I've had my Anet A8 for over a year, running the stock fw. I updated to Marlin. When I use the LCD menus PREPARE > PREHEAT PLA, the system does not preheat (not extruder, not bed). If I print a part from the SD card, Preheat happens. If I Use Pronterface and select preheat, it works. So I don't think this is a HW issue, but something with Marlin. I've been looking at the Marlin code, but can't find where the LCD command (interrupt?) is collected, or how to see what code is executed on that menu selection. Any thoughts or pointers into the code would be appreciated. I am also nearly C++ illiterate, but learning fast.
