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Not enough oil in the sump. On a modern engine this stops being a lubrication question and becomes a hydraulic one, which is why it appears among the causes of faults that look entirely electrical: oil pressure operates variable valve timing phasers, chain tensioners, cylinder deactivation and variable-displacement oil pumps. A phaser that responds slowly, or a timing chain that rattles on startup, can simply be short of oil.

It is the cheapest thing on any of those diagnostic lists and the most often skipped, because a dashboard warning usually only appears well below the point at which behaviour changes.

Where the level is genuinely low, the question is why. An engine consuming oil between services is burning it, leaking it, or diluting it — and diesel dilution from repeated failed particulate filter regenerations raises the level rather than lowering it, so a level that has risen is its own warning.

Check it properly: on level ground, engine off, warm but settled for several minutes. Checking immediately after switching off reads low because the oil is still draining back, which is how a healthy engine gets topped up past maximum — itself a hazard on a diesel.