Keep in mind high power connection(s) if loose can arc together but can brake open again once power is removed.
You could remove the box and disassemble it to look for corrosion or cracked solder joint(s) pr perhaps loose crimps on the wiring or signs of burnt traces on the PCB.
But if the trouble is with the electronics ( faulty processor or the likes ) that becomes much more involved and requires specific tooling and skill set.
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