What's the difference between fuses IGN A vs IGN B?
 

The fuses are numbered 49 for IGN A and 52 for IGN B, both 40 amps. Does anyone know what they are and their difference?

Interested in the back story? Long story short, my father's 07 Express 1500 randomly stopped working - It cranks but doesn't start (happened in driveway).

He has changed: fuel pump, body control module (under passenger side airbag), ignition coil, crank shaft sensor, distributor cap and rotor, ignition sensor module and it still didn't start.

After replacing these parts, he removed the alarm system the previous owner had installed, didn't help. Next he checked the fuses, all intact and functioning. After this, he took a spare fuse (thinking someone might have messed with his van) and placed it into slot 52 IGN B, the van started up perfectly! The car continues to function with a fuse in 52 IGN B (don't worry, no one is moving the van with this fuse in slot 52 IGN B).

Neither one of us are mechanics and I've hit a dead end - I don't know how to help him. Towing and/or dealership service is unfortunately not possible. Does anyone have any idea of what's going on to his van and what IGN A / IGN B are and why placing a fuse in IGN B worked?

Thank you so much in advance!