The average life of a Green Shield repeller unit is three to five years. If the green light ever stops flashing (either permanently on or off), the unit may have stopped working. Try replugging the unit in several times in different sockets to check.
If the light is flashing but you think the machine has stopped working, here’s a simple way to test it: dangle a paperclip on an elastic band or a piece of string, close to the machine (about 1cm away). If the unit is functioning properly, the paperclip should be pulled towards it once every second or so. Don’t forget, though, that the machine “rests” for one minute out of every four, so check over a period of five minutes just to be sure.
Note that you should unplug for 30 seconds before replugging in and testing with the rubber band and paperclip.
This video demonstrates how to test if your Green Shield electromagnetic pest repelling device is working.