i had to replace one of two number plate light bulbs twice in 6 months (of course i changed both of them )and again they have turned black on the inside loosing 50% of their light output, this always happen and with the sidelight bulbs too and in all the other cars ive had its the same why do they go black its not like there getting a lots more than 12 volts.
I've had no problem with LED bulbs from Amazon - fitted them 2 or 3 years ago - funny capless things in the side lights, festoon in the number plate and interior lights. I wonder if some people don't realise they are polarity-sensitive, fit them and, when they don't work claim they are rubbish and throw them away. Fit them the other way and they're fine. I've also...
The black, of course, is the tungsten from the filament evaporating and condensing on the glass. I'm surprised that your bulbs go black that quickly (mine certainly don't); perhaps the regulator on your alternator is playing up.
Have you thought of replacing them with LED bulbs? They won't go black.
I can't be the alternator it always happens on all 5 cars I've had although the bulbs did last longer than Months. yes thought about led but seems only cheapish ones from Amazon are available for number plate and side lights and read loads of bad reports on them
I've had no problem with LED bulbs from Amazon - fitted them 2 or 3 years ago - funny capless things in the side lights, festoon in the number plate and interior lights. I wonder if some people don't realise they are polarity-sensitive, fit them and, when they don't work claim they are rubbish and throw them away. Fit them the other way and they're fine.
I've also had bulbs from here:
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