As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMB, THE PROPELLER GAUGE INDICATION STARTED DECREASING. THE PILOT CYCLED THE PROPELLER CONTROL LEVEL, BUT GAUGE CONTINUED TO DECREASE TO ZERO. PILOT ELECTED TO RETURN TO DEPARTURE. NO EMERGENCY DECLARED. AIRCRAFT LANDED WITHOUT INCIDENT. MAINTENANCE REPLACED THE NP INDICATOR. OPS CHECKED GOOD.