As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PDX - ON TAXI OUT FOR FLIGHT 2456 RECEIVED A RIGHT PACK CAUTION MESSAGE. FLIGHT RETURNED TO THE GATE. MAINTENANCE PLACED THE RIGHT AIR CONDITIONING PACK ON MEL. MEL CLEARED ON 10/23 BY PERFORMING TROUBLESHOOTING IAW FIM 21-53-14, WITH NO DEFECTS NOTED. PERFORMED OPERATIONAL CHECK IAW AMM 21-51-00, NO DEFECTS NOTED. COULD NOT DUPLICATE DISCREPANCY. MEL CLEARED AND AIRCRAFT RETURNED TO SERVICE. NO PARTS REPLACED.