Motion Sensor Not Working

Common Symptoms

• No detection when walking directly past the sensor
• Sensor triggers false alarms constantly
• Zone shows open or faulted on the security panel
• LED indicator not blinking during a walk test
• Sensor shows offline or no signal in the security app
• Detection range suddenly reduced

Most Likely Causes

1. 1

   Low or Dead Battery (Wireless Sensors)

   Wireless PIR motion sensors run on CR123A or AA batteries with a typical lifespan of 1–3 years depending on trigger frequency. A low battery causes erratic detection, false triggers, and eventually complete failure. The sensor may not report a low-battery warning before failing entirely — proactive annual battery replacement is recommended.

2. 2

   Blocked Lens or Dirty Cover

   The PIR lens is a small Fresnel lens that focuses infrared radiation onto the sensor element. Dust, spider webs, or film buildup on the lens diffuses incoming IR and reduces detection range and sensitivity. A dirty lens is one of the most overlooked causes of reduced detection performance.

3. 3

   Sensitivity Set Too High or Too Low

   Many PIR sensors have a small sensitivity adjustment dial (1–4 on most models). Too high: pets, sunlight, or heat vents trigger the sensor. Too low: a person can walk past without registering. The correct setting depends on the detection zone size and whether pets are present.

4. 4

   Improper Mounting Angle

   PIR sensors are designed to detect motion across their field of view, not directly toward the sensor. The optimal mounting height is 6–7 feet with the sensor angled down 15–20 degrees. Too high or too flat and the sensor may miss low-movement targets; too low and range is severely reduced.

5. 5

   Temperature Differential Too Low (Summer / Hot Rooms)

   PIR sensors detect the temperature contrast between a moving body and the background. In very warm rooms (above 85°F) where the ambient temperature approaches body temperature, the contrast drops and detection becomes unreliable. This is why coverage degrades in hot garages or sunlit rooms in summer.

6. 6

   Interference from Pets, Heat Vents, or Direct Sunlight

   Pets moving through the detection zone are the most common false alarm source. HVAC vents blowing warm air, a fireplace, or direct sunlight entering through a window and falling on the detection zone all create the temperature changes that PIR sensors respond to.

7. 7

   Sensor Out of Wireless Range or Lost Pairing

   Wireless sensors use 433 MHz or Z-Wave radio to communicate with the panel. Thick masonry walls, large metal objects, or interference from other wireless devices can cause dropouts or complete signal loss. A sensor that shows 'offline' in the app has lost its pairing with the panel.

Quick DIY Checks

1. 1Replace the battery in wireless sensors first — even if it hasn't been that long. Use a fresh CR123A or AA (check the label on the sensor back). This resolves the majority of erratic detection and false alarm issues in wireless PIR sensors.
2. 2Clean the lens with a soft, dry cloth. Never use glass cleaner or solvents — they can leave a residue that further diffuses the IR lens. A clean lens often restores full detection range immediately.
3. 3Check the mounting angle: the sensor should be at 6–7 feet height, angled downward at 15–20 degrees toward the detection zone. Remount if the sensor is too high, too flat, or aimed at a wall rather than across the room.

1. 4Verify no heat vents, direct sunlight, or heat-producing appliances are within the detection zone. Move the sensor if needed — even 12 inches can take it out of a vent's heat plume and stop false triggers entirely.
2. 5Run a walk test with the panel in test mode: enter your alarm panel's test mode (consult your manual — typically code + 5 or code + TEST), then walk through the detection zone. The panel will log the zone trigger. This confirms whether the sensor is communicating with the panel at all.
3. 6Adjust the sensitivity dial if accessible: open the sensor cover and look for a small rotary dial or jumper marked 1–4 or L/H. If pets are causing false alarms, reduce sensitivity. If detection range is too short, increase it one step at a time.
4. 7Re-pair the wireless sensor to the panel if it shows as offline: consult your panel manufacturer's instructions for adding a zone. Typically involves putting the panel in enrollment mode and triggering the sensor. Sensors showing offline after a power interruption often recover with a simple re-pair.

Repair vs Replace

Recommended Tools & Parts

• CR123A Lithium Battery (4-pack)

  CR123A 3V lithium batteries used in most wireless alarm motion sensors. Longer shelf life and more stable voltage than alkaline in cold environments. Always use lithium for security sensors.

  $8–$15 for 4-pack

  Buy on Amazon →

• Honeywell 5800PIR Wireless PIR Sensor

  Wireless PIR motion detector compatible with most Honeywell/Resideo Lyric and Vista panels. 90° x 55° coverage, pet immunity up to 80 lbs, 12-month battery life.

  $35–$55

  Buy on Amazon →

Links are Amazon affiliate links (tag: fixitfastai-20). Prices are estimates.

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