As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW RECEIVED CO WARNING IN FLIGHT. THEY MADE A PRECAUTIONARY LANDING. UPON VISUAL INSPECTION OF EXHAUST SYSTEM FOUND CRACK IN AFT MUFFLER IN THE TRANSITION FROM FORWARD MUFFLER. THE MUFFLER WAS REPLACED NO CO WARNING WAS NOTED ON RUN UP.