E39 bulb failure sensor.

I don't think it has a sensor. Probably just monitored by the lighting control unit/body electrics control unit.

Is the third brake light working?

All lights are working as they should, inc that one, but display still indicates failure of r/h rear bulb. Mike. In the UK

I've heard that can happen if you replace the bulb with one that has either too high or two low a resistance. The sensor assumes the out of range resistance implies a faulty bulb, when in fact it doesn't.

Have tried swapping the bulbs from the l/h light unit to the r/h unit. Fault still shows on the r/h unit, which is why I suspect the sensor may be at fault. Mike. in the UK.

Could be. Or could be the socket or a connector. It doesn't take a lot of resistance to trigger the thing.

I don't know what it's line on the E39, but on the older cars it's a box with reed relays in it, and the reed relays go bad and the solder joints get cold too.

--scott

Double check the 3rd brake light in the rear window. If it has an issue, it will show as the right rear on the pictogram in the instrument cluster.

Brett Anderson KMS