What was found
SFO - FLT 1801 - DURING APPROACH TO SFO WHEN LANDING GEAR EXTENDED, UNSAFE INDICATION ON ALL 3 LANDING GEARS. ACCOMPLISHED EMERGENCY GEAR EXTENSION CHECKLIST AND OBTAINED 3 GREEN LIGHTS FOLLOWED BY LIGHTS RETURNING TO RED WHEN EMERGENCY GEAR EXTENSION LEVER WAS STOWED. AIRCRAFT LANDED SFO WITHOUT INCIDENT WITH GEAR EXTENSION LEVER NOT STOWED. CHECKED LANDING GEAR PROXIMITY SENSORS AND BITE CHECKED PROXIMITY SWITCH CONTROL UNIT WITH NO DEFECTS FOUND. CHECKED S1-89 SWITCH AND THROTTLE SWITCHES S1-319 AND S1-320 WITH NO DEFECTS FOUND. ACCOMPLISHED LANDING GEAR RETRACTION CHECK WITH NO DEFECTS FOUND. SYSTEM GROUND CHECKED, NORMAL OPER. UNABLE TO DUPLICATE PROBLEM AND ACFT RETURNED TO SERVICE. (X)