Printer hot end running away !

My printer a German RepRap Protos V3 has always beeen reliable ! And then !! The hot end switches itself on when I power up , without a command ! then rushes up to 360 degrees loads of smoke etc etc ! I have replaced the thermistor , same . I have replaced the 1.4 ramps board and another thermistor same ! Any ides what happening ? Hope so .

... check the MOSFET, if its working ...

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Viktor
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Unplug the hotend heater from D10 and Stick a multimeter into both D10 pins. (do not use another gnd pin, must use gnd pin on D10)

check for 12v, if its there at power up the issue is with ramps/mega

If its not there. check for shorts to gnd on the heater wires (remember that ramps switches the gnd wire, so any short to gnd will turn on the hotend)
A common failure of hotends is the thermistor wires (in this case gnd) short to the heater block. The heater cartridge also shorts to the heater block causing power to flow.

So check for continuity from both hotend heater wires to the heater block and both thermistor wires to the heater block.

Rerouted the hotend cables off the Mosfet then noticed this sender cable !!! Yup it the little things in life !! Thanks for the clue !!