Why does my security light come on by itself or stay on?

Whether the trigger is heat, a setting, a stuck override or water in the fitting — and how to tell which, without guessing.

Difficulty: beginner
Time: 45 min
Read: 4 min
Safety: warning

Short answer

Random triggering is nearly always heat movement in the sensor's view: a flue, a heat pump, a car cooling on the drive, sunlight on a wall, or an animal. A light that stays on permanently is usually the dusk-to-dawn override latched on, a sensitivity or time dial at maximum, or water in the sensor housing.

A passive infrared sensor is looking for a moving warm shape against a cooler background. Almost everything people describe as a fault turns out to be the sensor doing exactly that on something they had not thought of, and the rest is water or a setting.

Safety

Isolate the circuit at the consumer unit and prove it dead before opening any mains fitting, and do not rely on a wall switch. Do this in daylight, from a properly footed ladder with a stand-off, and never while standing on wet ground with the circuit live. If the fitting is damaged, water has got inside, or the fault is in the wiring, stop and call an electrician.

What you'll need

Voltage tester
Ladder with stand-off
Small screwdriver for the sensor dials
Silicone sealant for the top edge (optional)

Step by step

Watch it for an evening before touching anything.Note the times it triggers and what is happening. A repeating time each day, a pattern in the wind, or triggers when the heating fires all point straight at the cause.
Check for heat sources in the field of view.A boiler flue, an extractor outlet, a heat pump, a tumble dryer vent, a hot car bonnet on the drive, or a metal surface radiating heat after a sunny day. All read as moving warmth to a PIR.
Check for animals.Cats, foxes and hedgehogs trigger domestic sensors constantly. If the light comes on at consistent times overnight, this is very likely the answer, and masking the lower part of the lens usually fixes it.
Look at what moves in the frame.A branch warmed by the sun, washing on a line, a flag, or a bin lid. Movement plus a temperature difference is all it takes.
Mask part of the sensor lens.Most PIR heads have a sliding shield; where they do not, a small piece of electrical tape on the lens works. Block the direction that is causing the triggers rather than reducing overall sensitivity.
Turn the sensitivity down one step at a time.The dial marked SENS. Small changes, tested over an evening. Turning it to minimum tends to mean it never triggers for people either.
For a light stuck on, check the override first.Most PIR fittings latch into permanent-on if the wall switch is flicked off and on quickly. Switch off for a full thirty seconds, then on. That resets the override and fixes a large share of stuck-on lights.
Check the TIME and LUX dials.TIME at maximum can be fifteen minutes or more, so repeated triggering looks like permanently on. LUX at the sun symbol makes it operate in daylight too; set it towards the moon symbol so it only works after dark.
Look for water in the sensor.Condensation behind the lens, or a corroded PIR head, causes erratic and permanent-on behaviour. Isolate, open it, dry it, and seal the top edge of the fitting with a bead of silicone — leaving the bottom open so any water can escape.
Suspect the sensor itself if nothing else fits.PIR heads are consumable and fail after some years. A replacement sensor head, or a whole fitting, is usually cheaper than the time spent diagnosing further.

Tips

Note the dial positions with a photograph before you change them, so you can get back to a known state.
Seal the top of an outdoor fitting, never the bottom. Water that gets in must be able to get out.
If several outdoor lights misbehave together, suspect the circuit or a shared sensor rather than each fitting.

Common mistakes

Turning sensitivity to minimum to stop false triggers — It stops triggering for people as well, so the light quietly stops doing its job. Masking the offending direction keeps the useful detection.
Sealing all round an outdoor fitting — Condensation and driven rain always find a way in. If the bottom is sealed too, water collects inside and destroys the electronics.

If it doesn't work

Light on permanently, day and night

Cause: Override latched on, or LUX set to daylight operation — Fix: Switch off at the wall for thirty seconds and back on to clear the override, then set LUX towards the night setting.

Comes on repeatedly in the small hours

Cause: Cats or foxes crossing the detection field — Fix: Mask the lower portion of the sensor lens so it looks at chest height and above across the approach.

Triggers whenever the heating runs

Cause: Boiler flue plume in the sensor's view — Fix: Rotate or mask the sensor away from the flue. Warm moist air moving across the field is a textbook PIR trigger.

Flickers or hums, and trips the circuit occasionally

Cause: Water ingress or a failing driver — Fix: Isolate and stop using it. A fitting that trips a breaker is an electrical fault, not a settings problem, and needs replacing or an electrician.

Questions people ask

Can a PIR see through glass?

No. Infrared at these wavelengths does not pass through ordinary glass, which is why a sensor behind a window does not work, and also why a conservatory does not trigger one outside it.

Why does it work in winter but not summer?

A PIR detects the difference between body heat and background temperature. On a hot day with a sun-warmed wall or path the contrast shrinks and sensitivity falls. That is physics rather than a fault.

Should I replace the sensor or the whole fitting?

If the lamp is LED and integral, usually the whole fitting, because the driver and the LED array are not separately serviceable. On a fitting with a separate PIR head, the head alone is cheap and easy to swap.