As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WHEN PITOT HEAT IS ENGAGED, IT RENDERS THE STALL WARNING HORN INOPERABLE. STALL WARNING RESUMES WORKING PROPERLY WITH PITOT HEAT DISENGAGED. THE GROUND ON THE STALL WARNING UNIT WAS CLEANED. ALL OPERATIONS WORK PROPERLY. NOTE: THIS IS THE THIRD FINDING COME ACROSS WITH THIS AGE OF UNIT. (GL03200409830)