A diagnostic program (also known as a test mode) is software that determines the operational status of a computer system. Ideally, it provides guidance for handling a detected issue or problem. Although called a program, it is not necessarily a computer program per se. In the more general sense of the word, it is a well-defined capability of a computer program. Such a capability is often found in a consumer electronic product and electronic game.

Often, the capability operates without user interaction. For example, a microwave oven may display code "F6" to warn of a shorted temperature probe or a garage door opener may flash an LED to warn of critically misaligned safety sensors and impending shutdown.

Early diagnostic programs were procedural instructions performed by humans, not by computer programs. See Built-in test equipment that enabled system operators to perform testing and diagnostics.

As computers became smaller, more mobile, computer programs were used to perform diagnostics tasks. In the late 1960s, Volkswagen introduced the first on-board computer system with diagnostics provisions.

The diagnostic program for a device or system may be independently located or integrated within. These Methods of Operation are arranged, more-or-less, in order of increasing complexity and their increasing value of diagnostics information.