Rear light and brake light issue

My wife and I haven't had our C3 very long but were advised by another driver recently that our brake lights were permanently on.
I'm pretty clued up, and immediately thought of a failed or sticking brake light switch. Anyway, whilst checking it out, it became obvious that the brake lights were working fine. The problem was that when I turned the lights on, the brake lights came on too. So now had a dangerous situation whenever I braked, no warning was given as the brake lights were already on permanently. Clearly there was a short somewhere.

I found it quite by accident. A previous owner had replaced the top bulb with a single contact bulb, instead of the twin contact stop tail bulb that should have been installed. The single contact bulb was bridging the two contacts causing a short. When removing the bulb it was obvious that there were two connections, not one.
A new stop/tail bulb solved the problem immediately.
Hope it helps anyone with the same problem. It had been like this since we bought the car, but we hadn't used it at night before.

Great advice and information Spannerdemon.

The pins on the side of bulbs are supposed to be configured so you can't use the wrong bulb, however, modern bulb holders are made of pressed metal which is soft enough to allow the wrong bulb to be push in without much effort.

Missing Lincs wrote:Great advice and information Spannerdemon.

Yes, great information, thank you. I will also give it a place in the Tips and Tricks for the Citroen C3 for quick reference.

Hi All

I seem to be having the same issue i.e. rear brake light always on. I will check whether I have the right bulbs but I am fairly confident they are fine. Out of curiosity why do the brake light bulbs have 2 filaments or are double contact? thanks

One filament is 5 watts and is used for the tail light. The other is 21 watts and is used for the brake light.
If one filament blows the other one still works.
Some cars have independent brake and tail light bulbs but not many.
The day of the replaceable incandescent tail light bulbs is dying out with non-replaceable LED lamp assemblies. However, a lot of those still use 21 watt incandescent turn indicator bulbs?

mine was permently on at the mot the tester was kind enough to sort it out - it turns out the previous owner fitted the wrong lamp in the top brake light holder and that caused the problem as it was too large. so maybe worth checking.