| User-Defined Fault 1 | Fault 1 Signal given by the users through multi-functional terminals or PLC Programming Function |
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ERR21 – User-Defined Fault 1
What this means
ERR21 indicates that the VFD has received a user-defined fault signal.
In simple terms, this fault is not generated by the VFD itself.
It is being triggered intentionally by an external input, PLC logic, or multi-function terminal that has been configured by the user or system integrator.
Why This Happens
Many VFDs allow users to define custom fault conditions using:
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Multi-function digital inputs
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PLC logic
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External safety or interlock circuits
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Custom control schemes
When the configured condition is met, the VFD triggers ERR21 and shuts down.
This behavior is fully intentional and user-configurable.
Most Common Causes We See
1. External input assigned to User-Defined Fault 1 (most common)
Examples:
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Safety switch
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External interlock
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PLC output
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Controller logic signal
If this input changes state, ERR21 is triggered.
2. PLC or controller logic asserting a fault
If PLC programming or controller logic:
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Detects an internal condition
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Sends a fault signal to the VFD
The VFD will report ERR21 even though it is functioning normally.
3. Wiring issue on the user-defined fault input
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Loose terminal
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Broken wire
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Incorrect NO vs NC logic
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Intermittent connection
This can cause ERR21 to appear intermittently.
4. Misconfiguration or leftover settings
ERR21 may appear if:
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User-defined fault logic was enabled unintentionally
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Parameters were copied from another setup
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A previous configuration was not fully cleared
What to Check First
Step 1: Identify what is assigned to User-Defined Fault 1
Check the VFD parameters to determine:
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Which terminal or PLC function triggers ERR21
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What condition causes the fault to assert
Step 2: Verify the external signal state
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Confirm the external device is in its normal, non-fault state
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Check continuity or signal level if applicable
Step 3: Inspect wiring to the assigned terminal
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Power down the VFD
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Verify terminal tightness
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Check for damaged or pulled wires
Step 4: Disable User-Defined Fault 1 if not needed
If you are not intentionally using a custom fault:
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Remove the terminal assignment
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Disable the PLC fault logic
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Reset the VFD
This often resolves ERR21 immediately.
How to Clear ERR21
Once the user-defined fault condition is removed:
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Reset the VFD
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Ensure the external signal is stable
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Resume normal operation
The VFD will not restart until the fault signal is cleared.
What ERR21 Is Not
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Not an internal VFD hardware fault
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Not a spindle motor issue
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Not a cutting load or RPM problem
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Not EMI related
ERR21 always originates from external or user-configured logic.
Summary
ERR21 means the VFD received a user-defined fault signal from an external input or PLC logic.
Most common causes:
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External safety or interlock input
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PLC or controller logic
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Misconfigured user-defined fault parameters
If you did not intentionally configure a user-defined fault, disabling this feature will usually resolve the issue.
