As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
UPON BLOCK IN AT GATE F/O SWITCHED TO APU POWER AND WE GOT A VERY STRONG ELECTRICAL BURNING SMELL THROUGH ACFT. ACCOMPLISHED APU DUCTING DECONTAMINATION AS PER AMM 21-00-01. REPLACED BOTH RECIRC FAN FILTERS AS PER AMM 21-25. NO DEFECTS NOTED.