Bouchon de réservoir desserré ou absent

The filler cap left loose, cross-threaded or off altogether. It is the first thing to check on any evaporative leak code and the commonest single cause of one — the cap seals the tank, and the system tests the tank as a sealed vessel.

The standard names it directly, which is unusual: one code exists specifically to say the cap is loose or off, precisely because the fault is so common and so cheap. A cap that has not been turned until it clicks is enough, and so is one cross-threaded after refuelling.

Two things separate it from a cap that is genuinely faulty. A loose cap seals perfectly once tightened, so the code clears and stays clear; a perished seal or a non-original cap of the wrong pressure rating produces the same leak with the cap correctly fitted. Check the seal's condition while the cap is off — soft and uniform is serviceable, hard or flattened is not. The monitor only runs under particular conditions, so a code can persist for a drive cycle or two after the cap is tightened, which is worth telling an owner before they buy a cap they do not need.