WiremanWireman · Light won’t turn on: troubleshooting for electricians · 2023 NEC ·
Light won’t turn on
A dead fixture on a plain switch. Work from the bulb to the switch to the fixture box.
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How it works
A switched light is a simple loop. The hot comes from the panel, goes through the switch, then on to the fixture on the switched hot, and the neutral carries the current back. The light only works when every piece of that loop is whole: the bulb, the socket, the switch, both conductors and every splice between them. Testing at the switch and then at the fixture tells you which piece is open.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- Did you try a bulb you know works?
- Yes, and now it worksA burned-out bulb
- Known-good bulb, still deadIs anything else on that circuit dead too?
- Yes, other things are deadThe whole circuit is out, not just this light
- Only this lightWhat controls the light?
- One plain switchTest: Measure at the switchExpect: One screw reads about 120 V all the time (the line). The other reads about 120 V with the switch on and near 0 V with it off.
- No voltage on either screwNo power reaching the switch
- 120 V in, nothing out with it onA failed switch
- 120 V on both screws with it onTest: Measure at the fixtureExpect: About 120 V on both readings.
- 120 V on bothTest: Inspect the socket and fixture wiring
- Socket tab is pushed flatSocket tab not touching the bulb
- Socket or wires are scorched or brittleHeat damage in the fixture
- All looks fine, or it’s an integrated LEDThe fixture or its LED driver has failed
- Black to ground 120 V, black to white about 0 VOpen neutral to the fixture
- About 0 V on bothA break between the switch and the fixture
- 120 V on bothTest: Inspect the socket and fixture wiring
- Two or more switches (3-way or 4-way)A 3-way or 4-way circuit
- A dimmer, smart switch, timer or motion sensorAn electronic control
- One plain switchTest: Measure at the switchExpect: One screw reads about 120 V all the time (the line). The other reads about 120 V with the switch on and near 0 V with it off.
- It’s an LED fixture with no bulbIs anything else on that circuit dead too?
- Yes, other things are deadThe whole circuit is out, not just this light
- Only this lightWhat controls the light?
- One plain switchTest: Measure at the switchExpect: One screw reads about 120 V all the time (the line). The other reads about 120 V with the switch on and near 0 V with it off.
- No voltage on either screwNo power reaching the switch
- 120 V in, nothing out with it onA failed switch
- 120 V on both screws with it onTest: Measure at the fixtureExpect: About 120 V on both readings.
- 120 V on bothTest: Inspect the socket and fixture wiring
- Socket tab is pushed flatSocket tab not touching the bulb
- Socket or wires are scorched or brittleHeat damage in the fixture
- All looks fine, or it’s an integrated LEDThe fixture or its LED driver has failed
- Black to ground 120 V, black to white about 0 VOpen neutral to the fixture
- About 0 V on bothA break between the switch and the fixture
- 120 V on bothTest: Inspect the socket and fixture wiring
- Two or more switches (3-way or 4-way)A 3-way or 4-way circuit
- A dimmer, smart switch, timer or motion sensorAn electronic control
- One plain switchTest: Measure at the switchExpect: One screw reads about 120 V all the time (the line). The other reads about 120 V with the switch on and near 0 V with it off.