As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IN CRUISE, UPON TRANSFERRING THE BLEEDS TO THE APU, A HEAVY GAS & FUME SMELL BECAME NOTICEABLE. TRANSFERRED THE BLEEDS BACK TO THE ENGS & SHUT DOWN THE APU. THE SMELL WENT AWAY. FLT 5259 REMOVED APU COVER & INSPECTED FOR LEAKS WITH APU RUNNING. NO LEAKS NOTED AT THIS TIME.