Abstract: Disclosed is a method for determining operation of a valve in a gas tank system comprising a plurality of gas tank and valve arrangements, each comprising a gas tank and at least one valve arranged at a passage downstream said gas tank. The method comprises: opening the valves at the passages in a first set of gas tank and valve arrangements; determining a first pressure value at a gas transportation arrangement downstream all of said opened valves; closing the passage at all gas tank and valve arrangements, except for one gas tank and valve arrangement; waiting a pre-determined amount of time; determining a second pressure value at said gas transportation arrangement downstream the gas and tank and valve arrangement whose valve have not been closed; and determining whether said at least one valve which has not been closed operates properly based on said first and second determined pressure values.