As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHILE PERFORMING "BEFORE START" CHECKS WITH GPU CONNECTED AND POWERING SYS, A HEAVY ELECTRICAL BURNING ODOR AND SMOKE CAME FROM OVERHEAD ELECTRICAL PANEL. CREW THEN SHUTDOWN SYSTEMS IAW COMPANY SOP'S AND ALERTED MX. MX FOUND RT AUXILLIARY FUEL PUMP QA8 SWITCH BURNED. R & R QA8 SWITCH WITH SAME PN MS35058-23. OPS CHECKED GOOD, WORK PERFORMED IAW WDM 28-21-00.