Headlight bulb codes look arbitrary until you know what they describe: the base shape, the number of filaments, and the electrical rating. They say nothing about how bright a bulb is or what colour it appears. Two bulbs with the same code from different makers will fit the same housing and draw broadly the same power — the difference is in the glass, the filament and the coating.
The common fitments
H7 — PX26d base, single filament
The most widely used dipped-beam bulb on European vehicles. Single filament, so a car using H7 for dip typically uses a separate bulb for main beam. The retaining method varies by housing: some clip, some use a wire spring, some a twist collar.
H4 / 9003 — P43t base, twin filament
One bulb, two filaments: dip and main. Common on older vehicles, vans and motorcycles. Not interchangeable with H7 despite similar appearance.
HB3 / 9005 — P20d base, single filament
Frequently used for main beam on vehicles that use a separate dipped bulb. Also appears as a DRL or fog fitment. HB3 and 9005 are the same bulb under European and American naming.
HIR2 / 9012 — PX22d base
An infra-red reflective bulb: a coating on the glass reflects heat back onto the filament, so it runs hotter and brighter for the same power draw. Physically similar to HB3 but with a different base — they are not interchangeable. This catches people out regularly.
H15 — PGJ23t-1, twin filament
A combined daytime running light and main beam bulb, common on VAG group vehicles. One filament runs continuously as a DRL, the other on main beam. Because the DRL filament is lit whenever the car is running, H15 bulbs tend to fail sooner than a dip-only bulb.
H16 — PGJ19-3
Usually a fog light or DRL fitment. Related to the H11/H8/H9 family in appearance, but with its own base — again, not a substitute for them.
Practical points for the workshop
- Always replace in pairs. Light output falls off gradually. A new bulb beside an aged one gives an obviously uneven beam and the second will usually fail soon after.
- Don't touch the glass. Oil from skin creates a hot spot on halogen glass and shortens life significantly. Handle by the base, or use nitrile gloves.
- Check the beam pattern after fitting, particularly on any vehicle where the housing has been disturbed.
LED retrofits and UK road legality
This is worth being direct about. In the UK, a halogen headlight unit that has been fitted with an LED retrofit bulb does not meet the construction and use requirements for road use, and such a vehicle would not pass an MOT with them fitted to a dipped or main beam headlamp. LED retrofit bulbs are sold for off-road, show, competition and export use.
Our OSRAM LEDriving H7 LED bulbs are supplied on that basis. If a customer needs a road-legal upgrade, the answer is a higher-performance halogen in the correct fitment, not an LED retrofit.
Stocking common fitments
H7, H4 and HB3 cover a large share of the UK car parc, and buying in ten-packs is considerably cheaper per bulb than twin packs. See the bulbs and lighting range for trade quantities, including XENCN H7 in twin and 10-packs.
