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It's been about a month since I've started using my Anycubic i3 Mega and everything has been going great. But today, I left it alone to print a 3DBenchy and it suddenly stopped mid-print. I didn't see it stop but once I came to the printer, the motors and the bed were at ambient temperature suggesting that it has stopped for quite a while. I was surprised that the Bed Temperature registered at over 200 ºC (although it wasn't at all hot) and tried to stop the print. However. I wasn't able to stop the print (It said stop failed) and I couldn't change the temperature, so I just switched the PSU off.

After leaving it for a few minutes, I turned the printer back on and the Bed Temperature still registered at absurd temperatures although it wasn't hot at all as seen on the Image Below. Image 1

I then tested the Hot End as the temperature seemed normal and it does seem to work and I was able to get some filament to be extruded out normally. Image 2

Any ideas on what happened here? It was working fine for an entire month and was working fine for the day, but mid-print it just suddenly broke. I've also uploaded a GIF to Imgur to show how the bed temp fluctuates a lot. Any help would be appreciated :)

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The first value, which is in your pictures 179 and 184 °C, is the measured value. The 0 °C value is the set value. It looks like the temperature reading from the hotbed is not working correctly. So there are two possible issues:

  • The cable on the backside leading to the hotplate might be damaged. This seems to be a common issue with this printer. You could check if the cables show any obvious damage.
  • The hotbed thermistor is broken. On the leads on the backside you can measure with a multimeter the resistance between the green an blue cables. Typical readings are in the range of 100 kΩ. If you touch the thermistor, usually it is placed in the mid area of the heatbed, the resistance should change.
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  • $\begingroup$ Sorry for the super late comment but I suspect that the hotbed thermistor is broken, do you know if this thermistor is easily commercially available to buy and if so the compatible types? Thanks! $\endgroup$
    – Bryan
    Apr 10, 2020 at 5:12

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