Troubleshooting a “no fire” issue

Troubleshooting a “no fire” issue

A new article about troubleshooting a no-fire issue is published here.

Quick troubleshooting when the machine is not firing the laser any more.

To begin with, make sure all wires and connections are good in the machine and make sure all cables are where they should be.
Read up on the grounding-article here

1: Start the machine

Does the machine home to top left position? if it does – go to #2
If it does not move – check fuses in the back of the machine, and inside the PSU – do this without power cable connected.

2: Do a test fire on the panel

If the machine fires, you dont have a problem 🙂
If the machine do not fire, go to 3.

3: Do a test fire on the test-button on the power supply

If the machine fires, you have a problem in the buttons, potentiometer or the cables.
You might have a water flow sensor connected, remove it and bridge the two connectors it was connected to with a small cable.
Test fire again.
If the machine do not fire, go to 4.

4: Load a file, or make a shape and try to engrave or cut it.

Only need to test for 5 seconds.
If the machine do not move or fire – you probably have a issue with the bridge rectifier (green in the picture below) and the internal fuse. It is a $1 replacement, soldering needed. Order a new one, 600V 6A is good for the rectifier.

If the machine moves, but no fire from the tube you probably have a issue with the flyback transformer (HVT). Marked red in the picture below.This is a $20-50 replacement depending on where you buy it and how quick you want it delivered. We sell them with EU delivery within 3-4 days here.
Or check out ebay or aliexpress to order a new one. Open your power supply and make sure your is visually identical to the listing on Ebay/aliexpress, there is different types.


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Comments (47)

  • Tom Reply

    Dear Sir

    Just read your post and saw your picture.
    My new K40 laser will not test fire.
    The seller was very helpful and sent me a new laser tube but it still will not test fire.
    However there is a beam inside the laser tube when I pushed the test fire button.
    They believe the water sensor is not sensitive enough to trigger the laser.
    And they are sending me a new water sensor.

    Is that a possibility?

    Please advise
    Thank you

    2018-01-08 at 20:37
    • HP Persson Reply

      You can remove the water sensor, bridge P+ and P- on the power supply to test it without the sensor.

      2018-01-21 at 10:42
  • Rob Reply

    Super-useful, thanks.
    I had a problem with the laser not firing. When I pressed the test button, I could see the ammeter register the current but there was no fire. I switched the whole machine off, then back on again (that one weird IT trick…lol) and it started working again. However, I find this very worrying. Is this possibly the flyback transformer?

    2018-01-21 at 09:25
    • HP Persson Reply

      It might be a loose connection. Take off the lid on the PSU and inspect the 3-pin connector for the flyback, make sure its crimped properly and you have good contact on all three pins. Also check the machine if it has a water sensor, connected to P+ on the power supply, if these fail the firing can be intermittent. Also check the other cables on the PSU so the crimps are good and no damaged cables.

      2018-01-21 at 10:41
  • Joel Brondos Reply

    My K40 laser stopped firing in the middle of a project (using LIghtBurn and Cohesion3D Mini upgrade board). e.g. the stepper motor controllers seem to be working fine, communicating with the computer.

    When I click on the Test Fire button, NO CURRENT registers on the analog meter. Just before the laser firing failed, I noticed that the amperage seemed a bit erratic and did not increase or decrease smoothly as I turned the potentiometer.

    If this is likely to be the flyback, I’m thinking I might just as well replace the entire power supply and not just the flyback. Can you recommend any good power supply replacements — ones which could also feed an LED lighting strip and bottle rotation device if I ever should decide on one of those?

    2018-02-19 at 16:23
  • Gabe Reply


    I can only fire from the test button on the power supply (step 3). Any tips on fixing the issues with the buttons, or wiring?

    2018-04-07 at 00:37
    • HP Persson Reply

      Make sure the cables from your panel in the lid are connected properly. Also, a test to make is to remove any cables on P+ and P- on the PSU and bridge P+ to P-(gnd) with a small cable to see if the laser is firing after this. A common problem is a flow switch not working which will disable the firing. (test fire on PSU bypasses this protection). This is probably your problem.

