Abstract: A method of detecting spray header valve and nozzle malfunctions, in a spray header including plural nozzles each having its own control valve, by supplying the spray header with liquid at a constant pressure head through a conduit while opening and closing the nozzle valves one at a time, and measuring the flow rate through the conduit each time one of the valves is open and when all the valves are closed. In response to detection of a malfunction at a particular nozzle, the valves of one or more adjacent nozzles are adjusted so that the adjacent-nozzle flow rates compensate for the malfunction, for example to maintain a desired cooling pattern across a surface being cooled by the spray header notwithstanding the malfunction at the particular nozzle.