Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
The 'sensor' is located to the right of the display on the front face of the machine.
Try wiping the sensor off with a clean dry cloth. Then after that is cleaned by wiping, try turning On the machine, and see how that works.
If it still does not work properly, please note exactly what the message about 'sensor' is, in full, and let me know.
Try it by all means. If it doesn't work, let me know.
And a full description of the error message and condition wouldn't hurt. It may take several back-and-forth communications before the problem is really tracked down and solved, but it can be done I hope.