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Water, debris, biological growth or off-specification fuel in the tank. On a diesel the property that matters is cetane number — how readily the fuel ignites under compression. Low-cetane fuel ignites more slowly, so combustion peaks later than the module predicted from a perfectly correct injection event, and the result is logged as an injection timing error on a mechanically sound engine.

Water is the more damaging half. Modern high pressure fuel systems rely on the fuel itself to lubricate pump and injector clearances, so water displacing that film causes rapid wear, and it also corrodes and narrows injector nozzles.

The timing of the complaint is the diagnostic. A fault that appeared immediately after a fill-up points here far more strongly than at any component, which is why where and when the tank was last filled is worth asking about before parts are ordered. Check the water separator and the filter's age; replacing an injector while leaving contaminated fuel in the tank simply feeds the new one the same problem.