What was found
(CAN) ACFT HAD AN IN-FLIGHT TURN-BACK DUE TO CABIN ALT WARNING AND CAB AUTO CAUTION ON EICAS. ACFT ARRIVED BACK ON THE GATE WITH OUTFLOW VALVE CLOSED AND CABIN PARTIALLY PRESSURIZED. MX REPLACED, TESTED OUTFLOW VALVE AND ACFT DEPARTED AGAIN ONLY TO RETURN WITH SAME PROBLEM. PRESSURIZATION SELECTOR PANEL AND CABIN PRESSURE CONTROLLERS WERE REPLACED AND WIRING CHECKED. A MX VERIFICATION FLIGHT WAS CARRIED OUT WITH SAME RESULT. MFG WAS CONTACTED WHO SUGGESTED CHANGING THE STANDBY ALTIMETER. THE STANDBY ALTIMETER INTERFACES WITH CABIN PRESSURE CONTROLLERS AND IF THE AIR DATA SIGNALS FROM THE RECEIVERS TO CONTROLLERS ARE INVALID THE CONTROLLER WILL "AUTO FAIL" AND STORE A CONTROLLER FAULT. THIS FAULT IS INHIBITED ON GROUND AND CAN ONLY BE CONFIRMED IF THE ACFT IS IN FLIGHT MODE. THE STANDBY ALTIMETER HAD BEEN REPLACED AS A SCHEDULED COMPONENT CHANGE PRIOR TO EVENT AND FAILED ON INSTALL.