Preheat Error (Hotend and Bed)  

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jer.n
(@jer-n)
Active Member

My MK3 printed fine over the weekend but this morning when I start a print, neither heatbed nor hotend will heat up. The thermistors react when I blow hot air at them but I get a PREHEAT ERROR message when I try to print. I'm running the latest firmware and don't see any obvious connectors unplugged. What would cause both to stop working at the same time?

Best Answer by jer.n:

Alrighty, so something went bad with my Einsy RAMBo. I swapped it for a new one and everything works again. I suspect from brownouts in the area (even though the printer is on a surge protector).

I checked these first and they weren't the cause but if you have the same issue, check these first: hot end cable and thermistor resistances, heatbed cable and thermistor resistances, test old firmware, power supply voltage, fuses.

Thanks everyone for the suggestions as it helped me narrow things down.

Posted : 12/10/2020 9:19 pm
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 -2
(@-2)
Illustrious Member

When either heater doesn't provide feedback the printer shuts down.  A bad heater or bad connection can cause this. Use an ohm meter to check the bed and extruder heater resistances. There should also be some error code being thrown which hints at the cause. 

It is always wise to get more than one opinion... as for trusting Prusa? No way man....
Posted : 13/10/2020 5:16 am
jer.n
(@jer-n)
Active Member

@tim-m30

I got 14.7 ohms for the hot end and 2.8 ohms for the bed. Looks like the bed is supposed to be 3.5-4 ohms. I'd think it would heat up slower with a lower resistance but it doesn't heat at all.

Posted : 13/10/2020 4:03 pm
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 -2
(@-2)
Illustrious Member

Lower R means higher power. And 2.8 seems quite low for a 24v heater (24v/6.25a == 3.9 ohms) and indicates a problem there. Check the cable connection at the bed. Also check for charring on the fiberglass around the screw connections. 

You may also want to read this: https://help.prusa3d.com/en/article/mintemp-and-mintemp-bed_2169

 

It is always wise to get more than one opinion... as for trusting Prusa? No way man....
Posted : 13/10/2020 4:23 pm
jer.n
(@jer-n)
Active Member

@tim-m30

You're right about increased power. The connectors are tight and I don't see any charring. Does the low resistance mean the cable went bad or something else is wrong?

The article you referenced talks about broken thermistors but I get a reading of 23C for both and it never drops to 0.

Posted : 13/10/2020 6:57 pm
jer.n
(@jer-n)
Active Member

Heatbed thermistor is 106 kΩ and extruder is 109 kΩ which are within the 80-125 kΩ that Prusa says.

Posted : 14/10/2020 1:33 pm
JoanTabb
(@joantabb)
Moderator

Hi Jer, 
I would consider re flashing the old firmware and seeing if the problem persists... 
if the problem goes away, 
I would then try a factory reset and then reflashing the new firmware, but this would require a complete re calibration of the printer... 
(Not a huge job) 
If you have a lot of different build plate profiles, it might be worthwhile checking the individual Live Z value foe each build plate to make reprogramming quicker

regards Joan

 

I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK...
Posted : 14/10/2020 1:48 pm
karl-herbert
(@karl-herbert)
Famed Member
Posted by: @jer-n

My MK3 printed fine over the weekend but this morning when I start a print, neither heatbed nor hotend will heat up. The thermistors react when I blow hot air at them but I get a PREHEAT ERROR message when I try to print. I'm running the latest firmware and don't see any obvious connectors unplugged. What would cause both to stop working at the same time?

And every now and then the culprit is also the power supply. I would check that on this opportunity (24V).

This post was modified 2 months ago by karl-herbert
Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 14/10/2020 2:13 pm
jer.n
(@jer-n)
Active Member

@joantabb

I flashed the old 3.8.1 to it and there wasn't any difference. I did a factory reset and it fails during initial setup at the bed heat check, "Heater/Thermistor Not connected"

It fails before it get to testing the nozzle. Thanks for the suggestions.

Posted : 14/10/2020 2:43 pm
jer.n
(@jer-n)
Active Member

@karl-herbert

PSU power cables both were 24.4V.

I told it to preheat and got 0V at the heatbed terminals so power isn't making it to the bed. I hooked 24V directly to the bed terminal and the bed started heating fine so I believe some relay on the Einsy RAMBo died. Will confirm.

Posted : 14/10/2020 7:55 pm
--
 -2
(@-2)
Illustrious Member

Sometimes the obvious avoids even old times.

Did you check the fuses?

 

It is always wise to get more than one opinion... as for trusting Prusa? No way man....
Posted : 14/10/2020 11:48 pm
karl-herbert
(@karl-herbert)
Famed Member

@jer-n

Have you checked the 24V connections on the einsy (corrosion, melted insulation). If the fuses are ok, i think the einsy has got some damage.

Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 15/10/2020 6:58 pm
jer.n
(@jer-n)
Active Member

@karl-herbert

@tim-m30

Fuses are good. 24V 'power in' and 'bed in' are good.  0V at the 'bed out' connection. There can be frequent brownouts with bad weather here so that may have caused it to fail over time. I ordered another board and will swap them out to see if that fixes it.

Posted : 16/10/2020 3:12 pm
jer.n
(@jer-n)
Active Member

Alrighty, so something went bad with my Einsy RAMBo. I swapped it for a new one and everything works again. I suspect from brownouts in the area (even though the printer is on a surge protector).

I checked these first and they weren't the cause but if you have the same issue, check these first: hot end cable and thermistor resistances, heatbed cable and thermistor resistances, test old firmware, power supply voltage, fuses.

Thanks everyone for the suggestions as it helped me narrow things down.

Posted : 19/10/2020 9:48 pm
karl-herbert
(@karl-herbert)
Famed Member

@jer-n

Thank you for the feedback and I'm pleased that the printer is running again 👍

happy printing!

Statt zu klagen, dass wir nicht alles haben, was wir wollen, sollten wir lieber dankbar sein, dass wir nicht alles bekommen, was wir verdienen....
Posted : 19/10/2020 9:56 pm
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