What causes FANUC alarm 120?

Part program contains an illegal or malformed command related to the alarm condition. Missing required address word or parameter value in the program block. Programmed value is out of the allowable range. Conflict between active modal codes and the commanded function. Post-processor configuration mismatch for this Fanuc series.

How do I fix FANUC alarm 120?

1. Note the block number shown in the alarm screen and locate that block in the program. 2. Review the command in that block against the Fanuc programming manual. 3. Check for missing words, incorrect values, or conflicting G-code combinations. 4. Verify the program is post-processed for the correct Fanuc series. 5. Correct the program and dry-run at low speed before resuming machining.

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FANUC Controller

Program/G-code · Intermediate · 15-45 minutes

Alarm 120

Fanuc Alarm 120 - Custom Macro Error

Alarm 120 is triggered when the custom macro program (G65/G66 or macro variables) contains a syntax error, undefined variable operation, mathematical error, or illegal macro statement. This is a programmatic issue rather than a hardware fault, indicating the control detected an instruction it cannot process within the macro code.

FANUCProgram/G-codeIntermediate15-45 minutes

120

Fanuc Alarm 120 - Custom Macro Error

Macro Alarm

Root Cause Summary

Alarm 120 is triggered when the custom macro program (G65/G66 or macro variables) contains a syntax error, undefined variable operation, mathematical error, or illegal macro statement. This is a programmatic issue rather than a hardware fault, indicating the control detected an instruction it cannot process within the macro code.

Safety

Macro stopped

Causes and Fixes

1
Review the program line number shown with the alarm. Examine all macro variable calculations (#i=#j+#k, #i=#j/#k, etc.) near this line.
2
Check for division operations where the divisor variable could equal zero. Verify conditional logic prevents this condition.
3
Verify SQRT operations have non-negative inputs and that all referenced macro variables (#100-#999, #1-#33) have been assigned values before use.
4
Use MDI mode to manually check suspect variable values. Execute #3000=100 (common variable display) to verify current values before re-running program.
Avoid modifying macro variables used by machine builder or probe cycles (#500-#699 range often reserved).

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