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ErrorLib — Industrial Fault Code Library

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4120

Manual Switching Disabled

Mitsubishi Electric · MELSEC-Q/L Common Instruction

What does 4120 mean?

This fault occurs when the manual switching enable flag (SM1592) is OFF (disabled) at the time the SP.CONTSW instruction is executed. This configuration prevents manual system switching operations, effectively blocking the SP.CONTSW instruction from executing and causing the system to remain in its current state.

Common Causes

  • The manual switching enable flag (SM1592) is OFF.

Repair Steps & Checklist

Click steps to track your progress.

  1. 1

    Verify the status of the manual switching enable flag (SM1592) in the CPU program. Ensure it is set to ON to enable manual system switching.

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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