What causes FANUC alarm 291?

Part program contains an illegal G-code or M-code combination for this alarm type. Missing or incorrect address word in the program block. Axis or parameter value out of range for the commanded function. Required option not enabled on this CNC for the commanded function. Conflict between active modal states and the commanded function.

How do I fix FANUC alarm 291?

1. Note the block number shown on the alarm screen and locate that block in the program. 2. Review the command in that block against the Fanuc programming manual for alarm 291. 3. Check for missing address words, incorrect values, or conflicting G-code combinations. 4. Verify any required CNC options are enabled if using advanced functions. 5. Correct the program and verify the fix before restarting machining.

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FANUC Controller

Program/G-code · Intermediate · 5-30 minutes

Alarm 291

Fanuc 291 - Illegal command in programmable mirror image

Alarm 291 occurs when the control detects an incompatible or illegal G-code command while programmable mirror image (G51.1) is active. Certain commands like tool changes, subprogram calls, canned cycles, or coordinate system commands cannot be executed during mirror image mode and must be placed outside the G51.1/G50.1 block.

FANUCProgram/G-codeIntermediate5-30 minutes

291

Fanuc 291 - Illegal command in programmable mirror image

ILLEGAL COMMAND IN PROGRAMMABLE MIRROR IMAGE

Root Cause Summary

Alarm 291 occurs when the control detects an incompatible or illegal G-code command while programmable mirror image (G51.1) is active. Certain commands like tool changes, subprogram calls, canned cycles, or coordinate system commands cannot be executed during mirror image mode and must be placed outside the G51.1/G50.1 block.

Safety

Verify corrected program or parameters before resuming machining

Causes and Fixes

1
Locate the G51.1 command in the program and identify all code between G51.1 and G50.1.
2
Review for tool changes (T codes, M06), subprogram calls (M98, M99), macro calls (G65, G66), or work offset changes (G54-G59) within the mirror image block.
3
Move any prohibited commands outside the G51.1/G50.1 block, executing them before G51.1 or after G50.1.
Tool changes must always occur outside mirror image mode to prevent coordinate system conflicts.
4
Verify G50.1 is properly programmed to cancel mirror image before any tool change or mode-incompatible command.

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