Deteriorating components, particularly crystals and vacuum tubes, cause reduction of safety margins and are a prncipal source of error in digital computing and pulse communication. Marginal checking varies voltages in logical circuit groups, inducing inferior parts to cause failure, while a test program or pulse transmission detects and localizes potential failure. In a digital computer, this can be automatically accomplished with the computer itself acting as the detector. In one trial on a 400-tube prototype system the application of this type of preventive maintenance for half an hour per day improved reliability 50 to 1. Results of preliminary tests on a full computer are discussed.