      2018-04-07 at 00:51
      • Tyler Reply

        I am also having a no fire issue, it will fire from the “TEXT” button
        I removed the water brake wires and still no fire, only fire is from the test button
        any other suggesting?

        2018-07-17 at 04:08
        • HP Persson Reply

          Bridge P+ and GND, pin 5 and 6 if you count from left. Put a piece of cable between them pins and it should fire from the software too.

          2018-07-17 at 21:50
          • Florian

            Hi all, if the laser fires bridging pin 5-6 – what is the issue? Which part do i have to replace?
            thanks from munich!

            2019-01-10 at 06:08
          • HP Persson

            Flow sensor is often the problem if installed on that spot. The potentiometer setting the power might be another issue. Cheap fixes.

            2019-01-10 at 09:19
  • Fran Reply

    Hi. I have an issue with my K40. When firing at low power settings, it works well (ampmeter reads 3-4 mA and laser fires), but when I increase the power, laser stops firing, and ampmeter shows a very low readout (1-2mA).
    Could it be the flyback is damaged?

    2018-04-11 at 15:28
    • HP Persson Reply

      Do a test, on your power supply you have connections marked IN, 5V next to each other. Remove the cables on these two and put in a cable between them, then do a test-fire again. It might be your potentiometer (power knob) beeing faulty, this test shows if it is – you will see very high power so do not run it for long like this. Take a photo before removing cables, or mark them so you can put them back.

      2018-04-11 at 17:40
      • Fran Reply

        My machine is the one with digital panel, so no power knob. (also I have a cohesion3D mini board).
        I’ll make the test, and see if it fires at full power.

        2018-04-11 at 20:42
      • Fran Reply

        Hi. I made a video showing some test. Please take a look and tell me what you think. Thanks.

        2018-04-12 at 14:12
        • HP Persson Reply

          Hard to tell, it might be the tube. Have you tried bridging 5V and IN on the PSU? At the same time, quick loss of power can also be the flyback – but it´s often arcing on high power.

          2018-04-13 at 10:26
  • Pablo Verity Reply

    Also, my K40 won’t fire reliably much below 20%, but it does have some hours on it. For testing purposes, try at least that.

    2018-04-17 at 12:06
  • Jonathan Reply

    Have you ever heard of or experienced the actual laser tube being the cause of a power supply failure? I recently bought a new k40 machine and the tube stopped firing in the middle of a job. The supplier sent me a new tube and it didn’t work, so I’m now waiting on a new power supply for that machine. I ended up trying the first laser tube in my other k40 machine and it worked great for about a week and then stopped firing in the middle of a job, just like it did for the other machine. Any ideas?

    2018-06-11 at 22:46
    • HP Persson Reply

      Never heard of it, but it´s a common problem when the flyback dies in the PSU´s. Too warm in the psu compartment, tap water or bad coolant, too hot coolant can do that to the flyback. When you get the new PSU, make sure to upgrade it. Article is on this website 😉

      2018-06-11 at 22:56
  • bob Reply

    i heve this problem can you help me please
    see the link

    2018-09-22 at 12:37
    • HP Persson Reply

      Check all wires to and from the tube so they are seated properly and no damages. Also check the ground bolt, i have a article about it. If that doesnt help, its either the psu or the tube broken.

      2018-09-22 at 12:53
      • bob Reply

        Hello my friend
        they sent me a new power supply but is not easy to install
        can you please help me

        2018-10-08 at 14:40
        • HP Persson Reply

          Visit the facebook-group and search för HY-T50 to find schematics how to connect it. Link to the group can be found in the bottom of this page.

          2018-10-08 at 14:43
      • Alessandro Reply

        Hi HP, hi everyone, I’m Alex from Italy and since yesterday proud owner of not one but 2 k40 lasers :).

        I bought them used (well, 1 used 1 still in the shipping box, untouched) from a seller that works with wood but he prefers cutting wood by hand.

