As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ORD - CREW REPORTED HYDRAULIC SYSTEM MAINTENANCE MESSAGE AND THEN THE CENTER 2 HYDRAULIC OVERHEAT LIGHT ILLUMINATED. EMERGENCY DECLARED RETURNED ORD LANDED WITHOUT INCIDENT. AIRCRAFT REMOVED FROM SERVICE. REPLACED AIR DRIVEN PUMP FILTER MODULE PRESSURE RELIEF VALVE.