Direct answer
For the SimpliSafe Motion Sensor (Gen 2), zone geometry, travel direction, sensitivity, lighting, battery sleep and network delay matter. First, walk a repeatable path and compare event timestamp/first frame.
What this covers
- SimpliSafe Motion Sensor (Gen 2) (Gen 2 — the serial number is inside the battery compartment)
- SimpliSafe Gen 3 system component; sensors reach the Base Station over SimpliSafe's own sub-GHz radio rather than Wi-Fi, so a router problem and a sensor problem are separate investigations. Monitoring plan, cellular backup and camera generation vary.
- The specific fault this record addresses: misses motion or records too many events
What it does not cover
- ×The same symptom on a different SimpliSafe product line, which has its own record and its own cited source
- ×An on-screen error code whose meaning has not been checked against this exact model's manual
- ×Live electrical, gas, sealed-system, internal battery-cell or structural repair — a technician boundary, not a self-service step
Differential
What actually causes this, and how to tell them apart
These are ordered by how often each one is the answer, not by how easy it is to try. The point of the middle column is that two causes producing the same headline symptom rarely produce the same detail — that detail is what tells you which one you have before you change anything.
- 1
The subject crossed the beams too slowly, or walked straight at the sensor
What separates it Someone moving briskly across the room is caught and someone approaching slowly head-on is not. A passive infrared sensor detects heat moving BETWEEN its zones, so movement along a zone rather than across one produces very little signal.
How to confirm Walk across the sensor's face, then straight toward it, at the same pace. A large difference between the two confirms the geometry and is fixed by re-aiming rather than by any setting.
- 2
Mounting height or angle puts the coverage where nobody walks
What separates it Detection is reliable in part of the room and absent elsewhere, with a consistent boundary. A passive infrared sensor's zones fan outward, so a few degrees of tilt moves the far end of the coverage by a long way.
How to confirm Compare the installed height and angle against the manufacturer's specified figures. Height error is the single most common installation fault on this class of sensor.
- 3
The room temperature is close to body temperature
What separates it Detection degrades in hot weather and recovers when it cools. A passive infrared sensor works on the contrast between a body and its background, and that contrast collapses as the background warms.
How to confirm Compare reliability on a hot afternoon against an evening in the same room. This is physics rather than a fault, and it is why summer complaints about this sensor class are seasonal.
- 4
Glass, and anything else the sensor cannot see through
What separates it Movement on the other side of a window or an internal glazed door is never detected. Glass blocks the infrared wavelengths these sensors work in, completely.
How to confirm Test movement on both sides of the glass. Total failure through glass is expected behaviour and identifies the sensor type; it is not a range or sensitivity problem.
- 5
A heat source in view is producing events rather than a person
What separates it Alerts cluster around heating cycles, sunlight moving across a floor, a vent, an appliance or a pet. Anything that moves heat through the sensor's zones is a genuine detection.
How to confirm Correlate the event times against the heating schedule and the sun. A pattern that tracks either is environmental, and re-aiming away from the source is the fix rather than lowering sensitivity.
- 6
The sensor is not active in the mode the system is currently in
What separates it It detects during a test and never during normal use. Sensors are commonly active in some arming modes and deliberately inactive in others — a motion sensor disabled in Home mode is doing exactly what it was configured to do.
How to confirm Check which modes the sensor is enabled for, and which mode the system was in at the time. This is the most common non-fault on a system that appears to be missing events.
- 7
A deliberate delay between detections
What separates it The first movement is caught and repeated movement over the next minutes is not. Battery sensors enforce a quiet period after an event so they are not reporting continuously.
How to confirm Trigger it, wait, and trigger it again at intervals. A consistent gap is a designed power-saving behaviour rather than a missed detection.
Walk across the sensor's face, then straight toward it
ReversibleA passive infrared sensor detects heat moving between its zones, so crossing movement produces a strong signal and head-on approach produces almost none. A large difference between those two walks identifies geometry, which re-aiming fixes and sensitivity does not.
Check the mounting height and angle against the specification
ReversibleThe zones fan outward, so a few degrees of tilt moves the far end of the coverage a long way. Height error is the most common installation fault on this class of sensor and it produces a consistent boundary in the room rather than random misses.
