As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING APPROACH INTO DCA, THE RIGHT MAIN LANDING GEAR GREEN LIGHT DID NOT ILLUMINATE AFTER GEAR EXTENSION. CREW CONFIRMED VISUALLY THROUGH PERISCOPE THAT GEAR WAS DOWN AND LOCKED. AIRCRAFT LANDED WITHOUT INCIDENT. NOTE: EMERGENCY EQUIPMENT WAS CALLED OUT ON STANDBY AS A PRECAUTION. MAINTENANCE REPLACED THE RIGHT MAIN GEAR DOWNLOCK SWITCH. OPERATIONAL CHECK NORMAL. AIRCRAFT RETURNED TO SERVICE.