The diagram showed that ignition power goes through the before reaching E-16. She found the relay, swapped it with an identical horn relay—still no voltage. The diagram then led her to the ignition fuse (15A, position 12 in the cabin panel) . Blown.
Here’s a helpful story that walks you through understanding and using an in a real-world diagnostic situation. Title: The Silent Truck and the Talking Wires
One new fuse later: E-16 now had 12.3V. ECU was alive.
"Common rail, Ern. Starting fluid could bend a rod. Let's check the wiring diagram first."
Following the diagram further, she saw that the comes from an internal ECU regulator but passes through connector C-102 (a common corrosion point near the valve cover). She unplugged C-102—green crust inside.