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Ultimaker 3 Bed Temp Sensor Issues

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My school has recently purchased an ultimaker 3 and I am mostly in charge of it, and recently it has been displaying incorrect temp readings for the build plate. Even though the build plate is cool to the touch and not on, it says it is at 150C/0C. This is a problem because when I try to print, the temperature gets added to the 150 and tells the printer it is at max temp, it then stops printing. I have tried everything from updating software, factory resets, and temp calibration.

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This is a common problem and easy to fix (well I'm an electrical engineer).  Basically there is an increased resistance somewhere in the path of the temp sensor which starts at the circuit board under the printer and goes to the bed through obvious wires (in that black sheathing) and through a connector to the board itself.  The board is extremely unlikely to fail - the most common failure point is at either connector - wires can come loose and the connector itself can disconnect (barely) from the board.


I'd start by watching the temp while you poke around with your fingers where the wires connect to the board - it's at the back of the heated bed - 4 individual wires - the 2 smaller ones are the sensor wires.  Push and pull and if the temperature changes suddenly then great news - you found the problem area.  You can repeat for the circuit board but that requires removing the bottom cover (not that hard).


Assuming the problem is at the bed - you can take the bed all apart pretty easily by removing the 3 leveling screws.  Start by removing the glass, then undo the 3 leveling screws comlpetely.  Watch out for washers and really pause and stare at how it goes back together.  Lay the parts out in a safe spot where you won't bump them and lose what order they go.  Often a washer sticks to the plate (the grease acts as glue).


Once the wire is loose - remove and reinsert the 2 temp wires and power the printer up again to see if fixed.  Report back.

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