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The seal on an injector's fuel return connection, failed so fuel escapes from the low pressure return circuit. It is not the combustion seal at the injector's tip — that one leaks compression into the engine bay and sounds like it. This one leaks fuel, quietly, into the valley or down the back of the engine.

Its effect on the system is out of proportion to its size. The return circuit is how the module measures injector leak-off, so a leaking seal makes healthy injectors appear to be passing excess fuel — and the diagnosis condemns a set of injectors that are perfectly good. A leak-off test that fails across all cylinders equally should prompt a look at the return plumbing rather than a set of injectors.

Diesel smell with no visible drip is the practical sign, since the leak often evaporates off a hot engine. These seals are single-use and cheap, and any injector removed should have them replaced — reusing one that has taken a set is a common reason the fault returns immediately after an injector job.