Maintenance of onboard systems is almost constant, and varies in severity. Everything from fixing a stubborn replicator, to realigning the dilithium matrix is handled by technicians and engineers on a regular basis. Not all systems are checked centrally by Main Engineering; to do so would occupy too much computer time by routing every single process to one location. To alleviate that, systems are compartmentalized by deck and location for checking. Department heads are expected to run regular diagnostics of their own equipment and report anomalies to Engineering to be fixed.
All key operating systems and subsystems aboard the ship have a number of preprogrammed diagnostic software and procedures for use when actual or potential malfunctions are experienced. These various diagnostic protocols are generally classified into five different levels, each offering a different degree of crew verification of automated tests. Which type of diagnostic is used in a given situation will generally depend upon the criticality of a situation, and upon the amount of time available for the test procedures.
Level 1 Diagnostic - This refers to the most comprehensive type of system diagnostic, which is normally conducted on ship's systems. Extensive automated diagnostic routines are performed, but a Level 1 diagnostic requires a team of crew members to physically verify operation of system mechanisms and to system readings, rather than depending on the automated programs, thereby guarding against possible malfunctions in self-testing hardware and software. Level 1 diagnostics on major systems can take several hours, and in many cases, the subject system must be taken off-line for all tests to be performed.
Level 2 Diagnostic - This refers to a comprehensive system diagnostic protocol, which, like a Level 1, involves extensive automated routines, but requires crew verification of fewer operational elements. This yields a somewhat less reliable system analysis, but is a procedure that can be conducted in less than half the time of the more complex tests.
Level 3 Diagnostic - This protocol is similar to Level 1 and 2 diagnostics but involves crew verification of only key mechanics and systems readings. Level 3 diagnostics are intended to be performed in ten minutes or less.
Level 4 Diagnostic - This automated procedure is intended for use whenever trouble is suspected with a given system. This protocol is similar to Level 5, but involves more sophisticated batteries of automated diagnostics. For most systems, Level 4 diagnostics can be performed in less than 30 seconds.
Level 5 Diagnostic - This automated procedure is intended for routine use to verify system performance. Level 5 diagnostics, which usually require less than 2.5 seconds, are typically performed on most systems on at least a daily basis, and are also performed during crisis situations when time and system resources are carefully managed.