Level 1:

Level 1 diagnostic refers to the most complex kind of diagnostic normally carried out on a Starship. While an immense battery of automated tests are performed, Level 1 requires crew members to physically check each component, thereby guarding against check routine failure. Level 1 on a major system can take several hours, and the system usually has to be taken offline to execute it.

Level 2:

Level 2, like Level 1, includes the same battery of automated tests as level 1, but requires physical verification of only about half the components in a system, yielding a somewhat less reliable test, but this can usually be done in under half the time of Level 1.

Level 3:

Level 3 runs the same automated tests as Levels 1 and 2, but requires crew verification of only key components. Level 3 is intended to be performed in under ten minutes.

Level 4:

Level 4 is intended for use when trouble is suspected with any given system. While it doesn't included crew verification (i.e. all tests are automated), it includes a battery of tests more sophisticated than Level 5. This can be performed in under30 seconds.

Level 5:

Level 5 diagnostics are performed on all systems at least daily, and are intended to verify system performance. They are also performed regularly during crisis situations (e.g. Red Alert). These tests take 2.5 seconds or less.