While I am guessing this is not the cause, I thought I would offer the following as I had something similar happen to me (printer worked and then after some work on the board and flashing, the end would not heat up). Drove me nuts and took me way longer than I would like to admit to figure out. Turns out my thermostat was broken, so it simply read 0 degrees (not even room temp). This caused my printer to act exactly as yours when I tried to pre-heat.
Just throwing it out there as if it is that, it is a simple fix of swapping in new thermostat.
Good-luck.
Just throwing it out there as if it is that, it is a simple fix of swapping in new thermostat.
Good-luck.