Only volume up F12 not working-stumped!!

[redsquash]

Only volume up, F12, not working,
F10 and Fll mute and volume down work perfectly
but not f12 (volume up).
Have used
1 sudo killall coreaudiod
and tried
2 sudo kill -9 `ps ax|grep 'coreaudio[a-z]' | awk '{print $1}'`


3 I can increase the volume using the slider but that should not
be unnecessary.
4 Have restarted.
5 Have gone into system preferences sound and tickeded and unticked boxes
in a variety of permutations.
and
6 Have gone into system preferences viewer, F12 doesn't highlight but
then again F11 cause the screen to revert to desk top and back if pressed again.

so seems I cant check 2 keys but F11 decrease sound does work

7 Used the keyboard on a different iMac but F12 still wont increase volume

Is there another way to check if F12 is not working.
All internet searches give advice when all the sound keys are not working not when there is just one sound key not working.

It seems probable the F12 key is the problem, but how can I be certain its a F12 keyboard malfunction.
STUMPED !!!!

 

[Tarek]

I am not sure if Apple Diagnostics will cover this, but it does seem to identify a couple of keyboard problems in this article.

This is how you run it.

 

[Nermal]

If a different keyboard has the same issue then it's unlikely to be a hardware problem. The next thing I'd try would be to boot into Internet Recovery and see whether the same issue occurs there.