خوردگی ترمینال‌های جعبه فیوز

Corrosion in the fusebox itself, at the blade sockets or the bus bars. It is the one place people do not look, because the fuses test good and the box is assumed to be a passive connector.

Corroded sockets lose tension as well as conducting badly, so a fuse can be intact and barely making — which produces intermittent faults across whatever shares that section of bus. Several unrelated circuits misbehaving at once points at the box rather than at any of them. Water ingress is the usual cause: an engine bay box with a failed seal, or a cabin box under a leaking screen or heater matrix. Look for green deposits and check each fuse grips, and treat a wet fusebox as a reason to find where the water is coming from.