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The regulator holding rail pressure at its target, jammed at one position so that it no longer responds to what the engine is asking for. The distinction from a regulator that is electrically faulty is the whole diagnostic point: a stuck one obeys nothing because it cannot move, while a faulty one cannot be told what to do. A scan tool sees the difference clearly — commanded pressure changes, actual pressure does not follow, and the control circuit reports no electrical fault at all.

Which direction it sticks decides which code you get. Stuck closed, the pump's full output stays in the rail and pressure runs high; stuck open, fuel bleeds away and pressure collapses, most visibly during cranking when the pump has not yet built flow. A pressure complaint that appears only while cranking and clears once running is the open-stuck case, and it is easy to dismiss as a starting fault.

The usual mechanism is contamination rather than wear. Particulate from a disintegrating filter or a corroded tank, or varnish from fuel left standing for months, gums the valve to its seat. That matters for the repair: a regulator replaced without finding what jammed it meets the same contamination, so inspect the filter and the fuel itself before fitting the new part.