        I’m writing here because I cannot solve my problem: the power supply in both k40 was disconnected and the red wire cut, so i managed do reconnect everything but the laser doesn’t fire.

        Here is my step-by-step:

        1) water pump with tap water connected and on
        1.1) ground connected
        2) everything powered on
        3) laser head moves correctly using program
        4) laser doesn’t fire but, the strangest thing is that even the amperometer doesn’t power on: I have the older analog-amp version and the red, stop-button, digital power meter k40 but both doesn’t show life signs.

        Any idea?

        Thanks a lot

        2019-06-30 at 06:42
        • HP Persson Reply

          You can try with adding a small cable between GND and P+, see picture here (pin 5 and 6) and then test the machine again. Remove any other cables connected to 5 and 6 before testing. Picture:

          2019-07-01 at 00:09
  • Juan Reply

    Hi, i upgrade my K40 Láser PSU, and the Newone has diferent pin layout. Anyways i figure ir out how to connect all. BUT 🙂 … Láser dont work when i try to cut/engrave.
    If i press the PSU test switch it works perfect. If i press the láser panel test button it works perfect. Any Clue?



    2018-11-12 at 23:54
    • HP Persson Reply

      Make sure you have WP pin to ground, or connected to a flow sensor. It can also be marked as P, WP, Water protect and similar. Also important with the L cable from the controller board to the PSU, often marked L, LO or TH

      2018-11-13 at 08:56
  • Juan Reply

    Hi HP, thanks for the Fast Reply. I found the problem. Im using 2’PSU 1(Laser PSU) 2 (Switching PSU) all MUST share Ground. That was the problem in case someone has the same issue. Thanks for all the info you Give here every day !!

    Best Regards

    2018-11-13 at 13:03
  • t0r3r0s Reply

    greeatings i have problem with power supply on k40 laser .
    when i try test i have some voltage on my cable on bobine but it isnt big and cant tube work.
    is problem with bobine or something else ???

    2018-11-25 at 17:58
    • HP Persson Reply

      Can be broken tube or broken power supply, hard to tell – or even bad ground.

      2018-11-25 at 18:01
      • t0r3r0s Reply

        We replace tube, but same problem, i only cant test My bobine i have resistance between Black an yellow wire anything between red wire is it ok?

        2018-11-26 at 10:49
  • Jesse Reply

    Hey again! I have a quick (and probably simple) question when you have a minute. I’d like to upgrade to a 50 watt tube, so do you know if I also need to upgrade my power supply for this to work properly? Thanks for your time!

    2018-12-26 at 23:31
    • HP Persson Reply

      You can upgrade the laser tube only, but you will not be able to use the full 50w without a PSU upgrade. It will work though.

      2018-12-27 at 02:42
      • Jesse Meier Reply

        Thanks for your response! So I would get a little more power if I just upgraded the tube? Do you think it would be a marginal difference or worth it in your opinion? Definitely not looking to spend more money if there won’t be a decent difference.

        2018-12-29 at 20:57
        • HP Persson Reply

          The 40w tubes are about 30-32w real watts. The 50´s are actually closer to 40-45w depending on brand and tube quality/health. You will see a small increase of power for sure. But be aware runnung the PSU on the edge of what it can do will cause stress and strange things. Not damaging the tube though, but the PSU might go up in smoke.

          2019-01-02 at 03:19
  • Joseph Perreault Reply

    New k40, digital type, laser looses ALL power during short use time. Unplug plug physical ac plug and power restored. Happens randomly, low or high power same thing. Any idears, it just completely dies and switching of then on nothing, need to remove the AC plug to restore.

    2018-12-28 at 05:15
    • HP Persson Reply

      Sounds to me the PSU is overheating or has some kind of internal fault so it shuts down itself. Check for shorts or loose cables, and double check the ground wire.

      2018-12-28 at 18:39
  • John Reply

    Great stuff here! I have been reading trying to diagnose an issue I have. I think my PSU has failed as I have no response from either test button and no glow or response of any kind from the tube. I am not a fan of arcing the HVT to see if I’m getting high voltage to the tube. Everything seems to pin out power wise, potentiometer read correct, and switches and fuse are all good. Can I safely assume the PSU has failed since I am getting no response from the tube?

