I have the Dibond Mendel90.
All of a sudden the J-Head hotend has stopped working. Sadly I was not in the room, but it happened while it was printing. I came back and saw lots of extrusion prevented errors in pronterface. The hotend had stopped heating.
How could I figure out what has broken?

The most likely reason would be a bad connection at the IDC connector or the PCB.

Does it show the correct temperature? If so use a multimeter to check there is 12V at the heater terminals when told to heat. If there is then measure the heater resistor it should be ~ 5.6 Ohm.

If it doesn't show the correct temperature the most likely reason is molten plastic leaking onto the thermistor and shorting it out.

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[www.hydraraptor.blogspot.com]

Thank you for your reply. I checked the connections and the soldering points at that small pcb next to the extruder motor were very bad. I was ashamed when I looked at it and resoldered all connections. It works perfectly now and I have the impression that the extruder heats up a lot quicker now.