[Bug] EV3 Motor Detection is incomplete

[laurensvalk]

^{Originally posted by @KhuongAnNguyen January 6, 2026}

Hello. I am using the nightly build ev3-firmware-4563 (30-December-2025) on the latest Raspberry Pi Trixie OS.

The program compiled and ran; but sometimes gave random OSError when controlling the medium EV3 servo. For example:

1. OSError: [Errno 1] EPERM: The requested operation is not valid in the current state.
   motor.run_target(speed=1000, target_angle=-150*2, wait=True)
   print(motor.angle())
   wait(1000)
   motor.run_target(speed=1000, target_angle=0, wait=True) # This line triggers the error
2. OSError: [Errno 19] ENODEV: A sensor or motor is not connected to the specified port.
   ev3 = EV3Brick()
   motor = Motor(Port.D) # This line triggers the error
   I double-checked the cable, changed the port. Same issue from time to time.
   If I add "wait(200)" before the motor initialisation, the error seemed to appear less frequently.

What was really strange is that the exact same code ran 50% of the time, but randomly gave up with the above error.

Please help advise me. Thanks.

[laurensvalk]

Thank you for trying our new EV3 version early!

Automatic motor detection still needs some work, so this isn't entirely unexpected.

In this case, ENODEV means that it did not see the device when initializing it, whereas EPERM happens when it no longer sees it in the middle of an operation (even if it did see it before).

[BertLindeman]

Probably related
Running the robot arm h25 program on EV3 runs mostly OK.
But after running sometime, it gets:

Traceback (most recent call last):
  File "exampledoc_robotarm.py", line 134, in <module>
  File "exampledoc_robotarm.py", line 106, in robot_release
OSError: [Errno 1] EPERM: 

The requested operation is not valid in the current state:
--> Check the documentation for required conditions.
--> Check the line in your script that matches
    the line number given in the 'Traceback' above.

Source line where the error occurs:

    elbow_motor.run_target(60, -90)  # was -40)

Remark
The elbow motor in my case is an NXT motor not an EV3 motor.

Robot arm H25 program as it ran:

#!/usr/bin/env pybricks-micropython

"""
Example LEGO® MINDSTORMS® EV3 Robot Arm Program
-----------------------------------------------

This program requires LEGO® EV3 MicroPython v2.0.
Download: https://education.lego.com/en-us/support/mindstorms-ev3/python-for-ev3

Building instructions can be found at:
https://education.lego.com/en-us/support/mindstorms-ev3/building-instructions#building-core
"""

from pybricks.hubs import EV3Brick
from pybricks.ev3devices import Motor, TouchSensor, ColorSensor
from pybricks.parameters import Port, Stop, Direction
from pybricks.tools import wait

# Initialize the EV3 Brick
ev3 = EV3Brick()

# Configure the gripper motor on Port A with default settings.
gripper_motor = Motor(Port.A)

# Configure the elbow motor. It has an 8-teeth and a 40-teeth gear
# connected to it. We would like positive speed values to make the
# arm go upward. This corresponds to counterclockwise rotation
# of the motor.
elbow_motor = Motor(Port.B, Direction.COUNTERCLOCKWISE, [8, 40])

# Configure the motor that rotates the base. It has a 12-teeth and a
# 36-teeth gear connected to it. We would like positive speed values
# to make the arm go away from the Touch Sensor. This corresponds
# to counterclockwise rotation of the motor.
base_motor = Motor(Port.C, Direction.COUNTERCLOCKWISE, [12, 36])

# Limit the elbow and base accelerations. This results in
# very smooth motion. Like an industrial robot.
elbow_motor.control.limits(speed=60, acceleration=120)
base_motor.control.limits(speed=60, acceleration=120)

# Set up the Touch Sensor. It acts as an end-switch in the base
# of the robot arm. It defines the starting point of the base.
base_switch = TouchSensor(Port.S1)

# Set up the Color Sensor. This sensor detects when the elbow
# is in the starting position. This is when the sensor sees the
# white beam up close.
elbow_sensor = ColorSensor(Port.S3)

# Initialize the elbow. First make it go down for one second.
# Then make it go upwards slowly (15 degrees per second) until
# the Color Sensor detects the white beam. Then reset the motor
# angle to make this the zero point. Finally, hold the motor
# in place so it does not move.
elbow_motor.run_time(-30, 1000)
elbow_motor.run(15)
while elbow_sensor.reflection() < 32:
    wait(10)
elbow_motor.reset_angle(0)
elbow_motor.hold()

# Initialize the base. First rotate it until the Touch Sensor
# in the base is pressed. Reset the motor angle to make this
# the zero point. Then hold the motor in place so it does not move.
base_motor.run(-60)
while not base_switch.pressed():
    wait(10)
base_motor.reset_angle(0)
base_motor.hold()

# Initialize the gripper. First rotate the motor until it stalls.
# Stalling means that it cannot move any further. This position
# corresponds to the closed position. Then rotate the motor
# by 90 degrees such that the gripper is open.
gripper_motor.run_until_stalled(200, then=Stop.COAST, duty_limit=50)
gripper_motor.reset_angle(0)
gripper_motor.run_target(200, -90)


def robot_pick(position):
    # This function makes the robot base rotate to the indicated
    # position. There it lowers the elbow, closes the gripper, and
    # raises the elbow to pick up the object.