Confirm the sensor is active in the mode you are testing
ReversibleSensors are commonly enabled in some arming modes and deliberately disabled in others. A motion sensor that is off in Home mode is behaving as configured, and it is the most common non-fault behind a system that appears to be missing events.
Test through glass deliberately
ReversibleGlass blocks the infrared these sensors work in, completely. Confirming that movement on the far side of a window is never detected identifies the sensor type in one test and reframes every other symptom on this page.
Model-specific
What is true of this model and not of its siblings
Generic advice for the product line fails here. Each item below is a property of this specific model family that changes which of the steps above apply to you.
- Model scope
- Gen 2 — the serial number is inside the battery compartment. SimpliSafe Gen 3 system component; sensors reach the Base Station over SimpliSafe's own sub-GHz radio rather than Wi-Fi, so a router problem and a sensor problem are separate investigations. Monitoring plan, cellular backup and camera generation vary.
- Range and coverage
- SimpliSafe states the Gen 2 sensor has a range of 35 ft and a 90° field of view, intended to cover an entire room from one location. Beyond 35 ft is outside specification and no sensitivity setting extends it.
- Mounting height
- SimpliSafe specifies mounting in a corner, 6 ft from the floor. It states a shelf is acceptable only if it is still 6 ft from the floor, sitting flat and with a clear view of the room. Height error is the most consequential installation mistake on this sensor.
- Pet discrimination
- SimpliSafe states the Gen 2 sensor is engineered to tell the difference between people and pets up to 60 lbs, using a more advanced PIR sensor and detection algorithm than the Gen 1. An animal heavier than 60 lbs is outside what that discrimination is specified for.
- Sensitivity
- Two modes — Standard and Pet — adjustable in the SimpliSafe app. This is a mode switch rather than a continuous slider, so the tuning options here are narrower and more predictable than on a camera.
- Generation
- Gen 1 and Gen 2 are genuinely different hardware: SimpliSafe attributes the 35 ft range, the 90° field of view, the 60 lb pet discrimination and the app-adjustable sensitivity to the Gen 2 specifically. Procedures and expectations do not transfer between them.
- Power path
- Internal or removable battery. Every capability that costs current — night vision, higher stream quality, longer recordings, frequent events — is paid for in runtime, so a change in behaviour after a settings change is usually a power consequence rather than a fault. Cold weather reduces usable capacity sharply.
- Radio
- A manufacturer-specific sub-GHz radio to the base station rather than Wi-Fi. It travels further through structure than Wi-Fi does and is unaffected by router changes — which means a Wi-Fi problem and a sensor problem are genuinely separate investigations here.
- Symptom boundary
- This record covers SimpliSafe Motion Sensor (Gen 2) when it misses motion or records too many events. A different symptom on the same hardware, or this symptom on a different SimpliSafe product line, has its own record with its own causes and its own cited source.
Stop boundary
When this stops being a self-service repair
Continuing past any one of these costs more than the repair saves — in safety, in warranty, or in evidence a technician needs.
- !Detection is unreliable at all distances, in all lighting, on a device with good signal and full charge — the remaining variable is the sensor.
- !The device is relied on as the sole detection layer for a security or safety purpose, which no consumer motion system is specified for.
Evidence ledger
Sources behind this answer
Visible sources support the visible claims. Home Product Support records the publisher, the fact used and the review date; community reports can suggest an issue but do not become a published fact on their own.
Where the manufacturer's article stops. SimpliSafe Support publishes this as "Motion Sensor Not Detecting Motion". SimpliSafe's own article for a Motion Sensor that is online and failing to detect — the exact symptom, from the manufacturer. What that article does not carry is the model boundary — it is written for a product line, and Gen 2 — the serial number is inside the battery compartment is the scope applied here. Where a control label, terminal, indicator pattern or reset sequence differs on your unit, the manual for that exact model is the authority, not this page and not the article.
Exact questions
Common follow-ups
How far does the Motion Sensor Gen 2 reach?+
SimpliSafe specifies 35 ft with a 90° field of view, designed to cover a whole room from one corner. A subject beyond 35 ft is outside specification, and that is the answer to most 'it misses people at the far end' reports — sensitivity does not extend range.
