Abstract: A tamper-resistant, running time maintenance monitor for, and method of, indicating that equipment requiring periodic preventive maintenance is due for such maintenance. The monitor includes a keyboard for manually entering such user-selected data as an individualized access code, a warning time, a maintenance time, and at least some user-selected portion of a message individualized for the user. The entered data is stored and, if it is desired to change some or all of the data, e.g. the warning time, the maintenance time, or the individualized message portion, then the individualized access code must be entered. A running time sensor detects the running time, and a control unit accumulates the detected running time and generates warning and maintenance signals when the accumulated running time respectively matches the stored warning and maintenance times.