Skip to content
Sign in
Contact us!
  • There are no suggestions because the search field is empty.

"Contactor contacts are fused together" error on the GP20?

A diagnostic guide for identifying contactor, feedback, or control board issues.

Inside your GP20, a contactor acts as an electronically controlled safety switch for the motor. Under normal operation, the firmware closes this switch to provide power. However, for safety, the firmware will open this switch if an error occurs or if the Emergency Stop is pressed.

When the machine boots up, it performs a self-test to ensure the contactor isn't "fused" (permanently stuck in the closed position). If the system detects power reaching the motor driver when it shouldn't, it triggers this error to prevent the motor from running unexpectedly.

Note: These steps are only valid if the error appears immediately upon powering on the machine. If the error is intermittent or does not appear at startup, contact 3devo Support before proceeding.

Step 1: Generate a Data Log

Before opening the machine, we need to see what the firmware is reporting.

  1. Connect a computer to the machine’s USB port (use the port on the specific part,top or bottom showing the error).

  2. Open a serial terminal program (e.g. arduino-1.8.19-windows.exe).

  3. Power on the machine. This should trigger a restart and capture the boot sequence.

  4. Save the output as a .txt file.


Step 2: Internal Inspection

To diagnose the hardware, you will need to access the internal components.

  1. Power off the machine and unplug it from the wall.

  2. Open the case of the affected machine part.


Step 3: Check the Motor Driver (Invt)

  1. Plug the machine back in and power it on (be extremely careful with the case open).

  2. Locate the motor driver ,a black box labeled "invt" with a small digital display.

  3. Check the Invt display:

    • Display is ON: Power is successfully bypassing the contactor. The issue is likely with the contactor hardware or the control system.

    • Display is OFF: No power is reaching the driver. The issue is likely a "false positive" caused by the feedback system (the wires that tell the brain if the switch is closed)

Step 4: Isolation Test

  1. Power off and disconnect the machine.

  2. Locate the 4-wire cable (typically red-black-blue-white) labeled "Contactor" on the control board.

  3. Unplug this connector from the board, leaving the cable loose.

  4. Power the machine back on and observe:

    • Does the Invt display still turn on?

    • Does the machine display show the same error, or a different one?

Step 5: Multimeter Continuity Check (Optional)

If you have a multimeter, you can confirm a hardware failure:

  1. Ensure the machine is powered off and the contactor cable is disconnected.

  2. Set your multimeter to Continuity or Resistance  mode.

  3. Test the connection between contacts 13 and 14 on the contactor.

    • Continuity/Low Resistance: The contactor is physically fused and must be replaced.

    • No Continuity: The contactor is mechanically fine, suggesting a control or feedback error.

Next Steps

Once you have completed these checks, please send your Data Log and your findings from Steps 3, 4, and 5 to the 3devo Support team. This will allow us to ship the correct replacement part immediately.