BMW B38 (1.2 and 1.5)

The three-cylinder end of BMW's modular family, sharing its architecture with the four-cylinder B48 and the straight-six B58. All three are built around the same 500 cc cylinder, which is why a MINI Cooper, a 320i and an M340i share service intervals, sensors and a great deal of the diagnostic picture — a fault pattern learned on one is often worth reading across.

Three cylinders cannot balance themselves, so it carries a balance shaft; the vibration a driver notices at idle is normal for the layout rather than a symptom. It also introduced the first aluminium turbine housing fitted to a production turbocharger.

Its range is wider than the badge suggests. The same engine appears at 74 hp in a base 1 Series and at 228 hp in the i8, where it drives the rear axle of a plug-in hybrid — so a power figure quoted for "the 1.5" says very little without the car it came in.

The published fault picture is thin, which is worth stating plainly rather than filling in: this engine has not accumulated the documented, recall-backed failure modes of the N63 or the N47, and the honest answer today is that its common problems are not well established.