A method of diagnosing a check valve

A method for diagnosing a stop valve in a fuel line, said shut-off valve in the fuel line between at least one tank and at least one injection valve is arranged, comprising the steps of: closing the shut-off valve, adjusting a fuel pressure downstream of the closed shut-off valve, which is smaller than the fuel pressure upstream of the shut-off valve, monitoring the fuel pressure in the closed injection valve downstream of the isolation valve, wherein the barrier effect of the valve is recognized as insufficient when the pressure in the fuel line downstream of the valve increases.