Cause
Parasitic drain from an accessory
Something fitted to the vehicle continuing to draw current after it should have slept. Modern vehicles have a real quiescent draw — modules stay awake for minutes and wake periodically — so this is about a draw that never falls to the expected level.
A drain flattens the battery over days rather than hours, so the complaint is a vehicle that will not start after standing over a weekend while behaving perfectly in daily use. The low voltage then produces codes across many modules at once, each looking like an independent fault.
Aftermarket devices are the usual cause: trackers, dashcams wired to a permanent live, alarms and audio amplifiers. Measure the draw with the vehicle properly asleep, which means waiting for the sleep timers to expire rather than testing immediately — the commonest error in this test is measuring a vehicle that has not gone to sleep yet. Then pull fuses one at a time to find the circuit, and expect the answer to be behind the dashboard rather than in the fuse box.
Typically a moderate job.
Codes this can cause (2)
Codes where this is a likely explanation first, rather than merely a possible one.
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