As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTED AIR RETURN TO STL AND REPORTED STANDBY ALT LIGHT COME ON AND ATTEMPTED TO RESET SEVERAL TIMES WITH NO RESULT. MX PERFORMED GROUND OPERATIONAL CHECK AND FOUND VOLT/AMMETER ROTARY SELECTOR TO BE DEFECTIVE. REPLACED SWITCH WITH ALL INDICATIONS NORMAL.