    # Rotate to the pick-up position.
    base_motor.run_target(60, position)
    # Lower the arm.
    # elbow_motor.run_target(60, -40)
    elbow_motor.run_target(60, -90)  # Pickup was -40 - too high 
    # Close the gripper to grab the wheel stack.
    gripper_motor.run_until_stalled(200, then=Stop.HOLD, duty_limit=50)
    # Raise the arm to lift the wheel stack.
    elbow_motor.run_target(60, 0)


gripper_open = -70
def robot_release(position):
    # This function makes the robot base rotate to the indicated
    # position. There it lowers the elbow, opens the gripper to
    # release the object. Then it raises its arm again.

    # Rotate to the drop-off position.
    base_motor.run_target(60, position)
    # Lower the arm to put the wheel stack on the ground.
    elbow_motor.run_target(60, -90)  # was -40)
    # Open the gripper to release the wheel stack.
    gripper_motor.run_target(200, gripper_open)
    # Raise the arm.
    elbow_motor.run_target(60, 0)


# Play three beeps to indicate that the initialization is complete.
for i in range(3):
    ev3.speaker.beep()
    wait(100)

# Define the three destinations for picking up and moving the wheel stacks.
LEFT = 160
MIDDLE = 100
RIGHT = 40

# This is the main part of the program. It is a loop that repeats endlessly.
#
# First, the robot moves the object on the left towards the middle.
# Second, the robot moves the object on the right towards the left.
# Finally, the robot moves the object that is now in the middle, to the right.
#
# Now we have a wheel stack on the left and on the right as before, but they
# have switched places. Then the loop repeats to do this over and over.
while True:
    # Move a wheel stack from the left to the middle.
    robot_pick(LEFT)
    robot_release(MIDDLE)

    # Move a wheel stack from the right to the left.
    robot_pick(RIGHT)
    robot_release(LEFT)

    # Move a wheel stack from the middle to the right.
    robot_pick(MIDDLE)
    robot_release(RIGHT)

log

Traceback (most recent call last):
  File "exampledoc_robotarm.py", line 134, in <module>
  File "exampledoc_robotarm.py", line 106, in robot_release
OSError: [Errno 1] EPERM: 

The requested operation is not valid in the current state:
--> Check the documentation for required conditions.
--> Check the line in your script that matches
    the line number given in the 'Traceback' above.

[laurensvalk]

This should be fixed. You can try it when the actions finish building.

Feel free to re-open when this issue happens again!

[BertLindeman]

Needs a robotarm rebuild to test.
Tomorrow is Friday, so maybe Saturday.

[BertLindeman]

Trying to install EV3 firmware from Build 4733 hash 9fff3ad8 did not work as the firmware is no zip file.
Renamed the downloaded file to *.zip and installed.

Current program (did not build the robot-arm, just connected the motors).

#!/usr/bin/env pybricks-micropython

"""
Example LEGO® MINDSTORMS® EV3 Robot Arm Program
-----------------------------------------------

This program requires LEGO® EV3 MicroPython v2.0.
Download: https://education.lego.com/en-us/support/mindstorms-ev3/python-for-ev3

Building instructions can be found at:
https://education.lego.com/en-us/support/mindstorms-ev3/building-instructions#building-core
"""

from pybricks.hubs import EV3Brick
from pybricks.ev3devices import Motor, TouchSensor, ColorSensor
from pybricks.parameters import Port, Stop, Direction
from pybricks.tools import wait
from pybricks import version
print(version)
# Initialize the EV3 Brick
ev3 = EV3Brick()

# Configure the gripper motor on Port A with default settings.
gripper_motor = Motor(Port.A)

# Configure the elbow motor. It has an 8-teeth and a 40-teeth gear
# connected to it. We would like positive speed values to make the
# arm go upward. This corresponds to counterclockwise rotation
# of the motor.
elbow_motor = Motor(Port.B, Direction.COUNTERCLOCKWISE, [8, 40])

# Configure the motor that rotates the base. It has a 12-teeth and a
# 36-teeth gear connected to it. We would like positive speed values
# to make the arm go away from the Touch Sensor. This corresponds
# to counterclockwise rotation of the motor.
base_motor = Motor(Port.C, Direction.COUNTERCLOCKWISE, [12, 36])

# Limit the elbow and base accelerations. This results in
# very smooth motion. Like an industrial robot.
elbow_motor.control.limits(speed=60, acceleration=120)
# base_motor.control.limits(speed=60, acceleration=120)

# Set up the Touch Sensor. It acts as an end-switch in the base
# of the robot arm. It defines the starting point of the base.
# base_switch = TouchSensor(Port.S1)

# Set up the Color Sensor. This sensor detects when the elbow
# is in the starting position. This is when the sensor sees the
# white beam up close.
# elbow_sensor = ColorSensor(Port.S3)

