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Rust or sediment inside the fuel tank, which feeds everything downstream. It is a cause of repeated failures rather than a single fault — the pump, the strainer, the filter and eventually the injectors all meet whatever is in there, in that order.

The diagnostic signal is repetition. A fuel pump that fails twice, filters that block far short of their interval, or injectors that foul again after cleaning all point back at the tank. A second failure of the same part is the moment to look inside the tank, rather than fitting a third.

Steel tanks rust from the inside where condensation collects, and older vehicles and long lay-ups are the common cases; plastic tanks accumulate sediment instead. Inspect the strainer that comes out with the pump — what is on it is a direct sample of the tank's condition, and a heavily contaminated one means the tank needs cleaning or replacing before any new part goes in.