Blue Bird Body Company (Blue Bird) is recalling certain model year 2013-2014 All American rear engine school buses manufactured July 21, 2011 to April 30, 2014, and equipped with CNG fuel systems and delivered to or registered in the following “Cold Weather” states: Colorado, Pennsylvania, Utah, and Wyoming. In the affected vehicles, an O-ring in the pressure relief device for the compressed natural gas (CNG) system may fail at cold temperatures resulting in a natural gas leak.