Preheat error

The Preheat error message occurs when there is a problem with the preheat process before your print starts. Therefore, there is something preventing the printer from heating up in proper time.

Make sure to distinguish between the Preheat error and the Bed preheat error.

Preheat error - a problem with preheating the hotend.
Bed preheat error - a problem with preheating the heatbed.

Troubleshooting

Make sure that the ambient temperature is stable and there is nothing cooling the printer down, such as a nearby air-conditioning unit or an open window.
Make sure the thermistor and heater are properly plugged into the electronics board. You can doublecheck that by following these guides: Mini RAMBo electronics wiring (MK2S, MK2.5, MK2.5S) or Einsy RAMBo electronics wiring (MK3/MK3S/MK3S+).
Check if the fuses are not blown.
Make sure that your hotend assembly is correct.
If the issue persists, then there is something wrong with your printer's thermistor or heater. In order to find out which one of these two is malfunctioning, perform the simple test described below.

Bed preheat error

Make sure that the ambient temperature is stable and there is cooling down the printer, such as a nearby air-conditioning unit or an open window.
Make sure the heatbed thermistor is properly seated under the heated bed and secured with the golden Kapton tape. If not, then simply put it back and make sure that the thermistor cable has some space for movement.
Make sure that the thermistor and heatbed are correctly plugged into the electronics board. You can doublecheck that by following these guides: Mini RAMBo electronics wiring (MK2S, MK2.5, MK2.5S) or Einsy RAMBo electronics wiring (MK3/MK3S/MK3S+).
Check if the fuses are not blown.
If the issue persists, there is something wrong with your printer's thermistor or heater. In order to find out which one of these two is malfunctioning, perform the simple test described below.

Testing the thermistor and heater

You can also verify if the thermistor or heater are damaged using a multimeter

It is not very common, but there is a chance that only the thermistor is defective. You can check it using the following test:

Use a hairdryer to blow hot air at the heatbed or hotend.
In the meantime, check if there is any temperature change on the LCD screen.
If there is a positive temperature change it means that the heatbed or hotend thermistor is working. Which means there is something wrong with the heater or heatbed. In both cases contact our support and they will provide assistance.
If there is no change at all, the hotend or heatbed thermistor is defective.

11 comments

coolcut

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The heatbed on my MK3S stopped working and after testing the voltage with a multimeter at the einsy board heatbed connectors and then at the connectors on the heatbed I managed to figure out it was the cable.
The board was outputting stable 23.95V whereas the voltage at the heatbed connectors was fluctuating between 18 and 22V.
After removing the sleeve from the heatbed (and thermistor) cable I found that the cable was crimped and actually 95% severed by the connectors for the heatbed..

RMIL

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I am currently having this issue. I had to remove a clog and remove the nozzle. Naturally with the nozzle removed, I had to order new nozzles and wait for them to come in. I reassembled everything, except no nozzle inserted in the heat block. I tried to preheat to 280c so i could insert the new nozzle, but it gives me the preheat error at about 165c. I have tried several times and always at about 165c it beeps and gives me the preheat error. I checked the cables, connections, fuse, etc. What is odd to me is that it attempts to heat up when commanded and then caps out and gives me the error. Any ideas?

Giuliano - Official Prusa CS

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Hello. I advise to inspect the thermistor and make sure that it's all the way in the heater block and well secured with the grub screw. If that's already the case, please contact the support via email or live chat for troubleshooting.

simonlaube

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Power panic issues can also result in a preheat error. Check out the following forum page which solved the issue for me: https://forum.prusa3d.com/forum/original-prusa-i3-mk3s-mk3-hardware-firmware-and-software-help/hotend-and-bed-suddenly-not-heating/

jdavidnet

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Last night we suffered a power outage. I removed the failed print, used the preheat function to unload the filament, and I was going to move from a .25mm nozzle to a .40mm nozzle, and start a different print. While unscrewing the nozzle I damaged the working thermalcouple and since I had 2 spare, I started taking the printer appart to put in a new thermal couple. ( about this time I'm really wishing I had a new E3D Revo system ) Between the kids, breakfast and everything else I get the new replacement thermalcouple in, and ... well I got a PREHEAT ERROR. So I thought well, f' it, I must have just spent all of that time cable managing a dud of a thermalcouple.
I tried the troubleshooting mentioned above, and ther hair dryer does make the thermister go up. I also tried testing the voltage on the pins for the thermal couple, both without a heating load, and with a heating load. I tried two spare thermalcouples, one red, and one white (I think 1 is from E3D directly and the other from Amazon). Neither heated, and Neither had a voltage load across the pins.
I also updated the firmware to the lastest Firmware 3.11.0, and still no voltage or heat on the nozzle. The bed heats fine, but the nozzle thermalcouple just won't heat.
Is there something else I can try? Is my controller board shot?

jdavidnet

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BTW I'm I've now commited over 6 hours to this fix. Help!?

jdavidnet

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I contacted Pursa Support and they got back to me within 12-18hrs. I think it's the time difference.
They sent me a few other articles about potential issues, and I discovered that it was a blown fuse. And after further review on the help articles, I noticed that I had installed the wrong heater. I had installed the 12 volt heater. Now, after replacing the 5 Amp fuse, and putting in a 24 volt heater, everything seems to be working.
I just need to recalibrate my machine for the different Nozzle.

loadinglevelone

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I frequently get the bed preheat problem when printing high temperature filaments, such as PC. The problem is that the build plate is too slow to heat up at the higher temperature range. So at first layer the print is set to 110 degrees - which is fine. The printer preheats to this temperature without problems. Then, on the second layer, the gcode tells the printer to increase the temperature to 115 degrees. Well, on my Prusa this is so slow that it is unable to reach this temperature fast enough. Especially on very small prints, the printer may reach layer 20 before it reaches 115 on the build plate. I don't know what the logic is in the firmware, but this seems to trigger the bed preheat error and abort the print. There are a few tricks I do to remedy this problem: 1. Bigger prints. If the model is small I print 3-4 of them on the same print. This gives the printer longer time to reach the bed temperature between layers. 2. Pause print. I pause the print right after it hits the second layer and let the printer bed heat up before I continue. 3. Enclusure. Increasing the ambient temperature >30 degrees seems to help.