This article is for those who may be experiencing a random freezing of the print head in the middle of a print, OR for those that only have the ability to Home the printer (no other functionality: heating, motor controls, etc.).

NOTE: This article assumes that no other outside circumstances are causing the print head to stop mid-print. If this issue occurred overnight, for example, please first verify that there was not a power failure, electrical wall outlet issue/tripped fuse, or that a colleague did not stop the print after you left the office for the day.

How does the thermistor in the Creatr HS printer work?

We use what is called Negative Temperature Coefficient thermistors in our 3D printers. This means that the higher the temperature of the thermistor gets, the lower the resistance (measured in ohms, "Ω") output is for the thermistor reading. The firmware takes 1 reading roughly ever split second or so to make sure that the thermistor is providing a output. The HS thermistor needs to be in circuit to get an accurate reading. When heating, the firmware uses this reading to stabilize a the extruder at a certain set temperature.

What happens when the firmware does not read a thermsitor reading?

The firmware checks for this thermistor reading as a safety feature. If the safety feature wasn't in place, a faulty thermistor could be reading 0º, and the printer would keep heating a set extruder or bed temperature to an unsafe limit. Therefore, the firmware is set to put the printer in a lock-down mode that we call MIN/MAX Temp Triggered Error. The printer will stay in this lock-down mode only allowing you to home the printer. Program language speaking, only G28 (which homes the printer) is allowed, all G0 or G1 (movement and printing) commands as well as heating commands are ignored. If you try to run a printable .GCODE in either Stand-Alone or Connected to PC mode, the printer's firmware will not allow it until the motherboard is reading a proper thermistor resistance reading again. If you ever see an M999 error, it is telling you that your printer is locked up with a MIN/MAX Temp Trigger Error and will need the g-code command M999 entered in order to reset the error. However, if you manually enter M999 in the PC's g-code terminal while the thermistor is still not providing any reading, the firmware will take that next 0.0625 second reading and send the machine right back into lock-down mode again.

What if it's the thermistor of an extruder that you don't even use?

Regardless, of whether or not you use the extruder, the printer doesn't want human error to cause a fire - to put it bluntly. If you accidentally set the wrong extruder to heat in the print profiles used to slice a .GCODE or preheat the wrong extrduer, the firmware is designed to lock-down the printer's functionality until the "un-used" thermistor is sending a resistance signal again.

So how do you troubleshoot this?

The easiest way to tell if you may have a thermistor issue on the Creatr HS is to look at the temperature readings on the front display BEFORE preheating the printer:

The left number of the 3 pairs should NEVER read 0º (unless your printer is outside and it is below freezing ☺). Therefore, if you see 0/0 for any of these 3 numbers, you know that there is an issue with a thermistor OR the ribbon cable that goes through the drag chain (which relays the thermistor readings). Similarly, on any printer that connects to the PC, you can check the temperature reading in the 3D slicing software program.

At this point you'll want to determine if it is the thermistor or the ribbon cable (a damaged ribbon cable can mess up the signal of a perfectly good thermistor).

You may have had a print stop mid-print even if you currently read good ambient readings on all 3 thermistors. With the power on, you can freely move the print head around with your hand; do so for all 4 corners wiggling the ribbon cable in the drag chain while watching the ambient room temperature readings on the front panel display. If there is a short in this cable, the readings may be read when the print head is at one position and not be read when the print head is another position. If you notice this, then we are almost certain that it is a MIN/MAX Temp Trigger Error caused to a damaged ribbon cable. Here is a short video showing this test:

If you notice, the Right Extruder reading on the display starts out showing 44º/0 (the printer was temporarily preheated before making this video). However, just by moving the print head a small bit, the drag chain wire shorts out and causes the thermistor reading to drop to 0/0. Again, this, without a doubt is a MIN/MAX Temp Trigger Error caused by a damaged ribbon cable inside the drag chain.

Heated Bed? To check the heated bed's thermistor, home the printer and wiggle the wires that come from the heated bed and lead to the motherboard. If the same kind of short occurs as described above (the bed temperature suddenly changes to 0/0 when the wires are slightly bent), you know that it is the heated bed thermistor.

If you discover the cause of the issue and need a replacement, make a short video of this test (keeping it under 15MB, or uploading it to a file share website and sending the download link) and send it in a ticket to Leapfrog Technical Support. We'll also ask for your original purchasing invoice and shipping address.

If you see 0/0, the above check may not work. Therefore, remove  the side panels of the printer as well as the dust cover plate to expose the motherboard. Switch the motherboard connection of the thermistor that is giving a bad reading with a different thermistor motherboard connection: 

(You can see a better layout of the motherboard here: Layout of the Creatr HS Series Motherboard, Olimex Board, and Print Head PCB Connections.)

If the thermistor or ribbon cable are bad then the 0/0 reading on the front panel display should follow where you connected it. For example, if your left extruder is originally showing 0/0 and you switch the GREEN/BLUE wire connection with the ORANGE/YELLOW connection, logically the right extruder should show a reading of 0/0 (a rebooting of the printer may be required). IF the 0/0 reading stays on the left side (in this example), then the issue actually is with the motherboard. Make a short video (less than 15MB or upload it to a file sharing website) and send it in a ticket to Leapfrog Technical Support; additional troubleshooting may be required to come to a more definite conclusion.

Finally, if you think that it is the thermistor, you can disconnect the thermistor from the print head, unscrew it from the extruder block, and connect it to the motherboard directly (bypassing the ribbon cable and print head PCB).

You must make sure that all 3 thermistors are connected to the motherboard, so make sure that you disconnect the corresponding ribbon cable thermistor connect when plugging the actual thermistor directly into the motherboard. For example, if you suspect that the right extruder's thermistor is causing the issue, remove it from the print head and plug it directly into the slot that the ORANGE/YELLOW wire is plugged into:

Then, turn off the printer and turn it back on (this will send the M999 command that resets the printer and takes it out of lock-down IF the thermistor is not damaged). Again, a damaged thermistor will show a 0/0 on the display when connected to the motherboard like this. 

Here is a reading from Simpliy3D showing that the right thermistor reads ambient temperature just fine when we tested connecting the thermistor directly to the motherboard:

(The temperature reading indicated in yellow is the thermistor reading when plugging the thermistor directly into the right thermistor motherboard connection. The temperature reading indicated with the orange box is the heated bed's ambient temperature reading.)

If you get a 0/0 reading with the thermistor connected directly to the motherboard and you've tested it by using the CHECK 2 method, then we are certain that the thermistor needs to be replaced.

Okay... so your print head stopped and all of your thermistors (and ribbon cable) looks fine... now what?

In a 3D slicing software that has a terminal, customers will sometime see an error that says, "Firmware Unresponsive." If you see this error or have done the above and found that all thermistors are working fine, then please open a ticket with Leapfrog Technical Support explaining all that you've checked. In addition, if you have any other questions about the above information, feel free to open a ticket as well, where one of our Technical Support Agents will be happy to assist you further.