Most 3D printers have at the very least a layer fan that you can control from the LCD or GCode from your slicer. Some boards also have a secondary fan output that can be used to cool a hotend or the control box that will automatically turn on when the machine is in use. These fan outputs both use MOSFETs to control the on/off state of the fans.
These troubleshooting steps are assuming that you have everything wired to the correct connection points (plugs, terminals, etc) on your control board. If you have not done so already, check that everything is wired correctly before proceeding.
If you notice that one of these types of controllable fans is either not coming on or is stuck at 100% speed all the time there could be a short that caused the MOSFET to fail.
Tools Required #
Visual Inspection of the Control Board #
An easy way to confirm if you have/had a short or not on a fan output is to inspect the MOSFET that runs the fan header. Typically, on most boards, the MOSFET will be located near the fan header itself. If there is any discoloration, swelling, bumps, or cracks in the MOSFET then you can assume that there is/was a short on your fan wiring.
Some boards can have the MOSFET replaced by a professional with the right tools and skills. For some boards it is just recommended to replace the entire board as the cost to repair will exceed the price of a new board due to the time involved.
One concern with a short occurring is that the MOSFET can surge higher voltage into the IO pin on the CPU for the board when the short occurs. If this happens the CPU may still function but even after replacing the MOSFET the fan will still not function when told to turn on/off. If this is the case the CPU would also need to be replaced on the board. Without replacing the MOSFET first there is no easy way to determine if the CPU was also damaged in the incident.
Checking for Shorts #
The first thing to do is to check for shorts on your wiring. We will use a multimeter to check for continuity across the fan wires. 1. Unplug your printer from your wall power and computer
2. Locate and disconnect the fan wires or plug from the control board for the fan that is having the issue
3. If you have a plug on the fan there will usually be metal exposed that you can probe on. If your fan has bare wires just probe on the wires themselves.
4. Turn your multimeter onto continuity mode and touch the 2 probe leads together. If your multimeter beeps then you can move forward.
If it doesn’t beep then your meter may not have one or your probe leads are bad, refer to your multimeter manual for doing continuity testing before proceeding if you do not hear a beep.
5. Place the multimeter probes on each wire. If the meter beeps (most meters have a beeper, make sure yours does) then you have a short on the fan or the wiring
6. If the multimeter does not beep then it is safe to say there is no short present or it is no longer there
One important thing to note is that all it takes is a small wire fragment to bridge the connections between the 2 wires. If only a few strands caused the short they could get burned up when the printer is powered on and they will no longer be present. It is highly recommended to go over the entire length of the fan wiring to make sure all connections are in proper shape (no fraying, bad connections, exposed wire, etc) before installing a new board in the printer. It is also recommended to replace the fan as if the fan caused an intermittent short it could possibly cause the same issue in the future and damage another board.
Hopefully, this guide helps you identify and isolate issues with your fan wiring. This article can also apply to beds and hotend heaters as well. Their MOSFETs are typically larger than the ones used for fans but the same troubleshooting steps and concerns apply.
Affiliate Disclosure: The Amazon link to the multimeter is an affiliate link that gives us a small percentage of the sale if you purchase one. We use these inexpensive meters in our shop for doing testing as outlined in this article.