NOVELTY - The current and voltage of fuel cell stack are measured during driving of fuel cell vehicle, and based on the current, it is determined (S103) whether vehicle is being operated at constant current. The different factors are analyzed depending on whether vehicle is being operated at constant current, and it is determined whether the fuel cell stack is in normal state. A moisture supply into stack is calculated on determining that the fuel cell stack is not in normal state, and it is diagnosed (S115) whether fuel cell stack is in dryout state, based on calculated moisture supply.    USE - Method of diagnosing a fuel cell stack of fuel cell vehicle such as hybrid fuel cell vehicles, electric fuel cell vehicles, plug-in hybrid electric fuel cell vehicles, hydrogen-powered fuel cell vehicles, and all other types of fuel cell vehicles. Can also be used in motor vehicles such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, etc.    ADVANTAGE - The deterioration in the performance of fuel cell stack attributable to the dryout or the long-term deterioration of durability of the fuel cell stack is sensed efficiently, and the recovery operation is executed effectively.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) an apparatus for diagnosing a fuel cell stack; and    (2) a non-transitory computer readable medium containing program instructions for diagnosing a fuel cell stack.    DESCRIPTION OF DRAWING(S) - The drawing shows a flowchart of the method of diagnosing a fuel cell stack. (Drawing includes non-English language text)    Storing current and voltage of fuel cell stack in a queue in a memory (S101)    Determining whether vehicle is being operated at constant current (S103)    Determining whether long-term deterioration of durability of fuel cell stack has occurred (S105)    Analyzing current-output voltage characteristic curve (S107)    Diagnosing whether fuel cell stack is in dryout state (S115)