How high should I mount it?+
SimpliSafe specifies a corner, 6 ft from the floor. A shelf is acceptable only if it is still 6 ft up, flat, and with a clear view of the room. Height is the single most consequential thing about installing this sensor, because the detection zones fan outward from it.
Will my dog set it off?+
SimpliSafe states the Gen 2 distinguishes people from pets up to 60 lbs, using a more advanced PIR sensor and algorithm than the Gen 1. Above 60 lbs that discrimination is outside what it is specified for — and mounting height still matters, because it determines whether the animal is below the lowest zone.
How do I change the sensitivity?+
In the SimpliSafe app, between Standard Mode and Pet Mode. It is a two-position mode rather than a continuous slider, so there is no partial adjustment to hunt for — which also means placement and height carry more of the load here than on a camera.
How is the Gen 2 different from the Gen 1?+
SimpliSafe attributes to the Gen 2 specifically: a 35 ft range, a 90° field of view, people-versus-pet discrimination up to 60 lbs, and app-adjustable sensitivity. They are genuinely different hardware, so a figure or a procedure from one does not transfer to the other.
Why does it miss someone walking straight at it?+
A passive infrared sensor detects heat crossing between its detection zones. Someone approaching head-on stays inside one zone and produces very little signal; someone crossing the room passes through several and produces a lot. It is the hardest case for this sensor type, and aiming it across the route people actually take is the fix.
Will it detect through a window?+
No — glass blocks the infrared wavelengths a passive infrared sensor works in, completely. This is worth testing early: a total failure through glass confirms the sensor type and rules out range and sensitivity as explanations for everything else.
It works in winter and misses things in summer.+
That is the physics of the sensor, not a fault. It works on the contrast between a body and its background, and on a hot day the background approaches body temperature and the contrast collapses. Seasonal reliability on this sensor class is expected.
Does mounting height really matter that much?+
More than almost anything else. The detection zones fan outward from the sensor, so a few degrees of tilt moves the far end of the coverage a long way. A consistent boundary in the room — reliable on one side, absent on the other — is nearly always height or angle.
My pet keeps setting it off.+
Anything that moves heat through the zones is a genuine detection, so this is the sensor working rather than failing. Check the sensor's own pet-tolerance specification and its mounting height — height is what determines whether an animal stays below the lowest zone — before reaching for sensitivity.
It detected once and then ignored me for a few minutes.+
That is a deliberate quiet period. Battery sensors enforce a gap after an event so they are not transmitting continuously, which would exhaust the cell in days. Trigger it at intervals to see the gap — a consistent one is designed behaviour, not a missed detection.
It never triggers during normal use but passes every test.+
Check which arming modes the sensor is enabled for, and which mode the system was actually in. A sensor disabled in Home mode passes a walk test — because testing puts the system somewhere else — and does nothing the rest of the time. This is the most common explanation for exactly this pattern.
Alerts arrive whenever the heating comes on.+
A vent, a radiator or sunlight moving across a floor all move heat through the detection zones, and the sensor cannot tell that from a person. Correlate the event times against the heating schedule and the sun; re-aiming away from the source is the fix, where lowering sensitivity would cost you real detections too.
Does this answer apply to every SimpliSafe model?+
No. It is scoped to Gen 2 — the serial number is inside the battery compartment. SimpliSafe Gen 3 system component; sensors reach the Base Station over SimpliSafe's own sub-GHz radio rather than Wi-Fi, so a router problem and a sensor problem are separate investigations. Monitoring plan, cellular backup and camera generation vary.
What should I record before troubleshooting motion detection or missed events?+
Record the complete model identifier, exact message or indicator, software/firmware where visible, the operating stage and what still works.
Why is a factory reset not the first step?+
A reset can erase accounts, networks, maps, schedules or preferences without distinguishing motion detection, missed events. The ordered checks preserve that evidence.
When should I stop and use qualified service?+
Follow privacy laws and do not aim beyond authorized property.
Answer for the exact device and software version shown above.