01What it usually turns out to be
Many possible causes, and it needs a real diagnostic — but 12.2 V on a running engine is already far too low, so the charging path is suspect first.
02How I actually find it
Gather everything first: ISTA, service bulletins (SIB), then analyse the modules logging related faults, verify those components and reproduce the condition. That is what a normal diagnostic is.
03Why reading the code alone will not show it
Not necessarily — an intermittent cause does not always leave a code behind, which is why the symptom leads and the codes follow.
04What to do before bringing it in
Make an appointment and have the VIN ready.
Confirmed on cars in this shop.
This is usually diagnostics & electrical troubleshooting
Not sure yet? Send the symptom anyway — I will tell you what it involves for your car.
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