A burglar alarm that keeps going off for no obvious reason is one of the most frustrating problems a homeowner or business can deal with. It disturbs neighbours, erodes confidence in the system, and, if it happens often enough, risks a withdrawal of police response for monitored sites. The good news is that almost every false alarm has a knowable cause, and the majority can be diagnosed and fixed without replacing the system.
This guide walks through the most common causes of false activations, what you can check yourself, and when it is time to call a professional installer.
Why False Alarms Matter
UK police forces operate a three-strike rule, formalised under the NPCC Security Systems Policy. A monitored alarm with a URN (Unique Reference Number) that generates three false activations within a rolling 12-month window can have its police response level downgraded or withdrawn. Rebuilding that response requires the installer to investigate, fix the underlying problem, and apply for reinstatement.
Even for bell-only systems with no monitoring, repeated false alarms are a problem. Neighbours stop paying attention, the property stops being properly protected, and in some cases local authorities can take action under anti-social behaviour legislation.
The Five Most Common Causes of False Alarms
1. Insects and Spiders
The single most common cause of unexplained false alarms is a spider or insect crawling directly across the lens of a PIR (passive infrared) sensor. PIRs detect changes in infrared radiation, and a warm-bodied insect right on the lens looks exactly like a person walking through the beam from the sensor's perspective.
You will see this most often in lofts, garages, and outbuildings in warmer months, and in detectors mounted near ceilings where spiders spin webs. The fix is straightforward: vacuum or brush the sensor housing, clear any cobwebs from the surrounding area, and consider repositioning the detector away from known insect routes. Some detectors have "pet-immune" or anti-insect screens built in, and upgrading to these in problem areas is worth the small extra cost.
2. Pets
Standard PIR detectors cannot reliably distinguish between a cat, a small dog, and a person. If a pet is allowed into a protected area while the alarm is set, an activation is almost guaranteed. Two fixes exist:
- Confine pets to unprotected zones when setting the alarm. This works for quieter pets and single-room arrangements but is impractical for many households.
- Fit pet-tolerant PIRs that use a combination of two sensing elements and geometry to ignore animals below a given weight. Modern pet-tolerant detectors are effective up to 25 or 35 kilograms depending on the model and are the right choice in any household with a free-roaming cat or medium dog.
Retrofitting pet-tolerant detectors to an existing system is usually a quick and inexpensive job.
3. Draughts, Curtains, and Moving Objects
PIRs respond to fast changes in infrared. A warm radiator blowing air onto a curtain, sunlight moving across a sofa, or a loose draught from an open window can all produce false positives. Common culprits:
- Detectors mounted facing a window with direct sunlight.
- Curtains or blinds hanging directly in front of a detector's field of view.
- Radiators, heaters, or boilers within the detector's field.
- Ceiling fans, moving decorations, or helium balloons inside a protected area.
The fix is a combination of repositioning the detector, adjusting its sensitivity during commissioning, or moving the offending heat source. A good installer will have spotted most of these at the original survey, but homes change over time and what was a clean room five years ago may now have a radiator in the wrong place.
4. Low Batteries in Wireless Sensors
Wireless sensors run on long-life lithium cells that typically last three to five years. As a battery approaches end of life, the sensor's behaviour can become erratic. You may see random activations, supervision faults, or a sensor that reports fine during the day and then triggers at night when voltages dip.
Most modern wireless panels will flag low batteries before they cause false alarms, but this only works if someone is checking the panel's event log. If your system keeps reporting faults or activations from a single device, ask your installer to check the battery voltage at the next service. If a single sensor has started misbehaving after several years of reliable service, batteries are the first thing to suspect.
5. Mains Power and Environmental Issues
Less commonly, false alarms can be triggered by:
- Brief mains power dips that cause the panel to switch to battery backup. A failing backup battery in the panel itself can cause the system to behave unpredictably during a dip.
- Mobile phone signal loss on monitored systems using GSM signalling, which can cause the panel to report a loss-of-communication fault that some users mistake for a full activation.
- Water ingress in external sensors, sirens, or cable joints. This is particularly common on outdoor sirens more than a few years old.
- Insect nests inside external bell boxes, which can short-circuit the tamper switch and generate apparent activations.
What You Can Check Yourself
Before calling an engineer, there are a handful of checks worth doing. Most take less than ten minutes:
- Check the event log on the alarm panel or app. Every activation logs the device that triggered it. If the same device appears repeatedly, you know where to focus.
- Inspect that device. Look for cobwebs, insects, dust, or physical damage. Clean the sensor housing gently with a dry cloth.
- Check the surrounding area. Has a new radiator been installed nearby? Has a curtain been moved? Is a new pet using the room?
- Check the control panel display for any fault messages: low battery, supervision loss, mains loss, tamper, and so on. Most panels will tell you if a sensor is unhappy before it generates a full activation.
- Check the backup battery date. If your panel has not been serviced in the last 12 months and the main backup battery is more than three or four years old, it is overdue for replacement.
If the activations are coming from different devices at different times, or if you cannot pinpoint a single source, stop and call an engineer. Randomly adjusting settings on a commissioned alarm can create more problems than it solves.
When to Call a Professional
Call your installer if any of the following applies:
- The alarm has activated three times in the last 12 months and is monitored, because you are close to losing police response.
- Multiple devices are triggering and there is no obvious pattern.
- The panel is showing faults you do not recognise.
- The system has not been serviced in the last 12 months.
- You have recently changed the layout of the property, added pets, or had other building work done.
A diagnostic visit from a qualified engineer will usually identify the cause within an hour and either fix it on the spot or produce a clear recommendation. Most faults cost less to resolve than one callout charge from an emergency locksmith after a genuine burglary, so it is worth acting early.
Prevention: Regular Servicing Is Not Optional
The single biggest preventative measure is the annual service that insurance and inspectorate bodies already require. At a typical service visit, the engineer will test every sensor, check battery voltages, clean sensor housings, inspect external sirens, verify panel functionality, and update the event log. Around 80 per cent of the problems described in this guide will be caught at a standard annual service before they cause a false alarm.
If your system is out of service contract, or the installer is no longer trading, you can move the service to a different SSAIB or NSI approved company. The new installer will take over the system, issue a fresh maintenance certificate, and from then on the system will be covered under their agreement.
Getting It Sorted
At J&L Security, we diagnose and resolve false alarm problems on systems installed by us and by other companies, across Essex and Greater London. We are SSAIB approved, fully insured, and our engineers carry the common parts needed to fix most issues on the first visit.
If your alarm keeps going off and you want to get to the bottom of it, contact us or call 0204 538 5925. Read more about our burglar alarm services and other guides on alarm installation and maintenance.