801C12 is a fault stored in your BMW's memory (an ISTA fault code, not a standard OBD-II P-code).
What it means: The energy-management/gateway issued a forced power-down because the car couldn't go to sleep — a control unit or consumer kept the electrical system awake, which drains the battery while parked. Often a one-off after a battery change or a jump-start. If it keeps returning and the battery goes flat, have the quiescent (parasitic) current checked to find what won't let the car sleep.
