As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON TAKEOFF PILOT GOT A CARBON MONOXIDE WARNING ON PRIMARY FLIGHT DISPLAY. RETURNED AND LANDED WITHOUT INCIDENT. MAINTENANCE VERIFIED PRESENCE OF CARBON MONOXIDE DURING RUN-UP. VISUAL INSPECTION REVEALED SPOT WELD FAILED ON MUFFLER BODY WHERE THE SHROUD SUPPORT FLANGE IS ATTACHED. MUFFLER REPLACED WITH NEW UNIT.