Car lamps can sometimes be devious things. Many types are as alike as two peas in a pod. In other cases, the names are far more alike than the lamps themselves.

“Hello, I need a light bulb for my car” – how many times have dealers and spare parts stockists heard this vague request from their customers? If asked, they may have an idea about the wattage – if you’re lucky. 21 or 55 watts for example. But unfortunately there are always several with the same values, and they are not at all compatible with each other – how cruel. The wrong type won’t fit and in the worst case will damage the headlight if you try to put it in with too much force.

Lamp types with very similar type names are not always as easy to distinguish as the H4 (left) and HB4.


Lamps that look very similar cause confusion. Prime examples are dual-filament lamps for rear lights. They come in 21 and 5 watts, and 21 and 4 watts. You’ve guessed correctly: they are not interchangeable because the small detents at the base are in different places. Both types are also similar to the widely used P21W lamp which can often be found in indicators and brake lights. For indicators it’s also available in yellow and is called PY21W. The letter y stands for yellow, and this lamp won’t fit either instead of a normal white P21W. So each letter and number is important.

The H8, H9 and H11 types can also be confusingly similar. With their modern, angled plastic bases they also bear a striking resemblance to the HB3 and HB4 US types which are also permitted in Europe. So it’s probably not really helpful to figure out which replacement lamp you need simply by making a quick visual comparison. Once again, small differences in the base will render it useless.

At least the HB4, mentioned above but rarely used, and the very common H4 look quite different. Sometimes that doesn’t prevent less experienced sellers from touting an H4 for both purposes. Unfortunately that also happens with xenon lamps where the letters S and R make a huge difference. Don’t even think about replacing the common D2R with a D2S, and certainly not with a D3R. How cruel that the D4R looks very, very similar to the D2R. The D4R is a newer mercury-free lamp, but for technical reasons it can’t make use of this environmental advantage in old headlights.

So what’s the best thing to do? The basic rule is: the type names, such as H7, P5W or C5W, have to match one hundred percent. You’ll sometimes find the necessary information on the headlight itself, but in any case in the manual. To make it even more confusing, some manuals also unnecessarily mention the base name – Ba15s for example – which you can forget about straightaway though. Each type of lamp only exists with one single base.
The Vehicle Lamp Finder from Osram is a great way to find out which lamp you need. Go to for the correct information.


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