Repair Time: First Time? 1-2 hours. Done this before? 30-45 minutes.

 

Have your prints started looking stringy with the same settings you used before? Have you checked on your printer to find only a partial print and a message on the screen saying "MAXTEMP Error" or "MINTEMP Error" or "MINTEMP Error Fixed"? If you have any of these issues, your problem may be a faulty or intermittent hotend thermistor reading. Not to worry! It's simple to repair, and if you built your printer from a kit, you already have most of the tools you need.

 

Do You Have a MINTEMP or MAXTEMP Error?

Start by powering up your printer and looking at the display screen. What do you see there in the temperature reading/temperature command section? You should see at least one pair of numbers written like this "25/0." The number before the "/" is the temperature your printer is reading right now. The number after the "/" is the temperature target you're trying to hit. In the picture below, the top pair of numbers is for the hotend. The bottom pair of numbers is for the heated print bed.

Printer display screen showing MINTEMP error message.

For example, if you've just started a PLA print, you might see "25/205" for your hot-end thermistor and "25/50" for your bed thermistor. Both your hot-end and your bed are at 25°C (room temperature) at that moment, but you're targeting 205°C as the PLA print temperature and 50°C as the bed temperature.

 

If the current reading (the number before the "/") for either your bed or your hot-end is lower than 18°C or absurdly high (300°C for example) you have a thermistor problem. It's absolutely necessary to fix it, but it's not very difficult.

 

Hot-End Thermistor Troubleshooting

Think of a paperclip. What happens when you bend a paperclip in the same spot over and over? It gets brittle, and it snaps right where you bent it. The same thing can happen with your thermistor wires. Because the printer moves the same way so much, your wires get bent in the same spots thousands of times over the life of the printer.

 

Let's start by doing some troubleshooting. In this article, I'm assuming that it's your hotend thermistor that's misbehaving, but the steps would be similar for troubleshooting the bed thermistor. To test for a break, take the wire bundle going from your control board to the bad thermistor and wiggle it, bounce it, and bend it. As you're doing that, watch the thermistor reading on the printer screen. Does the reading change as you're flexing the wire? If it does, try to hold the wire bundle in a position where the thermistor reads correctly. This will tell you a lot about where your problem is.

 

If you are able to find a position where the thermistor reads correctly on the screen, look at which part of the wire bundle you're bending. Unwrap that portion if it's in any sort of wire wrap, and try bending smaller and smaller sections of the wire to find exactly where the thermistor reading changes as you bend it. As you narrow down where the break could be, look for discoloration or bulges in the wire (see picture below). Both discoloration and bulging are signs of breakage. If you've found a small section or spot this way, you'll need to repair the wire by stripping the wire and soldering in a patch.  Skip ahead to those instructions!

Thermistor wire with break inside.  Look for discoloration and bulging.

If bending the wire harness doesn't change the thermistor reading on the screen at all, you may have a bad thermistor. With the printer unplugged from power, gently remove the thermistor connector from the control board. Test the resistance of the whole thermistor wire harness with a multimeter by placing one probe in each socket of the thermistor connector you just removed. If your meter reads "OL," which stands for "overload," there's a break in the wire or thermistor somewhere. It's likely that you need to continue using the wiggle method mentioned above, but it is possible that your thermistor has broken. However, if your meter reads a value and it's something outside the range of 80-125kOhms, you likely have a bad thermistor.

In this case, you'll need to uninstall the thermistor wire harness connector from the control board (if you haven't already), take the thermistor wire out of the hot-end wire bundle, and remove the thermistor cartridge from the hotend. Skip down to those instructions!

If you're replacing the thermistor, check with your printer manufacturer to see if yours is still under warranty. If your thermistor is no longer covered by warranty, you have other options for about $10 to get your printer back up and running. The option I've used (and recommend) for e3D V6 hot-ends is this wire harness replacement kit from MatterHackers. It comes with enough wire to completely replace the thermistor wire harness on my Prusa i3MK3S, and it incorporates an extra connector to make it easier to replace the thermistor cartridge if it goes bad in the future. Please be sure to order the right part for your machine!

Thermistor Wire Repair

Here's how to solder in a wire patch to repair your thermistor wire:

  1. Identify where your wire is broken

  2. Unplug the printer!

  3. Snip the wire near the break using wire cutters

  4. Strip each end about half an inch (13mm)

  5. Confirm that you've found the break

  6. Cut a piece of heat-shrink tubing about 1.5" long and slide it over the thermistor wire

  7. Cut a patch of stranded wire the same gauge (that is, the same size) as your thermistor wire

  8. Loop the patch around each end of your thermistor wire and twist it to create a good physical connection

  9. Solder the patch wire to the thermistor wire to create a good electrical connection

  10. Slide the heat-shrink tubing over the bare wire and use your soldering iron (or a hair dryer or heat gun) to shrink the tubing and protect the wire.

  11. Power on the printer, and check that the thermistor is reading correctly. If not, head back to the troubleshooting step.

  12. All done!

 

Thermistor Cartridge Repair

Here's how to replace the thermistor cartridge. Your printer may be different!

  1. Unplug the printer!

  2. Unplug the hot-end thermistor wire harness connector from the control board

  3. If necessary, unwrap the hot-end wire bundle to remove the thermistor wire

  4. Remove the thermistor cartridge set screw.  Note on this step - Your set screw may be covered by burnt filament like mine was. I used a soldering iron to heat the hot end block and soften the burnt filament. Then I used a brass bristle brush, a utility knife, and a precision flathead screwdriver to scrape away enough filament to get access to the set screw.

  5. Remove and replace the thermistor cartridge with the new part

  6. If necessary, re-wrap the new thermistor wire into the hot-end wire bundle

  7. Plug the thermistor connector back into its proper spot on the control board.

  8. Power on the printer, and check that the thermistor is reading correctly. If not head back to the troubleshooting step.

  9. All done!

If neither of these procedures fixed your temperature error, I'm so sorry! I hope you're able to diagnose and repair your problem. You may also find some useful resources in the references below.

References:

Prusa MINTEMP Error Troubleshooting Guide

Prusa MAXTEMP Error Troubleshooting Guide

Prusa Multimeter Troubleshooting Guide

Filament Friday - Broken Solder Joint Repair for MINTEMP Error Fix

 

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