Hotend refuses to stop heating

I bought a Tronxy X5SA Pro a couple months ago, and it was working great. Then, out of the blue, a HUGE problem occurred. For some reason when i turn it on, the hot end automatically starts heating ( at warp speed!), then thermal runaway kicks in and throws up the “max temp” error.

So 2 questions:

  1. Why is the hot end heating automatically? It is not connected to my computer, and there is no SD card inserted. What is telling it to heat up?

  2. Why does it refuse to stop heating? The thermistor is reading correctly. If i take it out of the heat block the temp reading drops immediately. I have about 60 seconds before the temp maxes out and the printer freezes, and even setting pre heat and current hotend temps does nothing. It just refuses to stop heating. Just blows past the set temp.

Anyone know where to start looking for the problem?

Hi @Jeffopentax

Thanks for your post, Good to see you back.

Unfortunately, this is usually a sign the Mosfet (little tiny relay) is stuck in the closed position for the hot end, or the hot end cartridge is attached to the incorrect location on the mainboard. As this was working well up to a point, I will assume it’s not a wiring issue.

This is one way that the boards can fail. Essentially the thermal runaway is doing precisely what it’s supposed to. It’s telling you that the Heat circuit is NOT under control and needs your attention.

With only this amount of information to go on, I am thinking that you are going to have to replace the control board on the printer to regain control of the hot end.

there are some additional diagnostics you can do however it would require a volt meter if you have one. You can let me know if you do and I can give you an additional test to run.

This is something I have worried about. Do printer main boards not have a backup in the event the mosfet gets stuck closed? Even if it detects the runaway, what can the board do other than beep loudly to try to get someone’s attention? Or do I misunderstand the thermal runaway protection system, and it uses a secondary shutoff in the event of a thermal runaway?

Hi @MicroFarmModels

You have actually brought up a very good point.

Ironically I was actually just explaining this section while I was working on my book last night. I actually did not touch on this particular issue although it should be addressed.

Thermal runaway is built into all 3D printer firmware past Marlin 1.9X and has become the standard by which safety features are measured. 99% of the time it works, sometimes too well. If a heater cartridge becomes weak and does not heat up fast enough it trips an error and kills your print. It’s really doing what it’s supposed to.

I word it as this. For any heater circuit in your 3D printer, there is a control built in based on specific parameters it will succeed or fail. There must be a Heater cartridge and a thermistor mated as a pair.

Here is the actual Thermal Runaway section from Marlin 2.X, configuration_adv.h


  • Thermal Protection provides additional protection to your printer from damage
  • and fire. Marlin always includes safe min and max temperature ranges which
  • protect against a broken or disconnected thermistor wire.
  • The issue: If a thermistor falls out, it will report the much lower
  • temperature of the air in the room, and the the firmware will keep
  • the heater on.
  • The solution: Once the temperature reaches the target, start observing.
  • If the temperature stays too far below the target (hysteresis) for too
  • long (period), the firmware will halt the machine as a safety precaution.
  • If you get false positives for “Thermal Runaway”, increase
    #define THERMAL_PROTECTION_PERIOD 40 // Seconds
    #define THERMAL_PROTECTION_HYSTERESIS 4 // Degrees Celsius

//#define ADAPTIVE_FAN_SLOWING // Slow part cooling fan if temperature drops
//#define NO_FAN_SLOWING_IN_PID_TUNING // Don’t slow fan speed during M303


  • Whenever an M104, M109, or M303 increases the target temperature, the
  • firmware will wait for the WATCH_TEMP_PERIOD to expire. If the temperature
  • hasn’t increased by WATCH_TEMP_INCREASE degrees, the machine is halted and
  • requires a hard reset. This test restarts with any M104/M109/M303, but only
  • if the current temperature is far enough below the target for a reliable
  • test.
  • If you get false positives for “Heating failed”, increase WATCH_TEMP_PERIOD
  • and/or decrease WATCH_TEMP_INCREASE. WATCH_TEMP_INCREASE should not be set
  • below 2.
    #define WATCH_TEMP_PERIOD 40 // Seconds
    #define WATCH_TEMP_INCREASE 2 // Degrees Celsius

In order for the control of the printer to take over it must have control of the mosfet. Other than that its like a broken light switch, Its on and cannot turn it off.

It should shut off but 99% of the printers on the market do not have the control ability to shut down power to the power supply in case of such issues.

Thanks for that response. That is actually kinda scary. Even if that failure mode isn’t that likely. It wouldn’t even be that hard for a board to be built with a secondary power cutoff method, which could eliminate that failure mode to the greatest reasonable extent. Unless of course one would decide to go down the rabbit hole of having a backup for the backup, and backup for the backup’s backup, and so on. :crazy_face:

I agree with you 100%, the protection is there but the protection is not “complete” per se.

If the accelerator on your car gets stuck you can always turn off the key, however, it required you to be there to turn the key off. Same deal.

After some back and forth with Tronxy, they sent me a replacement board free of charge. That did solve the problem. However, in the process of “out with the old, in with the new”, the screen cable connector decided it didn’t want to be part of the board anymore (either poor quality, or i wasn’t delicate enough with it). Tronxy suggested soldering it back on. Tried that, but I suck at soldering so not surprisingly it didn’t work. A new board will be about $120 when it’s all said and done. I have a Tronxy XY-2 Pro I bought about 2 years ago. Does anyone know off hand if the board from that would work in the X5SA Pro?

thats more than frustrating, Check your local electronics store, you would be surprised where some hidden people are that love soldering jobs like that.

I needed a pinball machine fixed one time and I found him on Kijiji.

an absolute master…