As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: DCA-ORD AIR INTERRUPTED BACK TO DCA ACCOUNT DC ESSENTIAL BUS FAULT. FLIGHT DID NOT DECLARE AN EMERGENCY. AIRCRAFT WAS NOT OVERWEIGHT FOR LANDING. MAINTENANCE REPLACED #2 TANSFORMER RECTEFIER ( 1PU2 ) IAW AMM 24-32-51.