As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING A DUEL INSTRUCTION FLIGHT, ENGINE BECAME UNRESPONSIVE TO THROTTLE INPUT. THE AIRCRAFT RETURNED TO THE AIRPORT AND LANDED SAFELY. DURING THE SUBSEQUENT INVESTIGATION IT WAS DISCOVERED THE THROTTLE ARM SPLINES APPEARED TO BE WORN/ SHEARD. THE SPLINES ON THE THROTTLE SHAFT AND ARM ARE NOT VERY ROBUST AND DON'T ALWAYS MATCH UP EVEN WHEN NEW. THIS DESIGN RELIES ON TORQUE TO MASH THE SPLINES TOGETHER TO PROVIDE THE FRICTION / MECHANICAL INTERLOCK TO KEEP THE ARM FROM SLIPPING ON THE SHAFT.