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Reports how much oil is in the sump, and it is not the same thing as the oil pressure sensor — level says how much there is, pressure says whether it is reaching the bearings. An engine can have a full sump and no pressure, or low oil and normal pressure right up until it starves.

Its importance has grown because many modern cars have no dipstick at all. Where that is so, this sensor is the only way anyone can check the one fluid that destroys an engine when it runs out — which makes a fault here a genuine gap rather than an inconvenience.

Most are thermal: a wire is heated briefly and its cooling rate says whether it is immersed in oil or hanging in air. Others time an ultrasonic echo off the surface. Neither can read a moving surface, so both need the vehicle level and settled for some minutes after switch-off — which is behind a large share of implausible readings, along with sludge insulating a thermal element.

Read a wrong level as a finding rather than a sensor fault until proven otherwise. A falling level means oil is being burned or lost; and on a diesel a level that has risen means fuel dilution from failed regenerations, which is more urgent than a low level, not less.