This part of ISO 15500 specifies tests and requirements for the check valve, a compressed natural gas (CNG) fuel system component intended for use on the types of motor vehicles defined in ISO 3833. This part of ISO 15500 is applicable to vehicles (mono-fuel, bi-fuel or dual-fuel applications) using natural gas in accordance with ISO 15403 (all parts). It is not applicable to the following: a) liquefied natural gas (LNG) fuel system components located upstream of, and including, the vaporizer; b) fuel containers; c) stationary gas engines; d) container-mounting hardware; e) electronic fuel management; f) refuelling receptacles.