Alarm guide · Fanuc · Updated October 2026
Fanuc SV0421 alarm: excess semi-full error
Fanuc SV0421 means the difference between the motor position and the separate scale position is too large. It applies to axes with a separate detector.
By Suresh Sankaran, founder of Star Automations, servo motor repair since 1998.
What the alarm means
On a dual position feedback axis the two feedbacks must agree. Here they do not. Alarm numbers and wording can differ between series, so confirm with the manual for your unit.
Common causes
- Separate scale or linear scale fault
- Backlash or slip between motor and scale
- Wrong parameter for the scale
- Dirty scale or loose mounting
- Pulsecoder fault
Safe first checks
General guidance only. Follow the drive manual and your site's electrical safety rules. Power off, and let the DC bus discharge, before touching connectors.
- Note the exact alarm code and when it appears: at power-up, at start, or under load.
- Power off and turn the shaft by hand if safe: it should turn smoothly.
- Check the motor cable, encoder cable and connectors for damage and loose contacts.
- Check what changed recently: load, speed, settings, cables or a replaced part.
When to send it for repair
Check the scale and its mounting first. If the motor pulsecoder is suspected, bench test it.
We test every motor on our Universal Servo Motor Repair Equipment, which reads and aligns the feedback and runs the motor under load without the original drive.
See Fanuc servo motor repair, Fanuc encoder repair and Fanuc servo drive repair.
Questions
Is it always the scale?
No. It can be the motor feedback too. That is why both are checked.
How long does repair take?
Emergency repair is done the same day where the fault and parts allow, urgent in 3 working days and standard in 7 working days, counted from when the unit reaches our bench and the quote is approved. Every unit returns with a printed test report.
Can I get an exchange unit?
If we hold a matching unit, Servo Swap ships a tested exchange unit while yours is repaired. Mark "line down" on the request form and add the part number.
Related: SV0414 · SV0364 · SV0368 and SV0366
Brand names identify the units we repair. Star Automations is an independent multi-brand repair company and is not an OEM service centre.