As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DEN - ON TAXI OUT FOR FLIGHT 4014, RECEIVED A RIGHT PACK CAUTION MESSAGE. RETURNED TO THE GATE. MAINTENANCE FOUND A CONTROL VALVE 2 POSITION DISAGREE. CYCLED NR 2 PACK. NO FAULTS FOUND. OPERATIONAL CHECKS GOOD. AIRCRAFT RETURNED TO SERVICE. NO PARTS REPLACED.