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One of the short rigid pipes from the rail to an injector, restricted internally. It affects one cylinder only, which is what separates it from every other fuel supply fault — a restriction upstream of the rail starves all of them equally.

So the presentation is a single-cylinder misfire or contribution fault that survives an injector swap. Moving the injector to another cylinder moves nothing, because the pipe stayed where it was — and that test, which normally identifies a bad injector, points here when the fault refuses to travel.

Debris from a failed pump is the usual cause, which matters: if one pipe is contaminated the others and the injectors are too, so a pump failure means the whole high pressure side needs cleaning or replacing rather than one pipe. These pipes are single-use — the sealing cone deforms on first fitting, so one that has been removed should not be refitted.