SINUMERIK 840D sl Alarm 25000, active encoder hardware fault: what to check?

Question

I'm getting Alarm 25000 on my SINUMERIK 840D sl, indicating an active encoder hardware fault. The axis goes into follow-up mode, and I have to power cycle the machine. What should I check first?

Top answer · Written by InduQT from the fault guide and manufacturer manuals cited below · Oct 28, 2026

Alarm 25000 on a SINUMERIK 840D sl signifies a hardware fault with the active encoder, meaning its signals are missing, out of phase, or have a ground/short circuit. This causes the NC to switch the axis to follow-up mode, making the channel not ready, and requires re-referencing the axes after a power cycle. This alarm typically occurs at power-up, during motion if a connection is lost, or after maintenance involving the encoder, motor, or cable.

Likely causes and checks

1Encoder cable damaged

Points to it: The encoder cable may have an open circuit, short circuit, or be grounded. This often manifests at specific axis positions or when the cable chain flexes.
Check: With power off, check the encoder cable for continuity, short circuits and grounding.

2Encoder faulty

Points to it: The encoder itself may be defective, even if connectors and cables appear to be in good condition. This cause is suspected if the alarm persists after checking connections and cables.
Check: Check the encoder signals and replace the encoder if faults are found.

3Drive or encoder-module evaluation fault

Points to it: The drive or encoder module responsible for evaluating the encoder signals may be faulty. This is often indicated by accompanying SINAMICS encoder faults (2xxxxx) on the same drive.
Check: Evaluate the SINAMICS alarms in the SINAMICS S120 List Manual.

Next check: First, check the measuring-circuit connectors at both the motor and the drive or encoder module for correct contact, especially if the alarm occurred after recent work in those areas.

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