Burglar Alarm Keeps Going Off for No Reason? Causes, Fixes and When to Call Someone
By the DC Fire & Security engineering team — installing and maintaining fire and security systems since 2010. Updated July 2026.
Quick answer
A burglar alarm that goes off for no reason almost always has a reason — the usual four are a failing backup battery, an insect or spider on a PIR sensor, a loose door contact, and a tamper fault after weather or DIY. Silence it at the panel, note the zone number it shows, and work through the causes below. If it keeps happening, a £100 +VAT call-out finds most faults inside the first hour — and repeated false alarms on a monitored system will eventually cost you police response.
Before anything else: get the zone number
When the panel sounds, it logs which zone triggered. That number is the difference between fixing the fault and guessing at it — zone 3 every time means one sensor with a problem, different zones each time means power or panel. Write down what the display shows before you clear it, including any words like TAMPER, PSU, LOST or BATT. Engineers can work from that over the phone; nobody can work from 'it just goes off'.
The usual suspects, in order of likelihood
| Pattern | Likely cause | Fix |
|---|---|---|
| Random activations, often at night | Backup battery reaching end of life — they last about 5 years, and voltage dips overnight as temperature falls | Battery replacement — part of a service visit |
| Same zone, again and again | Spider or insect on the PIR lens, or a cobweb moving in a draught — genuinely the most common false alarm there is | Clean the detector lens and clear webs; if it persists the PIR is failing |
| Same door zone repeatedly | Loose or misaligned door contact — doors swell and drop with the seasons | Re-seat or replace the contact |
| After a power cut | Mains fail plus a tired battery — the system panics as voltage collapses | Battery replacement; check the fused spur wasn't switched off |
| In rain or high wind | Bell box tamper — water in the sounder or a lid working loose | Reseal or replace the bell box |
| Goes off when the alarm isn't even set | Tamper circuit: a lid, cable or sounder cover has opened the loop — tampers are live 24/7 by design | Find the opened tamper (often the bell box or a keypad lid) — engineer job if not obvious |
| Everything at once, keypad beeping, strange display | Panel fault or dead battery taking the system down with it | Engineer visit |
Silencing it without making things worse
Enter your user code at the keypad — that silences and unsets on effectively every domestic panel: Orisec (what we install), Texecom, Pyronix, Honeywell and the older badge-engineered boxes all behave this way. After silencing, most panels want the fault acknowledged or the event cleared; the display will prompt.
Two things not to do. Don't pull the bell box off the wall or cut its cable — it has its own battery precisely so it keeps sounding when disconnected, and you'll have bought a new tamper fault. And don't disconnect the panel's backup battery as a fix; on most panels that either changes nothing or triggers the sounder. If the alarm can't be silenced from the keypad, the code has been lost or the panel is in an engineer state — that's a call-out, not a crowbar moment.
The 20-minute rule and the neighbours
External sounders are required to cut out after about 20 minutes (the panel keeps its internal sounder and any signalling going). If an alarm near you has been ringing for hours, that cut-out has failed — the council's environmental health team deals with persistent alarm noise as a statutory nuisance and can, at the sharp end, arrange silencing at the keyholder's cost. Police attend for signs of a break-in, not for noise.
If it's your alarm doing this to the street: same fault list as above, but the clock on goodwill runs faster.
When false alarms start costing real money
On a monitored system with police response, false alarms are counted. Rack up enough of them and the police withdraw the property's URN — the response entitlement — and getting it back needs a clean record and paperwork. Insurers who discounted the premium for a monitored alarm read that the same way.
The fix is boring and works: an annual service (£90–£180/year depending on system) that replaces batteries on schedule, cleans detectors and catches tired parts before 3am does. Systems that false-alarm are almost always systems nobody has serviced in years.
When to stop DIYing
- The same zone keeps triggering after a lens clean — the detector is failing
- Any TAMPER or PSU fault you can't trace — those loops are live even when unset
- The backup battery is 5+ years old (if you don't know its age, it is)
- The installer has disappeared and you've no engineer code — takeovers are routine work for us, no rip-out needed
- You've silenced it three mornings running — £100 +VAT gets an engineer there, first hour included, and most of these faults die in that hour
Frequently Asked Questions
Why does my burglar alarm go off at 3am?
How long will a burglar alarm sound for?
Can I silence the alarm without the code?
Is the spider thing actually real?
Will the police come when my alarm goes off?
Why does it go off when it isn't set?
My installer's gone — can you take the system over?
Alarm driving you (and the street) mad?
We repair and take over intruder alarms across Luton, Bedfordshire, Hertfordshire and North London — most makes, including systems whose installer has vanished. £100 +VAT call-out including the first hour; most false-alarm faults are found and fixed inside it.
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