What causes FANUC alarm 021?

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 021?

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.

axismd.ai

FANUC Controller

Program/G-code · Intermediate · 10-20 minutes

Alarm 021

Fanuc Alarm 021 - Invalid Arc Command

Fanuc Alarm 021 occurs when the CNC detects an invalid circular interpolation (G02/G03) command in the part program. This is typically caused by incorrect arc endpoint specification where the calculated radius from start to arc center differs from end to arc center, missing or incorrect I/J/K values, or attempting arc moves on non-coplanar axes.

FANUCProgram/G-codeIntermediate10-20 minutes

021

Fanuc Alarm 021 - Invalid Arc Command

Illegal Circular Motion

Root Cause Summary

Fanuc Alarm 021 occurs when the CNC detects an invalid circular interpolation (G02/G03) command in the part program. This is typically caused by incorrect arc endpoint specification where the calculated radius from start to arc center differs from end to arc center, missing or incorrect I/J/K values, or attempting arc moves on non-coplanar axes.

Safety

Arc will not execute

Causes and Fixes

1
Check SYSTEM > PARAM > parameter 3410 to verify the circular interpolation tolerance setting (typical default is 0.005mm or 0.0002 inches). Record this value.
2
In MDI or EDIT mode, examine the flagged G02/G03 block. Calculate the start radius: SQRT(I² + J²) for XY plane, and end radius using endpoint coordinates relative to arc center. Verify the difference is within parameter 3410 tolerance.
3
Correct the arc command by adjusting either the endpoint coordinates (X, Y, Z) or the arc center offsets (I, J, K) to ensure both radii match within tolerance. Alternatively, use R format (G02 X_ Y_ R_) for simpler arcs under 180 degrees.
4
Reset alarm (RESET button), return to program start or flagged block, and test run in single block mode to verify correction.

DIY Feasiblehigh confidence

Call a technician if:

·Parameter changes required beyond axis names
·Machine crashes occur during testing
·Custom axis configuration needs modification

Prevention

Always verify plane selection before programming circular moves
Use consistent axis naming conventions
Test macro programs with various input values
Document any custom axis configurations

Common Mistakes

Programming Z-axis in G17 XY plane circular moves
Using parallel axes (X and U) in same circular command
Forgetting active coordinate rotation affects axis interpretation
Not checking macro variable values before circular interpolation

Related Alarms

020 022 023 510 511

Seeing Multiple Alarms?

Seeing another alarm with this one? Enter the code.

Get machine-aware diagnosis

AxisMD logs alarms against your actual machines, tracks patterns, and gives AI context your manual doesn't have.

Start free

Was this guide helpful?

Always verify with your machine manual or a certified technician before performing service procedures.