[SOLVED] on [EDIT 3]

I just finished changing my i3 Mega's PTFE tube and brass nozzle, reassembled it, always taking care not to damage the heating element and thermistor wires. Then I check the temperature and it was cooling down, despite the set temperature being higher (210 ºC). I turned it off, let it cool down and turned it back on. I set it again to 210 ºC and it only reaches about 135 ºC-140 ºC, not going over it. So I change the heating element and thermistor for fresh new ones (original ones, that came with the printer) and the same behavior takes place: not heating up beyond 140 ºC.

Any ideas on why this is happening or how to fix it? I checked the connectors and they all seem to be fine. Cooling fans run normally as well.

Printer: Anycubic i3 MEGA. All stock parts, except the new nozzle (standard brass nozzle) and the PTFE tube (a blue one with 1.9 mm internal diameter). I'm setting the temperatures through the printer's interface, as I have always done.


I did check some stuff with the multimeter and what I got is that:

  • The cables leading to the print head are fine;
  • The heating element's voltage is correct and the MOSFET seems to be working fine both by checking the voltage and from the LED that lights up when it is sending current to the HE;
  • The resistance of the HE is correct as well;

Form that, I am guessing the problem is the current fed to the HE. Later today I will try to heat both the nozzle and the bed to the (safe) max temperatures and see if the bed heating is problematic too. If so, the problem should be related to current.

If anyone has any other ideas, they are appreciated! :)


Heating up the print bed and the hotend at the same time didn't affect the bed at all. It heated up at the same pace as usual. Since the cables are fine, it shouldn't be a problem related to the power supply.

So I'm gessing the problem is the motherboard (Trigorilla 1.1). It didn't seem to have any burned components at a glance but I'm gonna check with a multimeter.


Turns out I messed up the heatbreaker, so the heatsink was cooling the hotend way too much... This is one way of learning I need to be careful with the heatbreaker, I guess. I'll leave a solution here for the newbies like me that end up in a similar situation.

  • $\begingroup$ Hi welcome to 3DPrinting.SE! Do you have a multimeter/tester to check the voltage and resistance of the heater element? Did you upload new firmware? Please look into phrase "Then I check the temperature and it was cooling down, despite the set temperature being higher", I don't understand what is meant here. $\endgroup$
    – 0scar
    Oct 18, 2020 at 21:55
  • $\begingroup$ Any chance you put a 24 V heater on a 12 V system? Have you used an alternative method to measure the heater temperature? $\endgroup$
    – Perry Webb
    Oct 19, 2020 at 15:45
  • $\begingroup$ If the voltage is correct and the resistance of the heater cartridge is correct, the current is a direct derivative ( I = U/R ). Is the heater cartridge making good contact to the heater block? $\endgroup$
    – 0scar
    Oct 22, 2020 at 21:22
  • $\begingroup$ Yeah, the heater cartridge is touching the block nicely $\endgroup$
    – Lurosset
    Oct 23, 2020 at 0:36
  • $\begingroup$ @0scar I meant the nozzle was set to 210C and it eventually started dropping below 210C and not heating back up $\endgroup$
    – Lurosset
    Oct 23, 2020 at 0:38

2 Answers 2


I recently had similar problem with my Prusa i3 MK2.5. It was not able to maintain the set tempereature. I measured the heater catridge (which was new) and its resistance was correct (about 4 ohms). So I changed the hotend MOSFET on the stock board with no result. So I changed the heater catridge for the older one and the issue was gone.

Since you have changed the heater, it is unlikely that you have multiple bad ones. Thus the only thing that remains is the MOSFET and the power supply. Make sure your power supply is stable under load and can deliver enough current.

More likely it is the MOSFET. It behaves like a resistor in the path for the current. If it is swithced off, it is like a large resistor (units or tens of megaohms) in series therefore no or very little current can flow. If it is switched on (units or tens of milliohms) the resistance is very small allowing the current to flow through the heater. If a MOSFET is bad, when it is switched on it can have higher resistance (units of ohms) and thus limiting the current and creating a voltage divider. Which you can measure.

You have to get to the bare wires that lead to the heater. Turn on the heater. Place your voltmeter lead on one wire and the other lead on the otehr wire. The voltemeter should show voltage close to your power supply voltage. If it is showing less, the MOSFET is bad and needs replacing.

For that you will need soldering tools and skills. You probably have an SMD MOSFET soldered directly to the board. I suggest removing it and replacing it with a THT MOSFET that you would place separately from the board with its own heatsink. When on the board, the board acts as a heatsink.

If you cannot do that yourself, ask someone to do it for you.

Before replacing the MOSFET, find whether it is a n-channel or p-channel MOSFET. It will most likely be an n-channel MOSFET but make sure it is.

If you replace it with THT MOSFET I would suggest these two: IRFZ44N (n-channel) or IRF4905 (p-channel). They are general purpose MOSFETs and should work well enough for drop-in replacement.

If it is not the MOSFET, power supply or the heater, then I have no idea.

You should be able to trace one of these loops: power supply -> (fuse ->) heater -> n-channel MOSFET -> power supply power supply -> (fuse ->) p-channel MOSFET -> heater -> power supply

Most likely will be the first loop.

NOTE: Checking the heater resistance might help but does not have to. At room temperature the resistance might be within limits (as with mine example) and when heated up the resistance increases with temperature and thus limiting the current. You would have to measure the resistance when heated up and disconnected from all circuits. (Heat up -> disconnect heater -> measure -> cooldown).

Hope that helps and good luck.

  • $\begingroup$ What about measuring the voltage and current on the heater when it is powered up. If the MOSFET is the issue and the heater didn't reach temperature, the voltage will be too low. The resistance of the heater will be R=V/I. Thus if the heater resistance is too high, the current will be low if not reaching temperature. At 70 ºC below the set point, the power to the heater should be open loop (maximum voltage). $\endgroup$
    – Perry Webb
    Oct 20, 2020 at 13:42
  • $\begingroup$ I have already suggested measuring voltage accross heater when powered up. That is (probably) the only way to check a MOSFET when in circuit. I also described measuring the heater resistance when heated up, or as you suggest, one can calculate it. But neither approach can give a deffinitive answer. The resistance will always raise with raising temperature. But can you say how much it should and how much is too much? You could say that only when comparing with another heater of the same type that is fully functional. $\endgroup$ Oct 21, 2020 at 15:54
  • $\begingroup$ The heater at temperature should be approximately R = (V^2)/P. So a 12V 40W heater should be about 3.6Ω, a 12V 30W heater 4.8Ω, 24V 40W 14.4Ω, 24V 30W 19.2Ω. $\endgroup$
    – Perry Webb
    Oct 21, 2020 at 18:28
  • $\begingroup$ Again, the resistance raises with raising temperature therefore, the maximum power of the heater decreases. One can either heat up the heater as much as it can go, disconnect it and measure the resitance with multimeter, or heat up the heater as much as it can go and then measure the voltage and current and calculate the resistance. Neither approach gives you an answer whether the heater is faulty, unless you have the datasheet of the heater or you can compare the values with a fully functional heater of the same type. $\endgroup$ Oct 22, 2020 at 13:45
  • $\begingroup$ @MStarha I did some new checks (on the edit of the original post). Thanks for the directions for troubleshooting! $\endgroup$
    – Lurosset
    Oct 22, 2020 at 18:52

There is a part of the extrusion ensemble called heatbreaker, responsible for sepparating the hot- and cold-ends. The problem is that I screwed the heatbreak up into the cold-end heatsinks and the heated block was now touching it. This made it cool off way too hard.

The solution was switching to a new, undamaged cold+hotend.


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