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4121

SP.CONTSW Instruction Executed in Incorrect Mode

Mitsubishi Electric · MELSEC-Q/L Common Instruction

What does 4121 mean?

This error occurs if the SP.CONTSW instruction was executed by the standby system CPU module while operating in separate mode or if the instruction was executed when the CPU was in debug mode. Both scenarios represent prohibited conditions for performing system switching, preventing the instruction from completing successfully.

Common Causes

  • SP.CONTSW instruction executed by the standby system CPU module in separate mode.
  • SP.CONTSW instruction executed in debug mode.

Repair Steps & Checklist

Click steps to track your progress.

  1. 1

    Verify the operating mode of the standby system CPU module. Ensure it is not in separate mode during SP.CONTSW instruction execution.

  2. 2

    Verify the operating mode of the CPU module. Ensure the system is not in debug mode when the SP.CONTSW instruction is executed.

Browse all codes in this manual (11)
Verified technical data. Last updated: April 2026

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Source: Mitsubishi Electric MELSEC-Q/L Common Instruction
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