Homing or the leveling probe fails (endstop, BLTouch/CRTouch, inductive or load cell)

Symptom

When homing, an axis keeps pushing against the end, stops too early, or the printer shows a homing or bed probing error. With a probe, leveling fails, the probe blinks or won't deploy its pin, or the nozzle touches the bed without the probe triggering.

Likely causes, from most to least likely

1. An endstop or probe with a loose connector or a damaged cable.
2. The probe is in an error state or stuck, for example a BLTouch blinking red or with its pin stuck down.
3. Firmware configuration that doesn't match the probe or the wiring: inverted logic, wrong pin or missing pull-up. Some compatible probes (clones) don't behave like the original.
4. Filament residue on the nozzle or bed, or a badly seated sheet, skewing the measurement (especially when the nozzle itself does the probing, as with a load cell).
5. A very tilted bed or a wrong probe Z offset: the probe may go lower than allowed without triggering (in Marlin, a “Probing Failed” error) or trigger earlier than expected.

Checks to tell which one it is

• Check the state of the endstops with nothing touching them: in Marlin with M119 and in Klipper with QUERY_ENDSTOPS. They should read open and, when pressed by hand, TRIGGERED.
• If one reads backwards (TRIGGERED without touching it, or open when pressed), the wiring or the configured logic is wrong; if it never changes, it's usually on another pin or disconnected.
• BLTouch on Klipper: with BLTOUCH_DEBUG COMMAND=pin_down and pin_up, the pin should go down and up; with touch_mode and QUERY_PROBE, it should change from open to TRIGGERED when you push the pin with a fingernail.
• BLTouch on Marlin: if M119 always shows the probe as TRIGGERED and it won't go down to probe, deploy it with M401 and check M119 again to review wiring and configuration.
• Look at the nozzle tip and the bed: no filament residue, and the sheet flat and well seated.

Before you start

Before taking anything apart, cleaning or adjusting it, switch the printer off and unplug it, unless a step says otherwise, and do not touch the nozzle, the heater block or the bed until they have cooled down. If in any doubt about electricity, heat or moving parts, stop and ask a professional.

Step-by-step fix

1. Turn off and check connectors and cables

   Turn off and unplug the printer. Check that the endstop or probe connectors are fully seated at both ends and that the cable isn't stripped, pinched or pulled tight anywhere along its path.

   Check: M119 or QUERY_ENDSTOPS change from open to TRIGGERED when triggered by hand.

   If that does not fix it: Test the probe carefully.

2. Test the probe before letting it reach the bed

   Move the print head away from the bed, start a homing or probing move and trigger the probe with your fingernail before it gets there: the motion must stop. Keep a hand near the power switch in case it doesn't. Don't touch the BLTouch pin with your fingers: skin grease affects it.

   Check: The printer stops when the probe triggers and homing completes.

   If that does not fix it: Clear the probe's error state.

3. Clear a BLTouch error state

   If the red light blinks or the pin stays down, in Klipper send BLTOUCH_DEBUG COMMAND=reset. If it's a clone and you get “BLTouch failed to verify sensor state” or “BLTouch failed to raise probe”, the Klipper documentation explains which option to try (pin_up_touch_mode_reports_triggered or pin_up_reports_not_triggered), always after testing the probe by hand and never on a genuine BLTouch.

   Check: The pin goes up and down normally and homing gives no error.

   If that does not fix it: If the error persists, contact the probe maker or replace the probe.

4. Fix the logic in the configuration

   If you built or configured the firmware yourself: in Klipper, add or remove “!” in endstop_pin when the state is inverted and check the “^” (pull-up) if it never changes; run RESTART after each change. In Marlin, check that the configured probe pin (Z_MIN_PIN or Z_MIN_PROBE_PIN) matches your wiring.

   Check: The state changes correctly when triggered by hand.

   If that does not fix it: Check the wiring against your board's diagram.

5. Clean the nozzle and bed

   Remove filament residue from the nozzle (careful: it may be hot) and from the bed, and seat the sheet properly. If you use a custom start G-code, make sure it doesn't interfere with the firmware's probing.

   Check: Bed probing completes without errors.

   If that does not fix it: Check the probe offset and the leveling.

6. Adjust the probe offset and level the bed

   If the probe fails while probing the bed, check that its Z offset is correct and, if the bed is very tilted, level it by hand first so the probe triggers within the allowed travel.

   Check: Leveling finishes without warnings and the first layer is at the right height.

   If that does not fix it: Contact your manufacturer's support.

Prevention

• Don't touch the BLTouch pin with your fingers and keep the probe clean.
• After changing or moving the probe, calibrate its offsets again.
• Leave slack in the cables going to the print head and keep the nozzle clean before each print.

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Printers it applies to

Any FDM (filament) 3D printer, from any brand.

Official sources

This guide summarises the official documentation linked here:

• Configuration checks (Verify endstops) · Klipper
• BL-Touch · Klipper
• Troubleshooting Tips (G29 Probing Failed; Won't Probe) · Marlin
• Probing failed · Prusa Research