GCodexWhat is G29?
G29 triggers the automatic bed leveling (ABL) routine. The printer probes multiple points across the bed surface, builds a mesh of height variations, and applies real-time Z compensation during printing to correct for bed warp and tilt.
Syntax
G29 [S] [L] [V]
Parameters
| Parameter | Description |
|---|---|
| (no args) | Run full probe sequence and build mesh |
| S0 | Disable bed leveling compensation |
| S1 | Enable bed leveling compensation (without re-probing) |
| L1 | Load mesh from EEPROM slot 1 (UBL mode) |
| V2 | Verbose output — print mesh values to serial |
Examples
G28
G29↑ Standard sequence — home first, then probe (always in this order)
G28
G29
M500↑ Probe and save mesh to EEPROM for future use
G28
G29 L1
G29 S1↑ Load saved UBL mesh and activate compensation
Important Notes
- Always run G28 before G29 — the printer must be homed before probing.
- G29 in bilinear mode probes a grid (default 3×3 or 5×5) and interpolates between points.
- UBL (Unified Bed Leveling) is the most powerful mode — supports 10×10 grids and manual mesh editing.
- Save the mesh with M500 after G29 to avoid re-probing every print.
- Baby step Z offset (M290) fine-tunes the first layer height without re-running G29.
Common Mistakes
- ❌ Skipping G28 before G29 — undefined position causes incorrect or dangerous probe moves.
- ❌ Not saving with M500 after probing — mesh is lost on power cycle.
- ❌ Running G29 after the bed has warmed up to a different temperature than when it was probed — mesh becomes inaccurate.
Frequently Asked Questions
Q: How many points does G29 probe?
In bilinear mode, the default is a 3×3 grid (9 points). You can increase this to 5×5 or higher in Marlin's Configuration.h (GRID_MAX_POINTS_X/Y). More points give a more accurate mesh but take longer.
Q: What is the difference between bilinear ABL and UBL?
Bilinear ABL probes a fixed grid and uses bilinear interpolation for compensation. UBL (Unified Bed Leveling) supports larger grids, allows manual point editing, stores multiple mesh slots in EEPROM, and uses a more accurate nearest-point algorithm. UBL is recommended for large beds or severe warp.
Q: Why is my first layer still uneven after G29?
The Z probe offset (M851) may be incorrectly set. The offset is the distance between the probe trigger point and the nozzle tip. Use baby stepping (M290) during the first layer of a real print to fine-tune. Also ensure the bed temperature matches what it was during calibration.