# FAQ: False positives on motion sensors **URL:** \ **Category:** FAQ\ **Created:** [July 30, 2015, 12:23am UTC](https://community.smartthings.com/t/faq-false-positives-on-motion-sensors/19876 "2015-07-30T00:23:28Z")\ **Posts on this page:** 1\ **Showing post:** 5
**Author:** ![JDRoberts](https://yyz2.discourse-cdn.com/smartthings/user_avatar/community.smartthings.com/jdroberts/32/56577_2.png) [@JDRoberts](https://community.smartthings.com/u/JDRoberts)\ **Post date:** [July 30, 2015, 4:32pm UTC](https://community.smartthings.com/t/faq-false-positives-on-motion-sensors/19876/5 "2015-07-30T16:32:27Z")
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: > [@FAQ: My Sensor (any brand) is Stuck on Closed (or Open)](https://community.smartthings.com/t/faq-my-sensor-any-brand-is-stuck-on-closed-or-open/25419): > > When any sensor gets “stuck” in one reporting state, open or closed, it’s almost always one of the following: Low battery. once the battery gets down to about 30%, the signal strength drops by a lot. It just may not be able to report. So you may want to try just changing the battery if it’s getting low. sometimes a new device gets delivered with a bad battery, it can happen. orphaned nodes. Zigbee does have a vulnerability to this. The fix is to unplug the hub for at least 15 minutes, including removing the batteries. This will cause all the zigbee devices to realize they’ve lost connection to the network, and then when you plug the hub back in the address tables will get rebuilt. You may not see an improvement until the next day, it can take a while for the changes to propagate… A motion sensor failing to report motion might be the same kind of network problem, but false positives are a different kind of issue. --- _[View the full topic](https://community.smartthings.com/t/faq-false-positives-on-motion-sensors/19876)._