It got down to 15° F last night. Woke up this morning and it was COLD in our house. 
Checked the furnace and it kept cycling, flames would light and then soon shut off, so I looked through the little window and observed the status LED. I was blinking red 3 times repeated. I had to look up inside the window, as the status LED is actually way up on the top of the circuit board. Why in the world wouldn’t York put the observation window in front of the status LED?)
My Furnace – click on picture for bigger view with labels