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For some reason whenever I turn on my printer, my nozzle says that it is at 120 °C even though it is not set at anything and my bed says that it is set at 180 °C even though it is not set to anything. I replaced my hotend and the problem is still there. Does anyone else know why this?

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    $\begingroup$ What printer do you have? What firmware are you running? Has anything other than the hotend been modified? Please edit in additional information so we can help you. The more information, the better. $\endgroup$
    – agarza
    Commented Jan 12, 2022 at 3:59
  • $\begingroup$ There are several hundred printer types out there, and the number of variants goes into the 4 digit range. misreading a temperature could have numerous reasons, and in some printers it is more common than others. $\endgroup$
    – Trish
    Commented Jan 12, 2022 at 7:06

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This is a common problem that affects many printer types, and is caused by the fact that most controller boards do not have over-voltage protection on thermistor (temperature sensor) inputs. If the thermistor wires come into electrical contact with the heater wires (with the printer switched on and the heaters running), this can ruin the analogue to digital convertor (ADC) circuits on the printer's microcontroller. The only solution is to replace the controller board.

This problem typically occurs when users are performing essential maintenance on their printers. For example, when trying to remove filament that has accumulated around the hot end during a print failure. Never try to remove filament with the printer switched on (and never use a wire brush to do so). If you need to pre-heat the hot end to soften the filament, always switch the printer off before attempting to remove the filament, and try to ensure that the thermistor wires are not damaged in any way.

There are controller boards that have over-voltage protection on thermistor inputs, although this is still fairly uncommon. The BigTreeTech SKR 2 (not to be confused with the SKR 1.4 V2) has such protection.

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