K2 Plus BM2355 at third CFS start purge with 0.8 mm nozzle

Flagship K Series Creality Flagship K2 Plus & CFS
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K2 Plus BM2355 at the third CFS start purge with a 0.8 mm nozzle

Summary

Creality K2 Plus consistently shuts down the nozzle MCU before the model starts printing when using the K2 Plus 0.8 mm nozzle profile with CFS. The failure occurs at the beginning of the third repeated extrusion-only purge block in the generated start G-code.

Removing only the positive extrusion commands from the third purge block allows the same file, printer, filament, and CFS path to print the complete test model successfully.

Environment

  • Printer: Creality K2 Plus with CFS
  • Physical nozzle: 0.8 mm
  • Nozzle size selected on the printer under Maintenance: 0.8 mm
  • Slicer printer profile: Creality K2 Plus 0.8 nozzle
  • Creality Print: 7.2.1.5476
  • Printer firmware: 1.1.6.1
  • Material used for the reproduction: PLA
  • Test model: 20 mm cube
  • Flow calibration disabled for the final comparison

Error

The printer displays BM2355. The relevant sanitized log message is:

MCU 'nozzle_mcu' shutdown: Stepper too far in past
This generally occurs when the micro-controller has been
requested to step at a rate higher than it is capable of obtaining.

Reproduction

  1. Install and select a 0.8 mm nozzle on the K2 Plus.
  2. Slice a simple 20 mm cube with the Creality K2 Plus 0.8 nozzle profile.
  3. Use PLA supplied through CFS.
  4. Start the print without flow calibration.
  5. The printer completes homing, leveling, nozzle cleaning, filament loading, and the earlier purge operations.
  6. Immediately before the model starts, the nozzle MCU shuts down with BM2355 at the beginning of the third repeated purge block.

The relevant repeated purge pattern is:

; FLUSH_START
G1 F60
M400
G1 E16.2 F199.559
G1 E1.8 F60
G1 E16.2 F199.559
G1 E1.8 F60
G1 E16.2 F199.559
G1 E1.8 F60
G1 E16.2 F199.559
G1 E1.8 F60
G1 E16.2 F199.559
G1 E1.8 F60

This pattern appears three times in the start sequence. The first two blocks complete. The failure is repeatable when the third block begins.

Comparison tests

Test A: generated start sequence

  • Three purge blocks present.
  • Result: BM2355 before the model starts.

Test B: reduced purge flow

  • The purge extrusion rate was reduced from approximately 14 mm3/s to 8 mm3/s.
  • All three purge blocks remained present.
  • Result: the same BM2355 at the same stage.

Test C: third purge extrusion removed

  • The first two purge blocks were left unchanged.
  • Only the ten positive G1 E... commands in the third purge block were commented out.
  • All later wipe, prime-line, and model G-code remained present.
  • Result: the print started normally and completed the full test model successfully in 20 minutes 21 seconds.

These results suggest that purge speed alone is not the trigger. The failure appears related to the third consecutive extrusion-only purge sequence or to nozzle-MCU scheduling/state accumulated across the repeated CFS purge blocks.

Additional observation

The printer UI correctly shows the installed nozzle as 0.8 mm, but the exported Klipper configuration contains:

[extruder]
nozzle_diameter = 0.400

This may be a static firmware safety value rather than the UI nozzle selection, but please confirm whether this is expected for a K2 Plus configured for a 0.8 mm nozzle and whether it can affect CFS start-code generation.

Expected behavior

The official 0.8 mm K2 Plus profile should generate a CFS start sequence that completes without BM2355. The printer should not require manual post-processing of every G-code file.

Workaround

Removing the positive extrusion commands from the third start purge block prevents the failure in the tested single-color PLA print. This is only a local workaround; a corrected Creality Print profile or firmware fix is requested.

Note about the diagnostic print quality

The successful comparison file was derived from an earlier fan-isolation test and intentionally kept the model and auxiliary cooling fans disabled. The completed 20 mm cube therefore showed heat-related surface defects, including softened corners, a seam bulge, and gaps in the top surface. These cosmetic defects are expected under the diagnostic cooling settings and are separate from BM2355. A permanent profile workaround should remove the third start purge while preserving the normal 0.8 mm PLA cooling settings.