Recovery after a restart or power loss

A restart, power cut or emergency stop makes Klipper forget two things: where the toolhead is, and where the parts on the bed are. klipper-auto-eject keeps the second, and can recover the first without touching a part.

flowchart LR
    P["Printing"] -- "every 30 s" --> S[("save_variables<br/>part outlines + height")]
    S -- "restart / power loss" --> R["Restored at startup"]
    R --> G{"AUTO_EJECT_GUARD<br/>at next print start"}
    G -- "guard_action: check" --> H["Smart home<br/>X/Y, then Z on a free spot"]
    H --> V{"Probe where<br/>the parts were"}
    V -- "nothing there" --> C(["Record cleared,<br/>print continues"])
    V -- "a tall part" --> J["Eject and verify"] --> C
    V -- "a thin part" --> E(["Print stopped,<br/>take it off"])
    G -- "guard_action: error" --> E2(["Print stopped<br/>until AUTO_EJECT_CLEAR"])

    classDef ok fill:#2e7d32,stroke:#1b5e20,color:#ffffff
    classDef stop fill:#c62828,stroke:#8e0000,color:#ffffff
    class C ok
    class E,E2 stop

What is saved

While a print runs, a background check every 30 seconds stores in [save_variables]:

Key	Content
auto_eject_objects	Each part's bounding box, centre and three verification points
auto_eject_height	The highest Z the print has reached, so travel moves clear the parts

It writes only when something changes (new print, or the height grew by 0.5 mm or more), so the SD card sees a few dozen writes per print. After a successful eject both are cleared. A failed or interrupted eject keeps them.

The toolhead position itself is not saved. With the motors unpowered the bed or gantry can move, so a stored position could be wrong. Z is always measured again.

Smart homing: AUTO_EJECT HOME=1

1. G28 X Y: X and Y never touch the bed. Your homing macro's Z lift still applies.
2. Pick the probe spot furthest from every stored part (at least home_clearance, 20 mm), preferring the centre when several are equally free. The probe, the nozzle and the whole blade (blade_offset, blade_width) must all be that far from the parts, because a nozzle probe such as Tap triggers on whatever touches first.
3. Measure Z there with the probe, so a part is never mistaken for the bed.
4. Continue with the normal eject.

Requirements: Z homes with the probe (endstop_pin: probe:z_virtual_endstop, e.g. Tap, BLTouch, inductive probe) and [force_move] enable_force_move: True. For a Z switch that sits off the bed, set home_method: "g28". Otherwise smart homing refuses, because a normal homing move could land on a part.

The guard: AUTO_EJECT_GUARD

Put it first in the slicer's start G-code (slicer-setup.md):

AUTO_EJECT_GUARD
PRINT_START ...

If the printer still remembers parts that were never ejected (a print without AUTO_EJECT, a cancelled print, a restart):

guard_action	What happens
"check" (default)	Smart home, then probe where the parts were. Nothing there: the record is cleared and the print continues, nobody has to confirm anything. A tall part still there: it is pushed off and checked first. A thin part such as a stencil: the print stops; take it off and start again
"error"	The new print stops with a message until you run AUTO_EJECT_CLEAR. Nothing moves
"eject"	Smart home around the old parts, eject them, verify, then the print continues

"check" needs a probe that can touch plastic (Tap, BLTouch, inductive probes do; an eddy probe only with its touch mode in verify_probe_gcode). Without one, or with no free spot to home Z, it stops like "error".

Thin parts. A 0.4 mm stencil is easy to miss: the bed itself is rarely flat to 0.4 mm. For parts thinner than twice verify_threshold, the probe also touches the bare bed on two sides of the part and compares with the lower of the two (a purge line next to the part raises a reading, it never lowers it), and the limit is half the part's height.

A cool nozzle. With a nozzle probe the nozzle touches the part if it is still there, so the check waits until the nozzle is at nozzle_part_max_temp (50 °C), below where PLA softens, with the part fan on to cool it faster. Homing Z on the bare sheet waits for nozzle_probe_max_temp (150 °C).

When a print ends without an eject

If a print ends (finished, cancelled or stopped) and its parts were not ejected, a dialog opens in Mainsail, Fluidd and the web page: take the parts off, then click Bed is clear. You don't have to: with guard_action: "check", the next print checks the bed itself.

A print that stopped before its first layer (the start G-code failed, say) printed nothing: its outlines are forgotten, and no dialog opens. Without layer numbers from the slicer, more than 5 mm of filament counts as printed.

Commands

Command	Use
AUTO_EJECT HOME=1	Home safely around the stored parts if needed, then eject
AUTO_EJECT HOME=1 SOURCE=saved	Same, using the stored outlines even if a new file's labels are loaded
AUTO_EJECT_CLEAR	You removed the parts by hand: forget them
AUTO_EJECT_GUARD	Start G-code check (see above)

Settings

Setting	Default
persist_state	True	Save outlines and height while printing
guard_action	"check"	"check", "error" or "eject"
home_method	"auto"	"auto", "probe" or "g28"
home_clearance	20.0	Minimum distance between the probe spot and any part
home_edge_margin	15.0	Distance kept from the bed edges when choosing the spot