In my 4 camera surveillance system two cameras stopped working

The screen should say a message similar to "VIDEO LOSS" if it does The power packs or multi power supply to your cameras are easiest to check for starters. Unplug the power to a working camera and plug it into the non working camera close to the dvr. If the camera that didn't work came on, then the power pack is most likely the problem. If it didn't have a message saying "VIDEO LOSS" then it may be the sensor that switches to day/night vision. I have had cameras that when it switches to night vision the screen goes black and unplugging in the power to the camera and re-plugging it in gives it a boost and then comes on. If switching the power from a working to non working camera don't work get the ladder out and unplug and re plug in the connectors at the camera in case it is a connection there. Last if that didn't work would be to take the camera that don't work off and plug it into the end cables to a working camera, if it works then I would suspect the cable to the camera to be bad. Just a process of elimination.

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Easiest way to check if power is coming to your camera is to cover the sensor on the camera with your finger (this is usually a green dot like thing on the front of the camera), when you cover this with your finger the camera will think its dark and the LEDs on your camera will light up. You can test this theory with the working cameras first and then go to your non working cameras and do the same.

If your non-working cameras do not show any LEDs it means that the power supply connected to these non-working cameras if faulty and needs replacing.

If you have a multimeter and know how to operate the basic functions like voltage measurement then the first thing i would check is the power supply going to the cameras that are not working. Or an even simpler way could be to swap a known good camera with the suspected bad one and depending on the outcome will tell you whether it is the camera or the wiring to the camera. If a good camera works after swapping it then it is most likely the original camera is defective in some way. Likewise if the good camera still does not work after the swap then you most likely have a wiring or connection issue that must be resolved.