Démarrages à froid répétés sur de très courts trajets

A usage pattern rather than a fault, and one of the most consequential entries in this vocabulary because of how many different failures it causes. An engine that never reaches full operating temperature never gets to clean itself.

What accumulates depends on the system, but the mechanism is the same everywhere. Soot builds on EGR valves, intake ports and turbocharger vane mechanisms because nothing is ever hot enough to burn it off. Particulate filters load faster than they regenerate. NOx and oxygen sensors coat over rather than running clean. Moisture condenses in the crankcase and the exhaust instead of evaporating — which emulsifies oil, corrodes exhaust internals, and is why NOx sensors on short-journey cars crack.

On a diesel it also dilutes the oil, because aborted regenerations wash fuel past the rings.

So a car with EGR, turbo, filter and sensor faults all at once often has one cause, and it is not a component.

The remedy is a change in use, not a part. An occasional sustained run — half an hour at steady speed, in a gear that keeps the engine working — genuinely slows all of it down. Cleaning or replacing parts without that simply restarts the clock, and a vehicle used only for short trips may be better suited to a petrol or hybrid than a modern diesel.