Check the dimmer or smart switch's own controls

Why this happens

Lutron's troubleshooting table for a light that will not turn on lists four causes: the front accessible service switch (FASS) is pulled out to off, the bulbs are burned out, the breaker is off or tripped, or there is a wiring error. Leviton's dimmer sheet lists a tripped breaker and a burned-out lamp too. Bulbs matter as well: DOE says LED bulbs must be designed for dimming, and Lutron notes that some bulbs turn on at a high light level but not at a low one. After an outage, Leviton says its Wi-Fi devices are designed to reconnect, but they keep trying over a period of time.

How to fix it

1. Set the dimmer to full brightness, then turn it on. Lutron notes that some bulbs will not come on at a low light level.
2. Look at the front of the switch for a small service tab. Lutron calls it the FASS. On Leviton Decora dimmers with a dim/bright bar, it is the air-gap switch at the bottom of that bar. If it is pulled out, the light has no power. Push it back in.
3. To restart a Lutron Caseta dimmer, Lutron says to pull the FASS tab out for about 10 seconds and then push it back in.
4. To restart a Leviton Decora Smart dimmer, Leviton says to lift the bottom of the dim/bright bar until the light or the LED at the bottom turns off, wait 5 seconds, and push the bar back in.
5. Check the breaker. At the panel, open only the hinged door. Never remove the metal cover behind it; that cover is for electricians. Keep your hands dry and do not stand on a wet floor. If a handle is in the middle or OFF, stand to the side, turn your face away, and push it firmly to OFF, then back to ON. Do this once. If it trips again or will not stay ON, leave it off and go to the breaker keeps tripping guide.
6. For a Leviton Decora Smart switch with no dim bar, Leviton's restart is to turn the breaker that powers the switch off for 5 seconds and back on. Skip this step if that breaker was tripped: resetting it has already restarted the switch, and a breaker that tripped again must stay off. Otherwise do this once, in the same careful way, and only if you are sure which breaker it is. If you are not sure, skip this step.
7. Make sure the bulbs are marked dimmable. DOE says the packaging will tell you. Before you take a bulb out to check it or change it, turn the light off at the switch, let a hot bulb cool, and pull the service tab out if the dimmer has one. Lutron and Leviton say the tab cuts power to the light. If a second dimmer controls the same light, pull its tab out as well. Push the tabs back in when you are done.
8. After an outage, check that your Wi-Fi is working and give a smart switch several minutes. Leviton says its devices reconnect on their own over a period of time.
9. Other brands differ, so use only a restart that your switch's own instructions describe on the front of the device. If the light is still dead, call a licensed electrician. Lutron's own next step is a wiring check, and that is not a homeowner job.

When to stop. Lutron says anything beyond routine bulb replacement needs the power disconnected at the main panel. If the front controls do not bring it back, or the dimmer is hot, discolored, buzzing or smells, stop and call a licensed electrician.

Start this troubleshooter over

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