Laser works in manual mode, won't fire when processing Gcode

I need some help.
I’ve read the other posts and have made all the recommend adjustments, but I just can’t get the laser to fire when running a Gcode job.
Works just fine when I flip to manual mode - burns the test wood all day long.

I created the job in LightBurn.
I’ve set all the variables based on what I’ve read here, Tool Type PWM Spindle with Rapid Auto Off and Dynamic Power Off.

Verified LightBurn Basic Device settings is set to “enable Laser Fire Button” is green, (on)
General Light Burn settings (Gear) beinner mode is off, Better for Diode = Inches/min

The Device is a JTech laser Pro, is set as GRBL-M3 1.1e or earlier.

It may be telling that when I manual enter M3 S10 then press Play, on the manual code entry screen it also DOES NOT turn on the laser.

It’s as if the laser just never gets the commands.

I’ve even tried removing the laser extension cables, Still no luck.

Please help what am I missing?

Hi, I had the same problem, when I changed LightBurn to GRBL it did work.


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I have the exact same issue and I’ve followed the lightburn AND Inkscape tutorials through jtech and still can’t manually or gcode turn on the laser. If I push the physical button to test it turns on full power…

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I found the cable between the main controller and the laser to be bad. It’s just a two conductor wire. I tested it with an ohm meter and one of the two wires was broken. Buggered it together and it’s working, Will need a new cable.

I am using Vectric VCarve Pro and I am having the same problem. With the small switch all the way to the left it works but the laser is always on and leaves a trail when it skips to another vector. With the switch all the way to the right ( which the install video says will only fire with the G code) nothing happens.

Having the same issue. The moving mode selector switch to left will fire the laser (at full power), confirming that the laser works. Trying to send gcode (M3 S10) through the MDI interface does not turn on the laser.

Confirmed the Tool settings match what was shown on

Checked cable with multimeter, both side sof the wire have good connection through the connectors.

Glad I’m not the only one with the issue, but eager to understand the problem.

I’m getting the exact same issue. Did anyone figure out the problem. I’ve checked my wires and I have continuity.

I had the same problem and ordered new cables from J TECH (5 ft instead of the 10 ft that came with the laser). Customer service at J TECH overnighted a new control module. The new cables and the New controller arrived the same day. I thought I would install the new cables first to see if that could be the problem before installing the new controller. I discovered that the cable going from my Onefinity controller to the J TECH controller had not “seated” correctly. When I reseated the cable every thing worked fine. J TECH’s customer service is fantastic. I shipped the new controller back to J TECH with expressions of gratitude for their great customer service. My problem had been “user error”.

I’m not having any issues running gcode to it using vcarve pro. Make sure the tool type is set to “PWM” and that you’re using the post processor for LASER. The laser tool should be setup as described here.

Hopefully this helps.


Update for my situation:

  1. I can actually burn using lightburn output either GRBL or GRBL 1.1 output, but could still not turn laser on using the M3 S10 strategy - meaning can’t accurately locate the images to be etched.
  2. The mode selector does cause laser to fire (at 100%).
  3. However, based on The M3 S10 XY problem - #2 by rblur01, if I turn machine off at the control box, turn it back on, but make sure to cancel the homing upon startup, the M3 S10 does indeed fire the laser at low power. This not homing trick seems to work for me. Might be worth trying for others who run into this.

Quick question is the value of max-spin set to 255 when your test fail?


I have a spindle and have to alternate from 24000 and 255 in order to have the PWM work correctly. Sometime I forget to set it and that the reason why PWM fail for me.

Yup (for me at least), max-spin is set to 255.

So here is what I have done so far to make it work. I followed this YouTube video to create your own tool for using the laser and was able to make it work.

If I use the laser tool provided by JTech I can’t get the laser to work. My guess now is that I have something set up wrong on my tool.

I’m using the same Post processor Onefinity laser .gcode for both

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Update… so I reached out to JTech regarding this issue and this is what I was told. Hope it help anyone still trying to figure this out.

Thanks for sending this over. We have found a bug in the vectric software that is not allowing the post processor to work in the current state for the laser module plugin. It works on the pocket and the quick engrave, but not the laser cut and fill. We are working on a solution now and will get back to you with the new post processor asap. Thanks for pointing this out to us and sorry for the inconvenience.

It sound weird to me that it can be related to the vetric post processor if people experience it also via manual entry in MDI panel.

I think there are two different issues here.

  1. Not being able to fire in the MDI screen. This issue can be a bunch of different things.
    a. The S value needs to be determined for the laser. It might be M3 S10, M3 S15, M3 S20, that turns it on. You need to go through to see which value works for you.
    b. You have to not home the machine to get it to fire in low power mode. If you home, it just doesn’t work. I believe this is in the bug fix list but I am not sure about if the newest version of firmware fixed it.
    c. The cable might not be seated correctly or it is bad. If you have 3.3V at the input of terminal H2 at the laser driver, then the driver can be bad. I would reach out to us at J Tech if you suspect something wrong with the driver. If you give a M3 S255 and you see 3.3V at the laser driver, then this can be the issue.

  2. Laser not firing on a program using the Vectric “laser module” plugin and the “laser cut and fill” toolpath. This is something we have seen not working on some customers depending on which version of the laser module you have. We have tested it here and are working on a modified post processor to fix this bug. It will depend on which version of vectric if it works or doesn’t work.

Don’t mean to hijack this thread with my own problems, but I’m having similar problems to where this thread started. Fan works, laser fires in manual (via the physical switch), but won’t fire on M3 S10, M3 S15, M3 S20, or M3 S255. I’m not homing on start-up, and tool is set to PWM (tried dynamic power on and off). I do not get 3.3v on H2 of the laser driver. I have tried all options with and without the extension cable (even though continuity check says the extension cable is good). Any other ideas?

did you ever figure out what was going on with your laser? I am have this same issue right now. I used it the other day and now it won’t work unless I manually switch it on.

Hey Eric,

have you tried this:

Welcome to the forum!

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Thank you for the info. I have not tried this yet but i am using Inkscape. Does this matter? Also is it weird that i ran a laser image program 2 days ago, it worked, but the same program doesn’t today?

And also thanks for the welcome.

I am usually just a lurker and reader then go do. I don’t typically have too many problems i can’t figure out. This one had me stumped.