# Initialize the elbow. First make it go down for one second.
# Then make it go upwards slowly (15 degrees per second) until
# the Color Sensor detects the white beam. Then reset the motor
# angle to make this the zero point. Finally, hold the motor
# in place so it does not move.
elbow_motor.run_time(-30, 1000)
elbow_motor.run(15)
# while elbow_sensor.reflection() < 32:
#     wait(10)
elbow_motor.reset_angle(0)
elbow_motor.hold()

# Initialize the base. First rotate it until the Touch Sensor
# in the base is pressed. Reset the motor angle to make this
# the zero point. Then hold the motor in place so it does not move.
base_motor.run(-60)
# while not base_switch.pressed():
#     wait(10)
base_motor.reset_angle(0)
base_motor.hold()

# Initialize the gripper. First rotate the motor until it stalls.
# Stalling means that it cannot move any further. This position
# corresponds to the closed position. Then rotate the motor
# by 90 degrees such that the gripper is open.
gripper_motor.run_until_stalled(200, then=Stop.COAST, duty_limit=50)
gripper_motor.reset_angle(0)
gripper_motor.run_target(200, -90)


def robot_pick(position):
    # This function makes the robot base rotate to the indicated
    # position. There it lowers the elbow, closes the gripper, and
    # raises the elbow to pick up the object.

    # Rotate to the pick-up position.
    base_motor.run_target(60, position)
    # Lower the arm.
    elbow_motor.run_target(60, -40)
    # Close the gripper to grab the wheel stack.
    # wait(2500)  # sim the time for the gripper
    gripper_motor.run_until_stalled(200, then=Stop.HOLD, duty_limit=50)
    # Raise the arm to lift the wheel stack.
    elbow_motor.run_target(60, 0)


def robot_release(position):
    # This function makes the robot base rotate to the indicated
    # position. There it lowers the elbow, opens the gripper to
    # release the object. Then it raises its arm again.

    # Rotate to the drop-off position.
    base_motor.run_target(60, position)
    # Lower the arm to put the wheel stack on the ground.
    elbow_motor.run_target(60, -40)
    # Open the gripper to release the wheel stack.
    # wait(2500)  # sim the time for the gripper
    gripper_motor.run_target(200, -90)
    # Raise the arm.
    elbow_motor.run_target(60, 0)


# Play three beeps to indicate that the initialization is complete.
for i in range(3):
    # ev3.speaker.beep()
    wait(100)

# Define the three destinations for picking up and moving the wheel stacks.
LEFT = 160
MIDDLE = 100
RIGHT = 40

# This is the main part of the program. It is a loop that repeats endlessly.
#
# First, the robot moves the object on the left towards the middle.
# Second, the robot moves the object on the right towards the left.
# Finally, the robot moves the object that is now in the middle, to the right.
#
# Now we have a wheel stack on the left and on the right as before, but they
# have switched places. Then the loop repeats to do this over and over.
while True:
    # Move a wheel stack from the left to the middle.
    robot_pick(LEFT)
    robot_release(MIDDLE)

    # Move a wheel stack from the right to the left.
    robot_pick(RIGHT)
    robot_release(LEFT)

    # Move a wheel stack from the middle to the right.
    robot_pick(MIDDLE)
    robot_release(RIGHT)

This time 3 NXT motors.

Running with this fix:

('ev3', '4.0.0b5', 'ci-build-4733-v4.0.0b5-51-g9fff3ad8 on 2026-02-19')
Traceback (most recent call last):
  File "issue_2536_EV3_MotorDetection_incomplete.py", line 134, in <module>
  File "issue_2536_EV3_MotorDetection_incomplete.py", line 93, in robot_pick
OSError: [Errno 1] EPERM: 

The requested operation is not valid in the current state:
--> Check the documentation for required conditions.
--> Check the line in your script that matches
    the line number given in the 'Traceback' above.

Not fixed I think.

To be sure trying current CI build.
It took longer (not minutes but a few rotations) and still:

('ev3', '4.0.0b6', 'ci-build-4739-v4.0.0b6-5-g44a4ef88 on 2026-02-20')
Traceback (most recent call last):
  File "issue_2536_EV3_MotorDetection_incomplete.py", line 143, in <module>
  File "issue_2536_EV3_MotorDetection_incomplete.py", line 104, in robot_release
OSError: [Errno 1] EPERM: 

The requested operation is not valid in the current state:
--> Check the documentation for required conditions.
--> Check the line in your script that matches
    the line number given in the 'Traceback' above.

Sometimes another motor is the culprit:

Traceback (most recent call last):
  File "issue_2536_EV3_MotorDetection_incomplete.py", line 142, in <module>
  File "issue_2536_EV3_MotorDetection_incomplete.py", line 88, in robot_pick
OSError: [Errno 1] EPERM: 

The requested operation is not valid in the current state:
--> Check the documentation for required conditions.
--> Check the line in your script that matches
    the line number given in the 'Traceback' above.

Did also see that the elbow motor was not recognized (ENODEV) and I needed to power-off/on the hub to get a running program again.

I am not authorized to re-open.