As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) DURING A TEST FLIGHT AFTER AN ENGINE CHANGE THE LT ENGINE BEGAN TO RUN ROUGH AT CRUISE POWER SETTINGS. AS A PRECAUTION, THE ENGINE WAS SHUT DOWN AND SECURED. AIRCRAFT LANDED WITHOUT INCIDENT. INVESTIGATION FOUND ONE SPARK PLUG BECAME LOOSE AND BACKED OUT OF CYLINDER HEAD. ALL OTHER SPARK PLUGS FOUND TO BE SECURE. MAINTENANCE CREW BRIEFED.