Direct answer: A hardwired smoke or CO detector with no green light usually means it is not getting house power, the breaker or branch circuit is off, the plug at the detector is loose, or the detector itself has failed. Start by checking whether other devices lost power too before you replace the detector.
Most likely: Most often, the detector lost AC power from a tripped breaker, a switched-off circuit, or a loose plug at the detector head. If the circuit is live and the detector still stays dark, the detector is usually at the end of its service life.
The green light is the quick clue that the detector sees house power. No green light does not always mean the detector is bad. Sometimes the unit is fine and the branch circuit is dead. Sometimes the detector is old, the battery drawer is not seated, or the head is not locked onto the mounting plate all the way. Reality check: a lot of homeowners replace the detector first and then find out the circuit feeding it is dead. Common wrong move: twisting the detector off and tugging on the wiring harness with the breaker still on.
Don’t start with: Do not start by opening wiring splices, probing live conductors, or buying a new detector just because the light is out.
A single smoke or CO detector has no green light, but the rest of the interconnected units still look normal.
Start here: Start with that detector’s battery drawer, mounting position, and plug connection before blaming the whole circuit.
More than one hardwired detector is dark, often after a breaker trip, outage, or recent electrical work.
Start here: Start at the breaker panel and check for other dead lights or devices on the same branch.
You hear periodic chirping, or the unit seems partly alive, but the AC power light is off.
Start here: Check the backup battery and battery drawer first, then confirm whether house power to the detector is actually present.
No green light, no test response, and no obvious sign of life from the detector.
Start here: Treat this as either lost branch power or a failed detector. Do the safe power checks first, then replace the detector only if the circuit is confirmed live.
When several detectors lose the green light together, the common feed is usually the issue. This is especially likely after a storm, outage, nuisance trip, or recent work on lights or switches.
Quick check: At the panel, look for a tripped breaker. Then check whether nearby lights, hallway fixtures, or another device on that circuit are also dead.
A hardwired detector can lose AC contact if the head is twisted on crooked, the quick-connect plug is not fully latched, or the battery drawer is keeping the unit from seating correctly.
Quick check: With power off, remove the detector head, inspect the plug, and reinstall it firmly until it locks flat on the plate.
Some hardwired detectors act odd or stay dark when the battery is missing, installed backward, or the drawer is not fully shut. Homeowners often miss this after a battery change.
Quick check: Open the battery compartment, confirm the correct battery is installed the right way, and make sure the drawer clicks fully closed.
If the branch has power, the harness is secure, and only one older detector stays dark, the detector itself is the likely problem. These units do not last forever.
Quick check: Look for an age date on the detector body. If it is at or beyond its service life and the circuit is live, replacement is the right move.
You want to separate a single bad detector from a power problem feeding multiple devices. That saves time and keeps you from replacing the wrong thing.
Next move: If you find other dead devices or multiple dark detectors, you have a branch power problem to correct first. If only one detector is affected and the rest of the house looks normal, focus on that detector itself.
What to conclude: Multiple dark units point to lost AC power. One dark unit points more often to a loose connection, battery issue, poor seating, or a failed detector.
A breaker can look on when it is really tripped halfway. Hardwired detectors often come back as soon as the branch is restored.
Next move: If the green light comes back and stays on, the detector likely just lost branch power. If the breaker is on and the detector still has no green light, move to the detector-side checks.
What to conclude: A restored light after breaker reset confirms the detector was not the main problem. No change means either the detector is not receiving power at its connection or the detector has failed.
This is the safest hands-on check and it solves a surprising number of no-light complaints. A battery drawer left ajar or a detector twisted on crooked can keep the unit from working normally.
Next move: If the green light returns after reseating the battery or detector head, the issue was a simple fit or battery-drawer problem. If the light stays off, the next question is whether the detector harness is connected securely and whether the unit is old enough to replace.
Once the easy fit issues are ruled out, the two most useful clues are a loose quick-connect harness and the detector’s age. Older units with confirmed power are usually not worth fighting.
Next move: If reconnecting the harness brings the green light back, the plug was likely not seated well. If the harness is secure, the unit is older, and the light still stays off, replacement is the most practical next step.
By this point, you have ruled out the common easy misses. If one detector stays dark on a live circuit, replacement is the clean fix. If several units are dark or power is still questionable, this is no longer a detector-only problem.
A good result: If the new detector shows a steady green light and responds normally to its test button, the old detector was the problem.
If not: If a replacement detector also has no green light, the issue is in the house power, interconnect branch, or wiring connection, not the detector head.
What to conclude: A successful replacement confirms detector failure. A second dark detector on the same spot points away from the detector and toward the circuit feeding it.
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Most of the time, the detector is not getting AC power from the house. Start with the breaker and whether other detectors or nearby lights are also dead. If only one detector is affected, check the battery drawer, detector seating, and harness connection before replacing it.
No. A dark power light can mean the branch circuit is off, the breaker tripped, the detector is not seated correctly, or the quick-connect plug is loose. Replace the detector only after those checks make sense.
It may still chirp or respond on battery backup, but the missing green light usually means it is not seeing house power. That is not a condition to ignore, because the detector is no longer operating the way it should on normal AC power.
Not automatically. If one unit is dark and the others are normal, start with that detector. If it is old and the circuit is live, replacing that detector is reasonable. If several units are the same age, it is smart to check their dates too.
That points away from the detector and toward the power feeding it. The branch circuit, wiring connection, or interconnect setup needs to be checked. At that point, stop DIY and call an electrician.
Usually the green light is the AC power indicator, so a dead battery is not the main reason it goes out. But a missing battery, wrong battery, or battery drawer left open can make the detector act dead or keep it from operating normally, so it is still worth checking.