S40 diesel preheating lamp issue

Sounds like a pain! I had something similar happen with my old Renault Megane, though it wasn't exactly the same. The 'preheating lamp' thingy stayed on forever, and it drove me nuts. Ended up being an electrical fault somewhere, but your glow plug theory sounds plausible. Does it affect how the car runs? Like, rough idle, engine misfire, anything like that?

I have noticed some rough idling, particularly during cold starts. Could you elaborate on the electrical fault your Renault Megane experienced? I am curious if there are any similarities I should be aware of during my own diagnostics.

Rough idling on cold starts screams glow plugs to me. My Renault was a wiring loom issue apparently, causing all sorts of random dashboard light errors. But since yours is pointing towards the glow plugs and you're getting those symptoms, I'd bet money on that being the culprit. Honestly, with modern diesels, messing with that stuff is asking for trouble. I'd take it to a workshop and let them sort it out. You don't want to start chasing ghosts.

Following further investigation, resistance testing of the glow plugs determined that one was defective. Replacement rectified the issue, at a cost of 440€.