As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING DEPARTURE CLIMB OUT OF NONESSENTIAL ELECTRICAL ITEMS. HE INSREASED VENTILATION AS THE CO INDICATOR GOT DARKER INDICATING CO INCREASE. HE RETURNED AND LANDED WITHOUT FURTHER PROBLEMS ALTHOUGH HE DID EXPERIENCE A HEADACHE AND NAUSEA. MAINTENANCE REPLACED THE MUFFLER AND HEATER SHROUD ON THE LT SIDE. OPS CHECK FLIGHT WAS NORMAL.