Heating Bed not working

Heat bed will not work, the alarm goes off and shuts the printer down.

  • check the wiring to the heated bad / make sure now frayed or broken wires.
  • check mainboard and make sure heater is still plugged in properly

Did you do any maintenance / move the printer before this stopped working?

Worse case scenario - if you are printing PLA, try printing without heating the bed / using an adhesion aid (I recommend AquaNet hair spray) while you try to figure out what is wrong.

Which printer are you talking about? To be clear, is the bed not heating up at all, or it’s heating up uncontrollably and causing the printer to shut down?

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OP more information is needed Lego has that right, I can’t understand what is happening, a Runaway (uncontrolled heating) has different solutions / troubleshooting issues than not heating at all.

Aerosol air sprays have very flammable propellents they offer a high risk around hot printers. Once all the stabilizers propellents anti clumping junk is ignored most hair spray (aquanet) uses a vinyl monomer. PolyVinylpyrrolidone in an alcohol. This is very similar to Polyvinyle Alcohol found in glue sticks.

Glue sticks are cheaper, safer, less toxic and more environmentally friendly.

Please consider this before you use hair sprays.


What Model Printer?

What Temp does the Thermistor for the Bed say at room temp?
How long passes before the Alarm goes off?
Does this Number increase in the normally 2-3 mins before the Alarm goes off?
Do You have VDC out to the Bed?
Is Wire Broken to Bed?
With AC Power off and Bed unplugged from Control board. What is the resistance of the bed in Ohms?

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110% !!! if you HAVE TO use Hair spray…

Take the Build plate off the printer and spray it outside, or standing in your Bathtub.

It’s unbelievable what Hair spray does the the Wheel bearings, Hotends and Power supplies of the Printers!!!
It’s a Fine Mist in the air, gets sucked up and stuck to everything, and everywhere the sun don’t shine!
Most hair Sprays are also Propelled by Propane, Butane, IsoButane, or Dimethyl Ether.
All x4 are VERY flammable


Printer is Ender3, has being rebuilt by PJ. Was working fine for a while. I tried to use PETG at 255 and the bed at 85. Turns out I did not notice that the filament was leaking in and around the heating block. I took the fan off to clean and put new nozzle on. When I started it up the heat bed alarm would go off right away and shut down the machine.
The room temperature before starting was about 27. I checked all the wires and the board and I don’t see anything loose. When I restarted this time it smells like something burning and the heat failure came on and I shut it down and unplugged cable. I think something is wrong for sure,

Excellent point. I’ve never thought of that.

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The room temperature as measured by what? What we’re trying to find out is whether the printer, if it’s been off for 1/2 hour, is measuring the room temperature correctly.

That is to say, with the printer fully cooled down, does the printer’s LCD show the bed and hot end temperatures at room temperature? If either of those temperatures does NOT reflect room temperature after 1/2 hour of cooling down, then you have a problem with the thermistor or thermistor circuit in whichever component is not giving an accurate temperature reading.

For these next tests, we will deliberately NOT use the stepper motors. Don’t home the machine or use the controls to move them. When the printer is first turned on, the X and Y axes should move relatively freely. Tell us if they aren’t.

With respect to the burning smell, unplug the hot end and unplug the bed. Turn on the printer. Is there still a burning smell?

If so, your power supply or controller board likely has a problem.

If not, plug in the bed. Is there a burning smell, without intentionally heating the bed? Is the bed actually getting warm? If so, your controller board has a problem.

If not, plug in the hot end. Is there a burning smell, without intentionally heating the hot end? Is the hot end actually getting warm? If so, your controller board has a problem.

The above checks for MOSFETS on the controller board that are powered on when they shouldn’t be.

If you’ve made it this far and have the printer powered up, but are not intentionally heating the bed or the hot end and there is no smell, then so far, so good.

Next, plug the bed back in and select Preheat PLA Bed. Your printer is different than mine, so I’ll leave it to you to find it in the menu system. For PLA the bed should be around 50C (I think) - the point is hot enough to feel it, but not hot enough to burn anything. If you have a burning smell at this point, can you tell if it’s coming from the power supply or controller or the bed? On my Ender 5 Pro that’s easy since they are a long way apart, but I’m not sure about your machine. If the smell is coming from the bed, check underneath the bed for something on the heater that shouldn’t be. Again, I don’t have familiarity with your printer. On my Ender 5 Pro, the bed is heated by a small heating element which has to supply heat for the entire bed. That’s the part that I’m wondering about. On my Longer LK1 beds, the heating element is a copper trace build right into the bed. It doesn’t get excessively hot in one place so I would never expect it to produce a burning smell. I don’t know which of the two heating systems the Ender 3 uses.

If there is nothing suspicious about the bed when it’s heating, then cycle the power on the printer (to turn the bed heating off) plug in the hot end and select Preheat PLA [Hot] END. This should try to bring it up to about 200C. If the burning smell starts again, is it coming from the power supply or the controller or the bed?

If the burning smell is in the hot end, check the hot end for debris. Something as simple as a human hair could be all it is. Obviously PLA shouldn’t smell at normal hot end temperatures. The hot end should be targeting 200C but is it overshooting? If so, it could be either a failed thermistor (which would likely not be reading the correct room temperature way back at the beginning of this marathon post) or it could be a MOSFET that is failing to turn off. That would trigger an alarm when the temperature continues to rise when the controller told it to turn off.


oh yea I use hair spray but not inside the enclosure where the printer is. everywhere the cloud goes, so goes the hairspray. I have removable metal sheet and magnet bed this is the way I use it.


Check all the wiring and do a PID tuning for your bed.