As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
NR 3 ENGINE SHUT DOWN DUE TO BACKFIRE AND DOUBLE SHORTED SECONDARY ON NR 15 CYLINDER. REMOVED NR 15 CYLINDER FRONT AND REAR SPARK PLUGS. NO VISUAL DEFECTS NOTED. TESTED PLUGS AND FOUND WOULD NOT FIRE. INSTALLED SERVICEABLE PLUGS. PERFORMED ENGINE OPS CHECK, CHECKED GOOD. REF NAC DC-6MM SEC IV ECO5.1 & ECO1.2.