This happens on DNX7220 models. You will have to disassemble it and reach Garmin GPS board inside. On the bottom of that board there is a small button. You will have to connect it to PC or notebook via rear mini USB connector. Keep that button depressed and power the DNX unit still keeping button depressed for about 10 seconds until you hear USB connect ding sound on PC. Depress button and wait for a few seconds to see if PC opens directory tree with Garmin files in it. When it does, simply turn off everything and put DNX unit back together. GPS will be reset after this procedure. If Garmin board fails to connect to PC then it needs replacement. When you open DNX unit, clean SDcard holder with contact spray. This usually happens when SD card is not working correctly because of the dust in the holder and DNX unit begins to mess with Garmin GPS unit's inner memory. Hope this will work! Good luck.
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