Symptoms of a guitar neck back bow:

• Fret buzz in the middle of the neck because the strings are too close to the frets.
• Inability to hear notes on certain (middle of neck) frets.
• Intonation issues as you play up the neck.
• No gap between the string and the 7th fret when holding the string down at the first fret.
  

  Reasons:

  • Truss rod adjusted improperly pulling the neck backward.
  • Changes in temperature and Excessive humidity causing the wood to warp. As moisture is absorbed into the neck fingerboard the neck swells and the back bow occurs.
  • Excessively heavy strings can cause a forward bow and as well stings too light could allow the neck to back bow. Therefore, the truss rod tension must be monitored at every string change.
  • Different string gauges than previously used can cause pressure to change on the neck. Heavier strings will cause a forward bow.
  • Changing ALL guitar strings at once will cause a sudden and unexpected loosening of the neck and it will drop back into a backwards bow. Be sure and change only ONE string at any given time to cause the least amount of disturbance to the neck pull as possible. Sudden relieving of string tension on any guitar neck will cause the rod to pull the neck into a back bow.
  • Other issues, such as frets being too tight in their slots or using water-based glue for the fingerboard. Truss rod too tight. Improper neck angle on a bolt-on style guitar neck.

  Repairs:

  Correcting a back bow is a delicate process, and if the guitar is valuable, it is highly recommended to consult a professional luthier.
  • Loosen the truss rod: This is the primary and least invasive method. The truss rod counteracts string tension; loosening it allows the strings to pull the neck forward into the correct slight relief.
    • How to: Loosen the strings slightly, then turn the truss rod adjustment nut counter-clockwise (lefty-loosey) in small increments (about a quarter turn at a time). Tune the guitar back to pitch, check the neck after a few hours, and repeat if necessary until the desired slight forward relief is achieved.
    • Neck massage: Advanced technique for experienced Luthiers only. Hold guitar down face up at about the 9th fret area with one hand. With the other hand pull up at the head-stock then push down several times to loosen the rod. Be careful as too much pressure or the wrong technique can break the head-stock. The reason for this extreme technique is the fact that the truss rod channel is dry with no lubrication. Sometimes the rod will bind in its channel and must be loosened for any truss rod adjustment to take effect.
    • Caution: Do not force a stuck truss rod, as a break can be very expensive to fix.
  • Heat and pressure treatment (for severe cases): For stubborn back bows that do not respond to truss rod adjustments (especially on necks with non-adjustable or single-action truss rods), luthiers may use controlled heat to soften the glue and gently force the neck into a forward bow. This is an advanced technique and should not be attempted by a novice, as improper heat and pressure application can damage the instrument.Haywire Custom Guitars addresses a guitar neck back bow by loosening the truss rod with a truss rod wrench, which will relieve tension and straighten the neck, unlike a "bow" which would require tightening. If the back bow is severe and the truss rod cannot fix it, it may indicate wood warping and a neck replacement could be necessary.