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A break in the wiring from the ignition switch, so a module does not receive the run or start signal. It is more specific than a general switch-wiring fault — this circuit tells modules the key state, and losing it means a module does not know the vehicle is being started or run.

The consequences are disproportionate to the wire. Modules that never see the run signal do not wake, do not run their tests, and report power or ignition circuit faults; a start signal that never arrives means no crank. A vehicle dead in one position of the key and alive in another is describing this circuit, and the pattern across positions localises it.

The switch itself carries current and wears, so pitted contacts inside it are as likely as a broken wire, and on vehicles with keyless systems the equivalent is a relay module rather than a mechanical switch. Check for voltage at the module in each key position rather than at the switch — that includes the whole circuit in the measurement, and the connector at the steering column is a common failure point through vibration and repeated flexing.