G-string not in balance

[Iggy]

Hi guys,
I have a small question, maybe someone can help me on this forum.
Five months ago I bought a second hand guitar (good build quality, small shop guitar).
When I tune the guitar and play the strings at the 12th fret, all of the string except the G-string are perfectly in tune.
The G-string starts to go out of tune after the fifth fret.
I really don't understand this.
Does anybody understands this phenomenon?
Is there something I can do about it to improve the situation?
The guitar sound further very good so I would be reluctant to let it go.
Thanks in advance.
Igor

[stanron]

A simple check you could do (may have done) first is to check with a capo at 1st fret. If the problem goes away it's the nut at fault. If the problem persists, does a change of strings make any difference, what strings do you use and what is the action like? A high action on an unwound or lower gauge G string can make it go sharp. If the string goes sharp it suggests the saddle is too far forward and if it goes flat, it is too far back. Before doing anything to the saddle I'd check the other stuff.

[Iggy]

Thank you for the reply.
I have checked a number of things you've suggested and am ashamed to tell the solution to the problem.
I've put new set of strings and the problem disappeared.
It' s stupid of course, I could had never guessed it.
But OK, thanks anyway.

Kind regards
Igor