As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING FLIGHT CARBON MONOXIDE WARNING ILLUMINATED. PILOT RETURNED TO LAND. MAINTENANCE DISCOVERED 2 SPOTWELDS ON THE SHROUD SUPPORT FLANGE OF THE MUFFLER HAD FAILED RESULTING IN OPEN HOLES IN THE MUFFLER CAN. REPLACED MUFFLER WITH NEW UNIT.