Today many cars have a control lamp which indicates the failure of a lamp. But sometimes it stays on even after you’ve changed the faulty lamp. There’s an easy solution to the problem: always replace lamps in pairs.

Let me tell you what’s annoying. Say the control lamp on the dashboard comes on and a quick check around the car identifies a dark rear light as the culprit. Luckily the lamp is easy to change. And, there we go, the rear light comes back on again.

But so does the control lamp. It just won’t go off. Turning the ignition on and off and starting the engine are just as useless as taking out the lamp and putting it back in. Is there perhaps something wrong with the electronics? Don’t even think about digging further or changing the lamp monitor. The remedy’s much simpler: replace all the lamps for that lighting function. So if for example the right rear light fails, the left rear light which is still working should also be replaced. Or the failed halogen lamp for dipped beam from one side needs to be replaced together with the one from the other side. Then the light bulb monitor should stay off. But why is that?

There are several technical ways of discovering a burnt out lamp. A very common one measures the current to the individual lamps. For those of you who are interested in the physics: the voltage drop is measured across a resistor. Then a transistor circuit compares the two currents (or voltage drops). If they don’t match, the lamp monitor assumes there’s a lamp that doesn’t work. But there may already be differences in power consumption if the lamps have been in use for different lengths of time. Shortly before a lamp reaches the end of its life the current increases because the filament has become very thin. And then there may also be tolerances between individual lamp types.

By the way, replacing lamps in pairs has another advantage. Often the second lamp of a pair will also fail shortly afterwards. That happens especially with high-quality brand lamps such as the ones from Osram. The reason is the narrow tolerances which also apply to the lifetime. Replacing both lamps will save you trouble, and having your headlights adjusted in a garage one more time. Longlife lamps such as the Ultra Life from Osram will save you even more hassle. They usually last three times as long as standard ones. To keep things simple they come in pairs. Just like pretty much all Osram lamps. And rightly so.

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