As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
SMOKE VAPOR OR TOXIC/NOXIOUS FUMES IN COCKPIT AND/OR CABIN. RETURNED TO FIELD IN IND TO "HOT CHARRED" ODOR IN COCKPIT. MTX DETERMINED SOURCE OF ODOR TO BE A MISSING WASHER ON THE LEFT WINDSHIELD HEAT CONTROLLER. WHEN THE W/S HEAT WAS CYCLING ON, THE ABSCENCE OF THE METAL WASHER CAUSED THE SURROUNDING PLASTIC TO HEAT UP AND MELT LEADING TO THE ODOR DESCRIBED.