"MOT ERR" — Troubleshooting Guide

"MOT ERR" appears for a specific motor when the STM control board cannot confirm that motor's movement. It is shown in two situations:

• The STM control board is not receiving a signal from that motor's encoder (sensor).
• The encoder readings do not match the movement the board expects — the motor is jammed, slowed down, or not turning.

Because of this, the cause is one of four things:

• the encoder inside the motor,
• the Cytron motor driver for that port,
• the STM control board inside the control box,
• or the simulator geometry/balance — too much load on the motor (binding, an overweight or off-center seat) can stop the motor from reaching its position and trigger the error.

The steps below help you find which one. Please go through them in order and, after each step, reply in your ticket with what you observed.

Step 1 — Run calibration with the rods removed (rule out load/geometry)

Disconnect all rods from the SFU arms so the motors carry no load from the moving part of the simulator, then power-cycle the platform to trigger a fresh calibration.

If the error disappears without load → the cause is on the mechanical side: binding, balance, or geometry (an overweight or off-center seat, a stiff joint). Re-check balance and free movement before reconnecting the rods.

If the error still appears with the rods removed → it is electrical/component-related. Continue to Step 2.

Step 2 — Motor swap test (is it the motor or the port?)

Swap the motor that shows "MOT ERR" with another motor that works without an error. Full guide: How to conduct the motor swap test

If the error follows the motor (it moves to the new port together with that motor) → the problem is the encoder inside that motor. Go to Step 4.

If the error stays on the same port (any motor on that port shows the error) → the problem is the Cytron driver or the STM board on that port. Go to Step 3.

Step 3 — Cytron driver test (for the affected port)

Test the Cytron MD13S driver of the affected port by pressing its MA / MB buttons. Full guide: Test Cytron MD13S 

Short version: open the control box, locate the Cytron board for that motor, then press MA and MB.

If the motor rotates in both directions → the Cytron driver is most likely fine. The issue is probably the encoder or the STM control board. Continue to Step 4.

If the motor does not rotate → the issue is most likely the Cytron driver or the motor itself.

Also compare the LEDs: the LEDs on the tested Cytron board should light up and behave the same way as on a neighboring, working Cytron board. A different LED pattern points to a fault on that board or its power.

Step 4 — Check the encoder in Monitor / SMC mode

The most common cause of "MOT ERR" is a broken encoder (sensor) wire.

1. Connect the motor to the port and power on the platform.
2. Launch Monitor / SMC.
3. Slowly turn the motor shaft by hand. Reference video: turning the shaft by hand.
4. Watch the green Feedback line on the SMC graph:
   • It moves smoothly → the encoder is working on that port.
   • It does not move → the encoder signal is not reaching the board.

If the line does not move:

• Inspect the encoder wiring for a broken wire (the most common fault) and try to locate the break visually.
• Open the motor cover and check the encoder disk. If it has shifted or is damaged, carefully reposition it, or replace it. Replacement video: replacing the encoder disk.

Still not resolved?

Send us a screenshot of the firmware / configuration and a calibration video:

1. Connect the platform to a PC and open https://dofreality.com/tools/
2. Send a screenshot of the firmware / configuration.
3. Record a video of the startup and calibration with the rods removed from the SFU arms, keeping the camera focused on the SFU screws.

Reply in your ticket with your result at each step, and we will guide you to the exact part that needs attention.