What was found
FLT 588 - DFW - DECLARED AN EMERGENCY, AIR RETURN, LANDING UNEVENTFUL. LOST ALL CABIN PRESSURE WITHIN 10 SECONDS AT 22,000. SYSTEM WENT TO STANDBY, BUT NO IMPROVEMENT. REGAINED CABIN WITH MANUAL MODE, CABIN MASKS DEPLOYED. REPLACED CABIN PRESSURE CONTROLLER PER MM 21-31-0, OPS CHECK OK. SERVICED PAX O2 BOTTLE PER MM 12-15-21. REPACKED PAX CABIN O2 MASKS PER MM 35-21-52. (M)