What causes FANUC alarm 421?

Incorrect dual position feedback conversion coefficients in parameters 2078 and 2079. Mechanical issues with the axis such as binding, excessive backlash, or worn components. Electrical issues with the servo motor, drive, or feedback devices.

How do I fix FANUC alarm 421?

1. Check the dual position feedback conversion coefficients in parameters 2078 and 2079. Ensure they are set correctly based on the machine specifications. 2. Perform a servo tuning procedure to optimize the servo loop gains and ensure the axis is properly compensated. 3. Inspect the mechanical components of the axis for any issues such as binding, excessive backlash, or worn parts that could be causing the excess error. 4. Check the servo motor, drive, and feedback devices for any electrical faults or wiring issues that could be contributing to the problem. 5. Review the G-code program for any commands or movements that could be exceeding the axis capabilities and causing the excess error. 6. If the issue persists, consult the machine manufacturer or a Fanuc service technician for further assistance.

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FANUC Controller

Servo/Axis · Intermediate · 30-60 minutes

Alarm 421

Fanuc Alarm 421 - SERVO ALARM: n AXIS EXCESS ERROR (DUAL)

Fanuc Alarm 421 occurs when dual position feedback systems (semi-closed and fully-closed loop) show excessive deviation between their position error values. This indicates mismatched scaling between the motor encoder and external linear scale, mechanical binding, or incorrect dual position feedback conversion parameters 2078 and 2079.

FANUCServo/AxisIntermediate30-60 minutes

421

Fanuc Alarm 421 - SERVO ALARM: n AXIS EXCESS ERROR (DUAL)

Excess Error (Dual Check)

Root Cause Summary

Fanuc Alarm 421 occurs when dual position feedback systems (semi-closed and fully-closed loop) show excessive deviation between their position error values. This indicates mismatched scaling between the motor encoder and external linear scale, mechanical binding, or incorrect dual position feedback conversion parameters 2078 and 2079.

Safety

Safety function - do not bypass
Contact service
This is a safety-critical function. Bypassing or disabling the dual check system can lead to serious injury or death.
Ensure the machine is in a safe state before performing any troubleshooting steps. Disconnect power to the axis drive if necessary.
Electrical shock hazard: Disconnect power before inspecting any electrical components.
Mechanical hazard: Be aware of potential pinch points and moving parts during inspection.
Refer to the machine's safety documentation for specific warnings and precautions.
Verify that all safety devices are functioning correctly after any repairs or adjustments.

Causes and Fixes

1
Navigate to SYSTEM > PARAM and verify parameter 1815 bit 0 (DPF) is set to 1 to enable dual position feedback for the affected axis.
2
Check parameters 2078 and 2079 for the affected axis. Calculate correct ratio: 2078/2079 should equal (linear scale resolution × ballscrew pitch) / (motor encoder resolution × gear ratio). Typical values are 2078=10000, 2079=10000 for 1:1 ratio.
Document original parameter values before changes. Parameter write enable (bit 0 of parameter 0000) must be set to 1.
3
Verify parameter 1825 (dual position feedback error detection width) is not set too tight. Typical value is 500-2000 detection units. Increase temporarily to 5000 to test if alarm clears during motion.
4
After parameter correction, perform reference return on affected axis and jog test at 10% rapid rate. Monitor DIAGNOSIS screen 300-399 (servo diagnostics) for position error values on both feedback systems.

DIY Feasiblemedium confidence

Call a technician if:

·Mechanical components require replacement
·Parameter calculations are uncertain
·Multiple related servo alarms persist

Prevention

Regular linear scale cleaning and inspection
Periodic backlash measurement and compensation
Maintain proper cable routing and shielding

Common Mistakes

Confusing parameter 2078/2079 calculation formulas
Ignoring mechanical wear indicators
Not backing up parameters before changes

Related Alarms

400 401 414 420

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