A light over top of a set of stairs works some of the time but not always. What could be a possible reason for this fault?

• A. Single pole switch shorted and makes contact all of the time
• B. Bad lamp
• C. Neutral connection came apart
• D. 3-way switch is miswired

Answer: (D)

In a stair light that is controlled from two locations, the switches form a 3-way circuit. For the light to work reliably, the hot feed must connect to the common on one switch, the two traveler wires must connect between the switches, and the common on the other switch must connect to the fixture. If the two 3-way switches are wired incorrectly, the path for current can be completed only in some switch positions, so the light comes on in certain positions and off in others. That pattern—works some of the time depending on switch position—fits a miswired 3-way setup well.

Other possibilities tend to produce more consistent results. A bad lamp usually fails outright or flickers rather than only sometimes working; a neutral coming apart would typically leave the circuit open in all switch positions; a single-pole switch shorted would tend to keep the light on all the time rather than intermittently.

To check this, confirm the common terminals on both 3-way switches are correctly wired (the hot feed on one common, the load to the fixture on the other common) and that the two traveler wires are on the remaining terminals. If needed, rewire them per a proper 3-way diagram to restore consistent operation.