What was found
(CAN) CABIN PRESSURE WARNING CAME ON DURING FLIGHT. CREW MENTIONED THERE WAS NO FAULT LIGHT ON THE CPCP. CREW TRIED MAN. THE DIFFERENTIAL PRESSURE WAS CLOSE TO 0. EMERGENCY WAS DECLARED AND AIR TURN BACK PERFORMED. DURING TROUBLE SHOOTING TECHNICIANS FOUND THE RECIRC FAN AFT OUTLET DUCT CRACKED OVER THE FULL LENGTH. DUCT REPLACED. TESTS SATISFACTORY. A/C RETURNED TO SERVICE. (TC# 20190604020)