As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
THE CREW REPORTED ON CLIMB OUT OF STL, AFTER SWITCHING TO ENGINE BLEEDS THEY NOTICED A STRONG ODOR SIMILAR TO "BURNED FIREWORKS". MX PERFORMED A GENERAL VISUAL INSPECTION OF NR 1 PACK SYS, PERFORMED HIGH POWER RUNS, AND PERFORMED A VISUAL INSPECTION OF THE RECIRCULATED FANS AND AIR CONDITIONING DUCTS IN CABIN. NO DEFECTS NOTED. R & R THE APU ANTI-SURGE VALVE WITH OPS CHECK GOOD. ACFT WAS APPROVED FOR RETURN TO SERVICE.