Siemens SINUMERIK codes
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Siemens SINUMERIK Controller

Program/G-code · Intermediate · 30-120 minutes

Alarm 107031

Siemens 840D 107031 - MCU: Command unknown

This alarm occurs when the CNC control encounters an unrecognized or invalid command in the program. Correct the syntax or verify if a required option is missing.

Siemens SINUMERIKProgram/G-codeIntermediate30-120 minutes

107031

Siemens 840D 107031 - MCU: Command unknown

MCU: Command unknown

Siemens SINUMERIK operator message 107031: MCU: Command unknown. This is an informational message from the SINUMERIK control indicating a system status or operation result that may require operator action.

Root Cause Summary

This alarm occurs when the CNC control encounters an unrecognized or invalid command in the program. Correct the syntax or verify if a required option is missing.

Safety

  • Refer to Siemens documentation
  • Ensure qualified technician performs repairs

Causes and Fixes

  1. 1

    SAFETY FIRST - Stop machine

    Ensure machine is in a safe state, spindle stopped, and program execution halted before making any changes.

  2. 2

    Locate error

    Check the line/block number in the alarm message and open the program in edit mode.

  3. 3

    Fix syntax

    Correct the invalid command or syntax error in the specified block. Verify changes against control documentation.

  4. 4

    Test program safely

    Clear the alarm, verify machine is in safe position, run program in single block mode or simulation first to verify the fix.

DIY Feasiblehigh confidence

Call a technician if:

  • ·If the alarm persists after syntax correction
  • ·If the required option cannot be enabled without assistance

Prevention

  • Always validate programs using simulation before execution
  • Confirm G code compatibility with control documentation and installed options
  • Use single block mode when testing modified programs
  • Maintain backup copies of working programs before modifications

Common Mistakes

  • Overlooking the specific block number in the alarm
  • Misidentifying the subprogram or file with the error

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