Always treat a sounding smoke, heat or carbon monoxide alarm as genuine until the cause is confirmed
Check the household, move to safety where required and follow the alarm manufacturer’s emergency instructions. Do not remove batteries or disable alarms simply to stop the noise.
MOST LIKELY CAUSES
Start with the alarm pattern and initiating unit
Symptom variations Match the exact sound, LED and interlink behaviour. ⌄
- Green LED off
- Alarm runs only on backup battery
- Several alarms lose green lights
- Problem began after electrical work
- Alarm chirps with no mains indication
Safe first checks Checks before reset, cleaning or replacement. ⌄
- Check for smoke, heat, fire or possible carbon monoxide first.
- Identify which alarm is initiating or chirping.
- Record the sound pattern and LED colour.
- Check the manufacture and replacement date.
- Do not disable the household alarm system without providing alternative protection.
Step 1 — Check the alarm still responds to its test button Confirm the alarm pattern and household safety before attempting reset or replacement. ⌄
A working backup battery does not prove the mains supply is present.
Step 2 — Check the consumer-unit circuit externally Confirm the alarm pattern and household safety before attempting reset or replacement. ⌄
Confirm the labelled smoke-alarm circuit has not tripped or been switched off.
Step 3 — Arrange electrical testing if the light stays off Confirm the alarm pattern and household safety before attempting reset or replacement. ⌄
Do not open mains wiring or bases unless competent to do so.
What the warning usually means How this domestic alarm condition develops. ⌄
No green mains light normally means the alarm is not receiving its normal AC supply, although indicator behaviour varies by manufacturer.
Further diagnostic checks Checks after household safety has been confirmed. ⌄
- Verify supply at the alarm base using safe isolation.
- Check looped live, neutral and interconnect terminations.
- Inspect for loose plug-in connectors.
- Confirm circuit protective-device status and recent alterations.
- Replace the alarm if supply is correct but the power indicator remains abnormal.
Information and equipment needed Useful items for accurate checking. ⌄
- Alarm make and model
- Manufacture and replace-by date
- User instructions and sound-code table
- Specified replacement battery where applicable
- Compatible replacement alarm and test method
Parts that may be involved Components commonly associated with this problem. ⌄
- Replacement mains smoke alarm
- Compatible powered base
- Backup battery
- Circuit protective device
- Wiring connectors
When immediate action is required Conditions that require urgent response. ⌄
- There may be smoke, fire or overheating.
- A carbon monoxide alarm pattern is sounding.
- Anyone has headache, dizziness, nausea or confusion.
- Multiple alarms have lost mains or interlink operation.
- The only alarm protecting an area has failed.
- The alarm cannot be restored to normal reliable operation.
Frequently asked questions Common domestic alarm questions. ⌄
Can a chirping alarm just be removed?
No. The warning should be identified and the battery or complete alarm replaced while maintaining protection.
Why do all alarms sound when only one detects something?
That is the purpose of interlinking. One initiating alarm warns the entire household.
How do I know which alarm started it?
The initiating unit usually shows a different LED or stored alarm-memory pattern described in its instructions.