Fanuc alarm 21: Position sensor polarity setting incorrect
αi SPM · Fanuc αi SPM
What it means
The polarity specified for the spindle position sensor (position coder) is wrong on the αi SPM. Detected when the polarity parameter and actual sensor signals disagree.
Likely causes
- Position sensor polarity bit set incorrectly (bit 4 of parameter No. 4001)
- Position coder feedback cable wired or defective
- Sensor fitted incorrectly after replacement
Replacement parts
We supply the following units for this alarm — tested, warranty-backed:
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Mi/MZi sensor feedback cable
amplifier · Common cause
Need the part urgently?
Send us alarm 21 with your machine details — typical RFQ turnaround is 24–48h.