The second (passive) encoder on the axis, often a linear scale, has a hardware fault.
What causes Siemens alarm 25001?
Scale dirty or contaminated. Scale cable damaged. Scale air purge not working.
How do I fix Siemens alarm 25001?
Check scale air purge pressure. Check the scale cable. Scale cleaning and replacement are tech jobs.
Alarm numbers and text can differ by control model, software version and machine builder. This page is a starting point from people who have worked on these machines, not a replacement for your machine builder's manual.
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