Calamine grippant les ailettes du turbocompresseur

Soot laid down on the pivots and unison ring of a variable-geometry turbocharger until the mechanism stiffens and then binds. It is the commonest thing actually wrong when a code names the turbocharger, and the turbocharger itself is usually in perfect condition — shaft, bearings and compressor wheel untouched while the vane gear seizes around them.

The vanes live in the exhaust stream, which is the dirtiest place on the engine, and the driving pattern that causes this is also the remedy. Short journeys and long stretches of light-load motorway running never get the exhaust hot enough to burn deposits away, so the mechanism cokes up gradually over years; an occasional sustained run under real load genuinely helps keep it free.

Caught while stiff rather than seized, it can often be freed off instead of replaced — which is the entire argument for taking an intermittent or erratic vane position code seriously rather than clearing it. Removed and cleaned is a very different bill from removed and replaced.

Which way it has stuck is readable from the driving and worth reporting: strong pull low down followed by a sudden cut to limp mode means stuck closed and overboosting; nothing low down with black smoke means stuck open. On a twin-turbo engine, assume both sides have accumulated the same deposits on the same journeys — freeing both while access is open is one job rather than two.