I bought one recently too.   It was working fine until  a 2014 Kia Cadenza came in for service.  It wasn't listed in the tool, so I performed an update and immediately after, I would get "Sensor triggered, but cannot be known" after getting no response from the tool.   Now it won't go into Update Mode, as you stated pressing "N" and power button at the same time only turns the tool on.    Serial number of my tool is T415485774

Hope to find a solution to the problem.