As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
OWNER REPORTED HIGH CARBON MONOXIDE INDICATED ON HIS COCKPIT MOUNTED CO DETECTOR. WE REMOVED THE MUFFLER SHROUD AND FOUND THE 6 TACK WELDS USED TO ATTACH THE MUFFLER SHROUD SUPPORT RING HAD FAILED/BROKEN. THIS WELD BREAKAGE CREATED A HOLE IN THE MUFFLER. WE FOUND EXHAUST SOOT IN THE CABIN AIR DUCTING!