This article is from the Fencing FAQ, by Morgan Burke with numerous contributions by others.
Weapon fails weight test.
1) The main spring is too soft.
2) Friction between the barrel and point is overwhelming the
Weapon fails shim tests.
1) The contact spring is too long.
2) Point and barrel are mismatched.
Hitting the strip produces a light.
1) The strip is not grounded, or is dirty/corroded.
2) The tip is dirty/corroded.
A touch to the guard produces a light.
1) The guard is dirty/corroded.
2) The exterior of the tip is dirty/corroded.
3) The body wire (in particular the ground) is faulty (test
against the ground pin of the body cord; if the lights
continue, the body wire or reel is at fault).
4) The contact between the clip and weapon is faulty or corroded.
5) The guard is loose.
6) The ground pin socket is loose in the weapon clip.
Epee produces lights when the tip is not depressed.
1) The tip is jammed shut.
2) Grit in the tip is shorting the circuit.
3) The blade wires are shorting to each other.
4) The scoring box is on the wrong weapon setting.
There is no light when a touch is made.
1) You are not hitting properly.
2) Friction between the barrel and point is preventing the point
3) Main spring is too heavy.
4) Contact spring is too short.
5) The barrel is loose.
6) Point contacts are dirty/corroded.
7) The blade wire is broken.
8) The blade wire is shorting to the weapon.
9) Something has come unplugged between you and the box.
10) The wires are improperly fastened to the weapon clip.
11) The body wire is broken.
12) The reel or floor wire is broken.
13) The scoring box is on the wrong weapon setting.