Grbl uses error messages to indicate issues with commands, syntax, or invalid G-code. These errors don’t stop the controller like alarms do — they simply reject the problematic command and wait for the next one.
Errors are returned in the format:
Where X is the error code number.
Error Code Reference
| Error Code | Name | Description |
1 |
Expected G-code word | Grbl expected a valid command (like G1, M3, etc.) but didn't find one. This is often due to a blank or malformed line. |
2 |
Bad number format | A number in the command was invalid or improperly formatted. |
3 |
Invalid statement | The line contains an unknown or unsupported command. |
4 |
Negative value | A negative value was used where it isn't allowed, such as a feed rate or spindle speed. |
5 |
Homing not enabled | A homing cycle ($H) was requested but homing is not enabled in the settings. |
6 |
Minimum step pulse time violated | A step pulse was issued faster than the configured minimum time ($0). |
7 |
EEPROM read fail | Grbl could not read its settings from EEPROM. May indicate hardware issues. |
8 |
Not idle | A command was issued that requires the machine to be idle, but motion or another operation is in progress. |
9 |
G-code lockout | A G-code command was blocked due to machine state. Common after a reset or alarm. |
10 |
Homing not set | The machine requires homing to establish its position before executing the command. |
11 |
Line overflow | A G-code line was too long for Grbl’s internal buffer. Shorten your lines. |
12 |
Step rate too high | Grbl could not generate step pulses fast enough for the commanded movement. |
13 |
Safety door open | A command was blocked because the safety door is open. |
14 |
Build info overflow | The build info string exceeded the allowed character limit. |
15 |
Setting disabled | A command depends on a setting (like homing or soft limits) that is disabled. |
16 |
Negative value in settings | A setting value was set to a negative number, which is invalid. |
17 |
Invalid jog command | A jog command is improperly formatted or unsupported. |
20 |
Unsupported command | The command is not supported by Grbl’s G-code parser. |
21 |
Modal group violation | Two conflicting commands from the same modal group (e.g. two motion modes) were used together. |
22 |
Undefined feed rate | A motion command was issued without a feed rate being set. |
23 |
Axis command conflict | Two axis words used inappropriately together, such as in arcs or jogs. |
24 |
Invalid target | The target position is invalid — for example, in an arc that can’t be generated. |
25 |
Invalid arc radius | Arc command contains a radius value that doesn't make geometric sense. |
26 |
Invalid G-code word | A word was used in the wrong context or is not allowed for the active command. |
27 |
Invalid line number | A line number was used incorrectly or exceeds limits. |
28 |
Value word repeated | A G-code word was used more than once on the same line. |
29 |
G59.x WCS error | A G59.1, G59.2, or G59.3 work coordinate system was used but isn’t supported. |
30 |
G53 with offset | G53 motion cannot be used with G54–G59 offsets. |
31 |
Invalid real value | A floating-point value is invalid (e.g. NaN, too many decimal places, etc.). |
32 |
Arc axis missing | An arc command is missing a required axis word. |
33 |
Arc format error | Arc command has incorrect or conflicting data (e.g., missing radius and offset). |
34 |
No axis word in motion | A motion command was issued without specifying any axis to move. |
35 |
G2/G3 not allowed | Arc motions (G2/G3) are not allowed in certain states, like jogging. |
36 |
Unused words | Extra G-code words were found that don’t apply to the current command. |
How to Handle Errors
-
Read the error code. This will tell you what went wrong.
-
Correct the G-code or command. Make sure formatting and syntax are valid.
-
Send next command. Grbl waits for the next valid line after an error — no reset is required (unless paired with an alarm).
Tips
- Ensure your G-Code is in the Easel G-Code Spec: Easel G-Code Spec