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Fault Tree of the System ECU

The standard pattern, applied until a common cause surfaces itself.

Applied subsystem by subsystem along the signal path, the pattern assembles the ECU's whole tree, with the microcontroller's transition condition telling a system failure apart from an input-signal failure at each node. Tracing the switch signal to the boundary and back reveals that the ECU both reads and powers the switch — so the microcontroller is, by construction, a common cause. The structure surfaced it.