I've been happily printing on my CR-10 for a couple of months but now there is an issue. As soon as I turn it on the hot-end begins heating. I have replaced the heater and the thermistor but the problem remains. The panel on the control box shows 0 for the commanded temperature, but the actual temp just keeps rising. Anyone know what could be wrong?
If you power the printer and it starts heating up the hotend, it most probably is caused by a faulty controller board. More specific, the MOSFET (as in an electronic switch component) controlling the current to the heater element is causing this. MOSFET devices usually fail short-circuit implying that when the MOSFET has failed, the current can freely run to the heater element without being controlled by the board.
If you are handy with electronics, you can replace the MOSFET, but it might be better to upgrade to a newer controller board as there may be other damage as well.