As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
JUST AFTER TAKEOFF, THE LOW ROTOR CAUTION LIGHT ILLUMINATED. 2 OR 3 SECONDS LATER, THE LOW ROTOR RPM HORN SOUNDED. ROTOR TACH INDICATIONS WERE NORMAL, PILOT RETURNED TO BASE. UPON LANDING, THE MECHANIC REMOVED AND REPLACED BOTH THE LOW ROTOR RPM AND ENGINE OUT SENSORS DUE TO CORROSION AND AC WAS OPS CHECKED AND RETURNED TO SERVICE. (K)