From UFStarfleet LCARS
Diagnostics are engineering and operations analysis programs used on any and all starfleet ships, stations and installations. Starfleet uses a standard diagnostics scale of 1 to 5. 1 being the most comprehensive examination of the system while a level 5 is a fast,automated examination. Diagnostics programs are pre programmed engineering analysis software programs and carry out an automated determination of system performance and at the same time identifies any malfunctions.
These diagnostics are intended to analyse and repair malfunctioning systems
Level 5 Diagnostic
This is an automated procedure that is mostly intended for routine use to verify system performance. Level 5 diagnostics require usually less the 3 seconds and are typically performed on all systems at least on a daily basis. Level 5 diagnostics are also performed during crisis situations when time and system recourses are carefully managed. During a Level 5 diagnostic the system may remain On Line
Level 4 Diagnostic
A Level4 diagnostic is an automated procedure used whenever trouble is suspected with a given system. The protocolis similar to a Level 5 but does involves a more sophisticated analysis. A Level4 diagnostic is also used during a shift change in order to generate an accurate report for the duty officer. During a Level 4 diagnostic the system may remain On Line and can be performed in under 30 seconds.
Level 3 Diagnostic
A Level 3 diagnostic is a protocol very similar to a Level 1 and Level 2 diagnostics but also involves crew verification of key mechanics and system readings. This is very complete scan and is only usedon computer systems. During a Level 3 diagnostic the system may remain On Line and can be performed in 10 minutes or less.
Level 2 Diagnostic
A Level 2 diagnostic refers to a comprehensive system diagnostic protocol which, like a Level 1 diagnostic, involves a very extensive automated routines but also requires crew verification of most operationalsystems. A Level 2 diagnostics takes around 1.5 to 2 hours and does require the systems to be taken off line before any tests can be performed
Level 1 Diagnostic
A Level 1 diagnostics is the most comprehensive type of systems diagnostics. Extensive automated diagnostic routines are performed but a team of crew members are required to physically verify the operation of system mechanisms and system readings. A Level1 diagnostics takes around 2 to 3 hours and requires the systems to be taken off line during the tests