As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: PILOT REPORTS FLIGHT RETURNED TO PHX ON ACCOUNT OF FALSE OVERSPEED WARNING. EMERGENCY DECLARED AND THE FLIGHT LANDED OVERWEIGHT WITHOUT INCIDENT. MAINTENANCE CONFIRMS THE AIRCRAFT WAS IN CLEAN/FLAP UP CONFIGURATION DURING THE OVERSPEED WARNING EVENTS. MAINTENANCE SUSPECTS THE FLAP HANDLE COMMAND SENSOR AT FAULT. REMOVED AND REPLACED THE FLAP CONTROL COMMAND SENSOR UNIT IAW AMM 27-51-17. OPS CHECK GOOD.