Mains (hardwired) smoke alarm not working / no light
A hardwired smoke alarm's power indicator is off or it isn't functioning — points at the alarm's supply (often a lighting circuit), the alarm itself, or end of life.
Safety first
Smoke alarms are life-safety devices — don't leave one out of service. Isolate before working; mains alarms are licensed electrical work.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Supply to the alarm lost
Most likelyThe alarm's supply (often shared with a lighting circuit) is off or tripped.
- 2
End-of-life / faulty alarm
#2The alarm has expired or failed and must be replaced.
- 3
Loose connection at the alarm base
#3A loose connection in the alarm's base plug/terminals.
- 4
Backup battery depleted (on a faulted unit)
Least likelyOn a unit that's lost mains, a depleted backup leaves it fully dead.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
Confirm the alarm's supply circuit is live (and not tripped); check the alarm's power indicator.
Supply present; indicator should be on.
Powered but not working — suspect the alarm (end of life/fault).
No supply / tripped — restore (find why if tripped).
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Mains alarm not working
→ step 2 - 2decision
Is the alarm's supply circuit live (not tripped)?
Yes→ step 3No→ step 4 - 3decision
Are the base connections sound and the alarm within life?
Yes→ step 5No→ step 6 - 4result
Restore the supply circuit (find why if tripped).
- 5result
Powered but dead — replace the alarm.
- 6result
Loose connection or end-of-life — rectify/replace.
Common mistakes apprentices make
- Leaving a mains alarm out of service.
- Not realising it shares a circuit with lighting (check that circuit).
- Overlooking a loose base connection.
- Keeping an expired alarm.
When to stop & escalate
Mains smoke alarm replacement and wiring is licensed electrical work and must keep the alarm in service. Replace expired/faulty units promptly.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
Related faults
Smoke alarm chirping / beeping intermittently
A smoke alarm chirps every minute or so — the classic low-battery or end-of-life signal, but can also be dust or a backup-battery issue on mains alarms.
Interconnected smoke alarms all sounding / not interconnecting
Interconnected alarms all sound when one triggers (by design), but you need to find which one, or they aren't interconnecting when they should — a wiring/wireless link issue.
All lights in one area out (others fine)
Every light in one part of the house is dead while power points and other lighting still work — points at that lighting circuit's protective device or a shared fault.