As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ORD FLT 6099 THE CREW REPORTED THE CABIN ALTITUDE ROSE TO 8600 FT. IT WOULD NOT HOLD IN EITHER AUTO OR MANUAL MODE EVEN WITH BOTH PACKS SELECTED ON ROSE TO 12000 FT DESENDED TO 10000. MAINTENANCE REPLACED THE MANUAL PRESSURIZATION CONTROLLER. OPERATIONAL CHECKS WERE GOOD.