board sensor issue

After 575 hours of carving I was having low board sensor readings and had to resort to masking tape etc. until it got so bad I couldn't carve anymore. I replaces the sensor and went from a 152 reading to 15 in 3 carves. I then replaced it again with somewhat better results, but it still went to a reading of 5 in less than a week. Obviously I was frustrated to say the least. Anyway began looking on the forum for answers and found them. I took one of them apart and replaced the window. Prior to putting the cover on I tested it and it read 152. I then put the cover or top on and it would not read at all. I took it back off and it was fine. I then put a piece of black tape on and it tested 150+ with a board, but would not go to below 10. I was able to carve, but had to jog to starting point etc. I then took the tape off and am running just fine with the unit open. Why???? What is wrong with it's being left open. Sure a lot easier to clean. Thanks and please help.:neutral:After 575 hours of carving I was having low board sensor readings and had to resort to masking tape etc. until it got so bad I couldn't carve anymore. I replaces the sensor and went from a 152 reading to 15 in 3 carves. I then replaced it again with somewhat better results, but it still went to a reading of 5 in less than a week. Obviously I was frustrated to say the least. Anyway began looking on the forum for answers and found them. I took one of them apart and replaced the window. Prior to putting the cover on I tested it and it read 152. I then put the cover or top on and it would not read at all. I took it back off and it was fine. I then put a piece of black tape on and it tested 150+ with a board, but would not go to below 10. I was able to carve, but had to jog to starting point etc. I then took the tape off and am running just fine with the unit open. Why???? What is wrong with it's being left open. Sure a lot easier to clean. Thanks and please help.:neutral: