A method is disclosed for diagnosing a valve assembly having valve members serially arranged along a flow channel of the valve assembly connecting at least one inlet and at least one outlet of the valve assembly. All of the serially arranged valve members of the valve assembly are opended to allow fluid to flow through the flow channel. The flow of fluid through the flow channel is measured by at least one sensor. At least one of the valve members is openend, and at least one sensor checks for fluid leakage caused by at least one faulted valve member. The at least one sensor may include a flow sensor, e.g., a mass flow sensor.