What was found
CLIMBING THROUGH FL 270 THE "CABIN PRESSURE" WARNING ILLUMINATED. CABIN WAS AT 10,000 AND 4.2 PSI DIFFERENTIAL WITH OUTFLOW VALVE FULLY CLOSED. BOTH PACKS WERE INDICATING NORMAL PRESSURE. CREW INITIATED AN EMERGENCY DESCENT WITH POWER ON TO FL 170 UNTIL CABIN CONTROL WAS REGAINED. CABIN ALTITUDE WAS MAINTAINED AT 8000 WITH A 3.4 PSI DIFFERENTIAL. FLIGHT ATTENDANT REPORTED A LOUD AIR LEAK AT AFT CABIN PLUG DOOR. LANDING AT CMH WAS UNEVENTFUL AND MAINTENANCE RESEATED THE PLUG DOOR. AIRCRAFT HAS RETURNED TO SERVICE.