MINTEMP & MINTEMP BED
The MINTEMP error (written sometimes as min temp) occurs when the temperature readout from the heatbed thermistor or from the hotend thermistor drops below 18°C. It is a safety feature that prevents the printer from overheating in case of a faulty thermistor, because by default, the printer is set to balance out any heat loss by pumping more power into the heatbed or hotend heater cartridge.
What does it look like?
When one of the thermistor cabes is down, it can be observed when the temperature on the LCD screen starts randomly jumping up and down during the Y-axis or X-axis movement.
It is important to distinguish between the MINTEMP error and the MINTEMP BED error.
MINTEMP - problem with the hotend temperature readout
MINTEMP BED - problem with the heatbed temperature readout
How to fix it
Make sure the temperature of the room where you operate the printer is not below 20°C. Also make sure the printer is not placed next to an opened window, air conditioning or in a way of a breeze that might cause a sudden temperature fluctuation.
As mentioned above, the error might have been triggered by a simple glitch. Try to reset your printer by pressing the X button under the knob and see if it will print afterwards.
Check if the thermistor is properly plugged into the RAMBo board: there there is a latch on the connector that needs to snap in. If your printer model is MK2, MK2S, MK2.5 or MK2.5S, check this page, if it is MK3 or MK3S, check this page. Also, make sure the textile sleeve protection of the extruder cable bundle did not snap and that it is still procecting the cables, especially around the spot where they enter the electronics casing.
On the other end of the hotend thermistor cable, check the wire where it goes into the heater block.
As for the heatbed thermistor cable, make sure it is firmly attached to the underside of the heated bed with the golden Kapton tape. If not, try to stick it back and make sure that the thermistor cable is not waring down by the movemend of the bed.
You can also test it with a multimeter (this artcle will guide you). If it fails to read the resistance on the metal inserts in the jumper connectors, then the wire is broken. At room temperature, it should have a resistance you can expect a resistance of 80 kΩ - 125 kΩ.
RAMBo board connector
This possibility is highly unlikely, as the board would not pass the final testing that we do in our factory. Still, if in doubt, try switching the Hotend and Heatbed thermistors on the board (both have 3 pins), if the MINTEMP does not turn into MINTEM BED or if the MINTEMP BED does not turn into MINTEMP, the electronics board connector is broken and a board is needed.