فَحَصَ الْعَامِلُ الْمِضَخَّةَ فَوَجَدَ الْخَلَلَ فِي الصِّمَامِ.
The worker inspected the pump and then found a defect in the valve.
Faʾ presents the discovery in close sequence after the inspection, potentially as its immediate result. Substituting wāw would connect the events without the same inherent force of succession.
This does not mean faʾ automatically proves strict logical causation in every context. Here the relation is temporal and inferential within a narrated inspection; the factual accuracy of the finding still depends on evidence.