A maintainable product is the result of a conscious and dedicated effort to incorporate design features that make preventive and corrective maintenance easy, safe, and economical in terms of time and resources. Any product can be maintained given enough time and money, but time and money are pervasive constraints. So it is the responsibility of the design team to design the product so that it can be maintained within these constraints.
An essential aspect of maintainability is determining when a product is malfunctioning and why. The hardware, software, or other documented means used to determine that a malfunction has occurred and to isolate the cause of the malfunction are collectively called diagnostics. As products become more complex, diagnostics becomes an essential and critical part of design. For such products, diagnostics is a driver of maintainability because identifying and isolating a problem to its root cause often accounts for the majority of repair time. A diagnostic capability encompasses more than built-in-test (BIT); it includes any automatic, semi-automatic, and manual testing, maintenance aids, technical information manuals, and the effects of personnel and training. So any action performed for the purpose of detecting and isolating malfunctions, including any equipments, data or knowledge used in the performance of such actions can be associated with a diagnostic capability.
Appendix C details many of the design guidelines proven to result in maintainable products.