Extrududer decoupled and bed heating up for no reason

Hello Forum-Members,
I've built a 3d printer with a ramps 1.4 and repetier firmware on an Arduino Mega.
The porblem is when I heat up the extruder, it works fine until the extruder reaches around 170°C then it gets shut down with the error message heater decoupled.
The strange think is when the heater is turned on it heats up, then at around 90° at the hotend, the heat-bed thinks it also gets heated and its value rises and rises fast and steady, but the bed doesnt heat up in reality just the board thinks so and I didnt told the bed to. Even if the heatbed is not connected to power and a normal 80k resistor is connected as a thermistor the board thinks the heatbeds temperature is rising the same way.
The board lets the heat-bed temperature rise up to 70°C and then it shuts down the hotend with the decoupled message, also the heatbed temp goes down then. I think there is some connection between the heat-bed temperature rising and the hotend geting turned off.
It must be some problem with the firmware, because changing the ramps board didn't change anything.


  • What is the target temperature in this test and what exactly is the output of firmware in console when the error happens including last temperature responses?
    Heating has 2 phases - full power until target-20°C normally and then the controlled range. Sounds like it happens at the switch between the 2, but without the info and maybe a graph of extruder and bed temperature hard to analyse. Maybe the allowed variance it too low so it kicks at that point to decoupled.

    That bed measures a temperature after 90°C is strange. Is the extrude close to bed temperature sensor when you test? Then it just feels the heat of extruder coming through. In that case try moving away e.g. z + 30 and see how it improves.
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