Yesterday evening I got a warning to check the right hand brake light. The handbook guided me through the dismantling process. Toolkit didn't have the necessary T20 tool. Got the whole fitting out. The handbook does not say which bulb is which, so I pulled them all it and checked them for continuity. All ok, but one was gettered. Changed it. Warning now gone. Is the CANBUS system designed to be sensitive enough to detect a failing filament?

Car is 10 1/2 years old and this was the first bulb changed involuntarily.

The system is pretty sensitive. Pinged up number plate light on mine. Both were working but one had slight corosion. I assume it senses a difference in resistance.

Alasdair