BY-SA 2.5, via Wikimedia Commons.