On my Disco 1, none of my interior lights work when the doors open (switch to DOOR) but they work when switched to ON. I've also noticed that I don't get door ajar alerts, nor does the alarm do anything when attempting to open the doors when armed. So it's a systemwide thing as opposed to a single switch or sensor.

Am I correct in thinking this is a BCU issue? I have a Nanocom and could check programming. What should I be looking for otherwise?

 

A D1 does not have a BCU. That began with the DII.

As for door switches and the alarm... the first step is test continuity with a multi meter. That can be a rather tedious task.

 

probably just dirty door sensor - plunger gets dirty - spray liberally with contact cleaner

 

If your 100% all of your door sensors and bulbs are working I'd look at the MFU. Swapped mine and fixed similar issues.