    2019-02-21 at 02:09
    • HP Persson Reply

      It´s 50-50%, it´s pretty rare with a PSU going totally dead, most of them get stuck at 2-3-4mA when the flyback burns. If your laser machine homes to 0x0, moving the steppers to zero position but cannot fire the tube i would test with the PSU first. But it´s hard to tell if its the tube or PSU actually. PSU is cheaper to swap though.

      2019-02-21 at 06:14
  • UHF Reply

    No fire issue – SOLVED

    I’m posting this to help others with the same problem. I moved from a pc to a laptop and the laser cutter no longer fired. It would fire when the test button was pressed. I discovered that the USB outer metal shield needs grounding (yes, another grounding problem). A pc has it’s USB connectors grounded through the mains, a laptop (usually) doesn’t. If your k40 has this issue, try grounding the outside of the USB connector – this could be modified on the k40.

    2019-03-07 at 02:05
    • Gabrijel Reply


      was your experience like this:
      1. started a job from laptop
      2. laser was working and suddenly stops. Repetier shows the machine connected but can’t communicate, Only thing that helps was unplug and plug back USB cable. Even disconnect and connect on Repetier doesn’t help.
      I also have this behavior:
      1. power on the machine and connect USB to my laptop
      2. connect Repetier
      3. fire a couple of “Test” fires on laser panel
      4. Repetier “connected” but just stacking commands. If I just try to disconnect and connect, error shows up saying that it is unable to communicate. Only re-plugging USB cable helped again.


      2019-03-18 at 15:15
      • HP Persson Reply

        As someone said in a earlier comment, if my tips in the article doesnt work – check your USB ground.

        2019-03-18 at 23:09
        • Gabrijel Reply


          shortly: connect GND from controller board to the casing of the engraver where also power plug “GROUNDING” is?


          2019-03-19 at 14:09
          • Gabrijel

            …tried to connect GND from the controller board to the casing… No luck:( Still getting communication problems. I also noticed squealing sound from my LPS and sometimes even popping sound (after I release “Test” button. The mentioned sound is also present when “Test” button is pressed). As long as I can tell, the laser power is OK and ammeter shows increase and decrease of mA if I turn the knob. I also tried “full power” and the mA was around 23mA. I really don’t know what to try next and what might be wrong…

            2019-03-19 at 23:33
  • Vidmo Reply

    Thanks for this guide. It helped me a lot.
    Problem started lke that: after 1 or 2 seconds of cutting laser went off. I powered off-on, then test shoot with the same result. Few times like that. I blamed PSU: broken fly-back or something like that. But after some time I figured out, that wobbling cables with pressed test fire starts laser.
    First thing I found was broken test button on PSU (I’ll have to change that). Then I found, that wires in ammeter’s connectors are oxidized and a little bit loose (I soldered them), then potentiometer connectors just slipped off (I soldered wires directly to pot). And finally I found, that crimp sleeves on PSU’s wires are loose (I also soldered wires’ endings and thrown away that useless junk).
    Wiring job in my K40 was shit.

    BTW. some PCs’ PSUs have quite high voltage: ca. 150V as ground potential when ungrounded. So never connect ungrounded PC to something, that is grounded, unless you want to fry some stuff…

    2019-04-17 at 22:53
  • Abraham Rodriguez Reply

    I have an issue with my Chinese 50w laser. Bought it brand new worked fine and while making a large order midway the laser working on a “job” it stopped firing but kept running the job. I was engraving at the time. Also when it stopped the red dot laser stopped working as well? I checked the board that goes to the red dot laser and noticed that one of the electronics that has the cool fins was hotter than normal and the red led light was not on anymore.

    I’ve ordered a new board but the waiting sucks. Any ideas to trouble shoot this issue?

    Test fire works, water sensor is good.
    Just doesn’t fire when job is running.

    Thank you,

    2019-04-18 at 04:31

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