As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) WHEN ACCOMPLISHING A O/H ON THE HEATER WE FOUND THE DIFFERENTIAL PRESS SW STUCK IN THE CLOSED POSITION. THIS IS THE FOURTH SWITCH FOUND DEFECTIVE IN OUR O/H SHOP. THE SUPPOSEDLY NEW IMPROVED SWITCH MFG HAS PRODUCED IS NO BETTER THAN THE OLD SWITCH THAT IT REPLACES.