I just bought a used guitar and when I tune it all strings are good except for the low E string, which I seem to be unable to tune to E2, which it is supposed to be. What could be wrong?
Taking a wild swing at this...
If you are using a meter & it can't quite grasp the pitch, there are a couple of things you can try.
Pluck the string with the soft part of your finger gently - fewer bright overtones means more fundamental frequency for the meter to listen to. If you pull the string too hard it will pitch-bend - it will start over-pitch [sharp] & drop down to pitch over a couple of seconds.
Rest your finger gently over the 12th fret & pluck as normal, which will create a perfect octave harmonic & tune to E3.
Do it the old-fashioned way & compare the A string to the E on the 5th fret.
It doesn't matter which octave it recognises, so long as it recognises the pitch itself - as it's almost impossible to be out of tune by that much. An octave under it would be like a slack rubber band & an octave over... well, you'd never get there in one piece.