Replaced 52 light switches (8 3-way) from the traditional to the modern style. Ran into an issue with one of the 3-way switches where switch A would only turn on the light if switch B was on. Very annoying. Obviously, there was something wired wrong on either both or one of the switches.
Found this gem detailing steps on how to resolve this.
Assuming that the switches worked previously, and that only the switch terminal connections are wired wrong, I advise the following procedure.
If you are comfortable using a voltmeter or a neon tester. Follow these easy steps (for 2 -3 way switches).
1) Turn the power OFF and physically remove both switches, and take off the wires from the switches only. You do NOT have to remove the GROUND WIRE. This will leave 6 exposed wires (3 in each box).
2) Turn the power ON and identify the HOT wire (it will be the only one of six). Measure between each wire and the metal boxes or GROUND WIRE until you find it.(remember this wire) Then turn power back OFF. Mark this wire
3) The HOT wire found in step 2 is the COMMON wire, and goes on the DARKEST screw of the switch. The other 2 wires can go on either of the two remaining screws. Wire this switch (#1) back up and put it back into the box. This switch (#1) is now correct and you can focus on the other switch. (#2).
4) Turn the power back ON and identify a HOT wire at switch box (#2). Then toggle the (#1) switch and identify another HOT wire at box (#2). The COMMON wire at this switch (#2) box is the wire that was NOT HOT .
(remember this wire) Then turn power back OFF. Mark this wire
5) With the power OFF, wire up the second switch, The COMMON, wire goes on the DARKEST screw of the switch. Then wire up the two HOT wires to the two other screws.
LINK to Original post