802831 is a fault stored in your BMW's memory (an ISTA fault code, not a standard OBD-II P-code).
What it means: Trailer socket microswitch fault: the switch isn't closing properly, reports an implausible signal, or has an open line. Causes: dirt/corrosion/moisture in the socket, a stuck or broken spring-loaded plunger, broken/corroded wiring. Fix: clean the socket with contact cleaner, plug/unplug a trailer connector a few times, inspect wiring; replace the socket assembly if damaged.
