"That's an impossible combination," he said. "Not absolutely impossible, but unlikely enough that I'd check the measurement before I touched the battery."
Marcus folded his arms. "The sensor is calibrated. We've checked its resistance at room temperature."
"Room temperature isn't the condition that makes the fault happen."
"Are you suggesting we don't understand our own prototype?"
Evan finally looked at him. "I'm suggesting that understanding a design and finding a broken connection are different jobs. You know what this car was built to do. I know what it sounds like when a part stops doing it. We might need both."
The younger technician lowered his eyes to hide a smile. Charlotte stood silently beside the glass barrier, watching the exchange. Marcus had excellent credentials and years of experience, but he had developed an instinct to defend his diagnosis before challenging it. Evan, who had nothing to defend, was asking questions nobody had thought to ask that morning.
He requested a multimeter, a nonconductive work light, the harness diagram, and a technician authorized to make the vehicle electrically safe before he inspected the relevant connections. Once the system had been secured and verified, he worked slowly along the wiring path, comparing labels, connector shapes, and routing clips. His hands moved with the efficiency of somebody who had learned that careless work could injure the next person who opened the hood. He asked the younger technician, whose name was Nadia Price, to record what they found as they went.
"Why write it down before you know what's wrong?" she asked.
"Because when you fix a car, you can accidentally erase the best evidence of why it broke."
Charlotte heard the sentence and felt a strange unease. Daniel Carter had once said almost exactly the same thing during a demonstration of his first experimental drivetrain. She could remember standing behind him at a workbench, pretending to be interested in the wiring when she was really curious about the confidence with which he spoke to her father. He had told her that a failed test was never wasted if the evidence was kept honest. Richard had laughed and called him painfully sentimental about machines.
For forty minutes Evan worked without mentioning Daniel again. He traced the return signal from the battery temperature sensor, watched the values during a carefully controlled bench test, and checked the connection seated behind the inverter housing. At first glance, the plug looked ordinary. Under the work light, however, its molded body showed a different sheen from the other connectors, and one contact had darkened from repeated heating.
"Who replaced this?" Evan asked.
Marcus crouched beside him. "That harness was installed during final assembly."
"Then final assembly used the wrong part. The cable jacket can tolerate the temperature here, but this connector body can't. When the contact heats under load, its resistance changes. The control unit sees a bad temperature reading and does exactly what it was told to do: protect the battery."
"So the pack never overheated," Nadia said.