Which one is common: "Diagnostic" or "Diagnostics"?

I'm translating the interface texts of an industrial control panel software and got stuck on this one. The module I'm referring to has some lists containing the sensors and switches for the digital and analog inputs and outputs of the machine. This module was called "Manual Control" in its current language but I've decided to give it a more proper term. In general those functions are mostly included in modules called "Diagnostics" in the plural form but when I've sent my suggestion to the software team they've decided to use the word in its singular form "Diagnostic".

I couldn't find any online reference to this matter so I'm asking it here. Which form is common and if possible could you explain why is it so?

Referring to technology, diagnostic appears to be the more common term. Note that "diagnostics" is a noun, while "diagnostic" is both a noun and and adjective:

From Dictionary.com:

• Computers:
  • a message output by a computer diagnosing an error in a computer program, computer system, or component device.
  • a program or subroutine that produces such messages.

From the OED:

• Computing:

• Of a program or sub-routine: designed to identify program errors or system faults and give information about them.

  • 1950 W. W. Stifler et al. High-Speed Computing Devices xvii. 437 In the computer proposed by the Raytheon Company, self-checking and diagnostic equipment is provided throughout.
  • 1953 Proc. IRE XLI. 1320/1 We discuss the use of three types of diagnostic and servicing programs which enable us to use the computer to diagnose its own troubles.

  • 1967 A. Battersby Network Analysis (ed. 2) viii. 140 Diagnostic routines are able to detect obvious errors in the input and print out comments on them—i think i have a loop is one.

  • 1985 Sci. Amer. July 13/1 This program, which will run only in the graphics mode, is diagnostic.

Ngram: computer diagnostic vs computer diagnostics, software diagnostic vs software diagnostics