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The large seal where the filler neck or the fuel sender unit meets the tank, perished so the sealed system leaks. It is a different seal from the filler cap's — that one is checked and replaced routinely, while these sit under a trim panel or beneath the vehicle and are rarely thought of.

The evaporative monitor tests the tank as a sealed vessel, so a hardened sender seal shows as a leak with a perfectly good cap fitted and no leak anywhere in the lines. A leak code that survives a new fuel cap is the pattern, and it is the point at which most diagnoses stall.

The sender seal is disturbed whenever a fuel pump is replaced, and it is single-use — a seal that has taken a set will not reseal on reassembly. So a leak code appearing shortly after a fuel pump job is usually this, and it is worth asking whether the tank has been opened before searching the whole system. Smoke-testing the tank finds it where a pressure decay test only proves a leak exists.