The present invention relates to a method for diagnosing at least one pressure relief valve of a liquefied gas fuel system (a) for a vehicle. said fuel system (a) has tank (10) for supplying fuel to the vehicle engine, and at least one pressure relief valve (22, 24) set to open at a predetermined pressure to evacuate evaporated gas. the method comprises the steps of: connecting a temporary pressure relief valve (22t, 24t) to said at least one pressure relief valve (22, 24), said temporary pressure relief valve being established to opening at a pressure above said predetermined pressure; increasing the pressure in the tank to a pressure above said set pressure; determining if said at least one pressure relief valve opens (22, 24); if said at least one pressure relief valve opens, determine at what pressure said at least one pressure relief valve opens; and, based on determinations that said at least one pressure relief valve opens and, if said at least one pressure relief valve opens, at what pressure said at least one pressure relief valve opens, diagnose the functionality of said at least one pressure relief valve. The present invention also relates to a system for diagnosing at least one pressure relief valve of a liquefied gas fuel system. The present invention also relates to a vehicle.
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