As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE FLIGHT CREW REPORTED THAT THE ROTARY TEST SWITCH FAILED TO TEST THE LANDING GEAR SYSTEM CAUSING THE FLIGHT TO BE CANCELLED. THE MAINTENANCE VENDOR TROUBLESHOT AND DISCOVERED A DEFECTIVE K78 RELAY ON PCB101. THE MAINTENANCE VENDOR REPLACED THE K78 RELAY WITH NEW. OPERATIONAL CHECKS WERE SATISFACTORY AND THE AIRCRAFT WAS RETURNED TO SERVICE.