As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
WITH THE APU RUNNING AND THE BLEED AIR OFF. THERE WAS A STRONG ODOR OF ELECTRICAL WIRES MELTING. ODOR WAS COMING FROM THE REAR OF THE CABIN. THE APU GEN WAS ON. CONFIRMED DISCREPANCY, FOUND SMELL COMING FROM A/C COOLING VENTS NOT FLOOR HEAT VENTS. CHECKED ALL DUCTS FOR LEAKS IN APU COMPARTMENT, NO LEAKS ON BURNED WIRES. FOUND WATER SEPARATOR SOCK DIRTY CLEANS IAW MM CH 25-51-00. PARA (4) OPS CHECK GOOD.