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A battery that can no longer hold charge, or a charging system not replacing what is used. It is recorded as a cause because low system voltage produces codes everywhere — modules browning out, actuators not reaching their targets, communication dropping — and each looks like an independent fault in its own system.

That breadth is the diagnostic signal. A set of unrelated codes appearing together across several modules, often after the vehicle has stood, is a supply problem rather than several coincidental failures. Checking voltage first costs nothing and prevents a long diagnosis of consequences.

Separate the battery from the charging system before replacing either. A battery that will not hold charge overnight and a charging system that never fills it produce the same low voltage — so measure resting voltage after standing, then charging voltage with the engine running and loads applied. Modern vehicles complicate this deliberately: a smart charging system varies output by design, so a voltage that looks low may be correct, and many require a replacement battery to be registered before the system will charge it properly.