As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIR RETURN. STRONG BURNING ODOR IN COCKPIT & CABIN, PERFORMED TROUBLESHOOTING IAW TSM 05-50-00-810-831-A. PERFORMED APU INSPECTION IAW 49-00-00-200-814 NO LEAKS NOTED. RAN NR 1 PACK FOR 20 MINS NO ODOR NOTED. NR 1 ENG PRECOOLER FOUND LEAKING AND RUPTURED AND REPLACED.