FAQ: False positives on motion sensors

Hi there,

I have started to get false positives with one of my SmartThings motion sensors.Anyone any idea what that could cause this? I observed it many times with definitelty no motion at all. It was during the day. The temperature is 99F. There is a security glass door which I thought was the culprit. However when I stand there myself seeing my own reflection it does not trigger a motion alert. I have several sensors all exposed to the same kind of temperature. Is is possible that this sensor went defective?

Thanks for posting. Have you shared these experiences with the support team?

I have not yet spoken to SmartThings about this due to a lack of time. It will take me more than a week to do so as I am currently traveling. In the meantime I am hoping for some feedback.



A few weeks ago I was having this happen all day with one sensor. Finally I just unconnected it. I will say the battery was really low. I haven’t changed the battery and reinstalled it to see if that was the problem yet or not.

It’s possible, but rare, for a motion sensor to become defective after it’s been in use for a few months. More typically a defective device is observed as soon as it’s installed, or towards the end of its working life.

There are three common reasons for false alerts on indoor motion sensors.

The first is a dying battery. This can give you all kinds of crazy results. Most often you wouldn’t see this until battery life registered below 50%, but it never hurts to recharge or put in new batteries and see if that fixes the problem.

The second possibility is an insect crawling along the face of the sensor. This is usually A spider who has set up a homebase nearby. These are somewhat more common in the summer months. Visually inspecting the area will usually determine if this is the problem.

The third problem is a classic summer problem: gusts of air of different temperatures. Very common if you run a fan or an air conditioner, but can also happen on a very warm day if clouds go by outside, as the temperature of the sunlight hitting the sensor may vary by 10 or even 15° depending on whether it’s bright or cloudy. The passing cloud problem is most often seen when the outdoor temperatures are above 90° F. At lower temperatures, the variance between cloud and no cloud is not as strong.

The air problem can affect all the motion sensors and one room, or sometimes just one, it just depends on the exact source of the temperature variance. I know one house where opening the bedroom door from a room that had an air-conditioner would cause a gust of air to blow across a motion sensor in the hallway, and could set it off because of the difference in temperature between the two rooms.

Diagnosing air gust problems requires diligent detective work. The passing cloud one can be particularly tricky to find, because it’s hard to re-create. But if you see direct sunlight falling on the motion sensor that is giving the false alerts, try closing the curtain or something else to block that light and see if that reduces the false alerts. If it does, it’s possible that it’s an outdoor temperature variance, again usually caused by clouds, that’s the problem.

People tend to get confused and think that it’s motion outside that is setting off the sensor, but PIR sensors cannot detect through glass. Instead, they are triggered by temperature variance, so when the light that comes through the window has changed temperature that’s what can set it off. That’s almost always a result of sunlight being very bright, and then being briefly blocked. That’s why this particular issue tends to be a summer problem. The unblocked sunlight isn’t bright enough in the winter to create the variance.

Well, those are just some environmental things you can look at. Support will be able to look at the devices with remote diagnostics to see what they see.

Contact Sensors

Also, it’s probably obvious, but this post addresses motion sensors, not contact sensors. Contact sensors can be affected by the battery issue, but not by insects or cloudy days. Contact sensors getting “stuck” (always showing open or always showing closed) are more commonly a network communication issue. Here is the FAQ for contact sensors:

A motion sensor failing to report motion might be the same kind of network problem, but false positives are a different kind of issue.


Also a bug could actually be inside. A fly can lay an egg inside the device thru some gap, and it could hatch into an impossibly large fly.

I would have said “BS” if somebody told me that, except it actually happened in one of my X10 Hawkeyes in the garage. yuck

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Thanks for this elaborate response! I appreciate it! As we live in AZ I think I may have that issue with the sun you mentioned. It happens mostly in the late afternoon when it is very hot. My other sensors on the other side of the house are not affected. So I infer it is sun/ temp-related. have repositioned the sensor a tad today hoping this will help.


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My motion sensor started reporting frequent false positives as well after I had moved some furniture around in the room. I stuck the motion sensor on the side of a large, heavy wooden wardrobe. Previously the sensor was facing a blank wall in the room. Now it is facing a wall which has a mirror on it. Could there be a reflection of the window in the mirror that’s causing the false positive motion alerts? The curtains have always been closed btw. Battery is brand new, one week old. Currently 89%. I’ve checked the sensor seems to be free of bugs/spiders etc.


Yes, reflections from mirrors and even regular windows can cause heat variations in a room which can then trigger a motion sensor. It’s not the reflection image itself, so it’s not the window issue, it’s just the fact that glass or mirrors can concentrate heat and throw it back in a way that confuses a PIR sensor.

Try just moving the sensor again away from the mirror and see if that helps reduce the false positives.

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Thanks for the quick reply! Will try it out and feedback here.

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That did the trick. I re-positioned the motion sensor so it is no longer point at a mirror or a window. Have not had any false positive alerts since then. Thank you for your help!

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BTW, since this FAQ was first written Mike Maxwell has created an excellent and very popular zone manager smart app which can significantly reduce false alarms by combining multiple motion sensors into one zone. It can really help when the false alarms are caused by microclimate variations as long as you’re willing to deploy two or three devices to cover a zone.