As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOTS NOTED STRONG SMOKE/ODOR. CANOPY WAS OPENED IN FLIGHT TO VENT SMOKE/ODORS. RETURNED FOR LANDING. CO2 DETECTOR HAD TURNED DARK TO THE UNSAFE ZONE AND REMAINED DARK TILL SHORTLY AFTER LANDING. ACFT INSPECTED AND FOUND INDICATIONS OF EXHAUST STAINS AROUND LT SECONDARY HEAT VALVE. VALVE SECURED IN THE OFF POSITION. ACFT TEST FLOWN SERVICEABLE.