Connecting BL-Touch

A warning before you start: Avoid touching the BL-Touch pin with your bare fingers, since it is quite sensitive to finger grease. And if you do touch it, be very gentle, in order to not bend or push anything.

Hook up the BL-Touch “servo” connector to a control_pin according to the BL-Touch documentation or your MCU documentation. Using the original wiring, the yellow wire from the triple is the control_pin and the white wire from the pair is the sensor_pin. You need to configure these pins according to your wiring. For example:

sensor_pin: P1.24
control_pin: P1.26

If the BL-Touch will be used to home the Z axis then set endstop_pin: probe:z_virtual_endstop in the [stepper_z] config section and add a [homing_override] config section to raise the z-axis, home the x/y-axis, move to the center of the bed, and home the z-axis. For example:

    G90 ; Use absolute position mode
    G1 Z10 ; Move up 10mm
    G28 X Y
    G1 X166 Y120 F6000 ; Change the X and Y coordinates to the center of your print bed
    G28 Z
set_position_z: 0.0

It’s important that the initial Z upwards movement in the homing_override is high enough that the probe doesn’t hit anything even if the probe pin happens to be in its lowest state.

Initial tests

Before moving on, verify that the BL-Touch is mounted at the correct height, the pin should be roughly 2 mm above the nozzle when retracted

When you turn on the printer, the BL-Touch probe should perform a self-test and move the pin up and down a couple of times. Once the self-test is completed, the pin should be retracted and the red LED on the probe should be lit. If there are any errors, for example the probe is flashing red or the pin is down instead of up, please turn off the printer and check the wiring and configuration.

If the above is looking good, it’s time to test that the probe responds to commands from the firmware. First run BLTOUCH_DEBUG COMMAND=pin_down in your printer terminal. Verify that the pin moves down, and that the red LED on the probe turns off. If not, check your wiring and configuration again. Next issue a BLTOUCH_DEBUG COMMAND=pin_up, verify that the pin moves up, and that the red light turns on again. If it’s flashing then there’s some problem.

Now, it’s time to test homing with a twist. Instead of letting the probe pin touch the print bed, let it touch the nail on your finger. So issue a G28, wait until it starts to move down, and stop the movement by very gently touching the pin with your nail. You probably have to do it twice, since the default configuration makes it probe twice. But be prepared to turn off the printer, to avoid damage, if it doesn’t stop when you touch the pin.

If that was successful, do another G28 but this time let it touch the bed as it should.

Calibrating the BL-Touch offsets

Follow the directions in the Probe Calibrate guide to set the x_offset, y_offset, and z_offset config parameters.

It’s a good idea to verify that the Z offset is close to 1mm. If not, then you probably want to move the probe up or down to fix this. You want it to trigger well before the nozzle hits the bed, so that possible stuck filament or a warped bed doesn’t affect any probing action. But at the same time, you want the retracted position to be as far above the nozzle as possible to avoid it touching printed parts. If an adjustment is made to the probe position, then rerun the probe calibration steps.

BL-Touch gone bad

Once the BL-Touch is in inconsistent state, it starts blinking red. You can force it to leave that state by issuing:


This may happen if its calibration is interrupted by the probe being blocked from being extracted.

However, the BL-Touch may also not be able to calibrate itself anymore. This happens if the screw on its top is in the wrong position or the magnetic core inside the probe pin has moved. If it has moved up so that it sticks to the screw, it may not be able to lower its pin anymore. With this behavior you need to open the screw and use a ball-point pen to push it gently back into place. Re-Insert the pin into the BL-Touch so that it falls into the extracted position. Carefully readjust the headless screw into place. You need to find the right position so it is able to lower and raise the pin and the red light turns on and of. Use the reset, pin_up and pin_down commands to achieve this.


  • If you are sure the wiring of the BL-Touch is correct and every attempt to probe with the BL-Touch reports “BLTouch failed to verify sensor state” then it may be necessary to add pin_up_touch_mode_reports_triggered: False to the bltouch config section. The BL-Touch v3 and many clones require this setting.

  • A BL-Touch v3 may not work correctly when its signal wire is connected to the Z end-stop pin on some printer boards. The symptoms of this problem are: the BL-Touch probe deploys, the printer descends, the probe contacts a surface, the BL-Touch raises the probe, the BL-Touch does not successfully notify the micro-controller, and the printer continues to descend. The Z end-stop pin on some printer boards have a capacitor to filter the signal which the BL-Touch v3 may not support. The simplest solution is to connect the BL-Touch v3 sensor wire to an available pin on the printer board that is not associated with an end-stop (and thus is unlikely to have a capacitor). An alternative solution is to physically alter the printer board to disable the given end-stop capacitor or to add a hardware “pull up resistor” to the BL-Touch v3 sensor wire.