Miles stared at the diagram. It wasn't just lines and squares. To a layman, it was chaos: a spiderweb of red (battery), yellow (ECM), black (ground), and blue (sensor return). But to Miles, it was a language. The 70-pin ECM connector (J1/J2) was the brainstem. The Crank/Run relay was the heartbeat. And the elusive 5-volt sensor supply circuit—that was the soul.

Lena climbed into the cab. The starter cranked. The C7 rumbled to life—that familiar, oil-lumpy idle. She pressed the throttle. The tach needle swept past 1,500… 2,000… 2,500. Smooth as a sewing machine. The engine didn't derate.

He dropped the truck into gear and smiled. For the first time in eighteen months, he held a real wrench.