Code 25021: Axis %1 zero mark moni toring of passive encoder
Issue Description & Safety Warning
The Siemens SINUMERIK 840D CNC has triggered a critical Axis %1 zero mark moni toring of passive encoder alarm. Immediate attention from a qualified operator or maintenance technician is required to prevent secondary damage to the equipment or spoiled parts during the manufacturing cycle. Always ensure standard safety and LOTO procedures are followed before accessing electrical cabinets.
Official Troubleshooting Steps
Parameters: %1 = Axis name, spindle number Definitions: Monitoring relates to the encoder that is not used by the position control. (NC-PLC interface signal DB31,... DBX1.5 = 0 (Position measuring system 1) or DB31,... DBX1.6 = 0 (Position measuring system 2)) More detailed explanations are si milar to those for alarm 25020. Reaction: Alarm display. Remedy: Please inform the authorized personne l/service department. The differences can result from transmission errors, disturbances, encoder hardware faults or from the ev aluation electronics in the encoder not used for position control. The actual value branch must therefore be checked: 1. Transmission path: Check the ac tual-value connectors for corr ect contacting, encoder cable for continuity, and also check for shor t-circuits and groundi ng (loose contact?). 2. Encoder pulses: Is the encoder power supply within the tolerance limits? 3. Evaluation electronics: Replace or rec onfigure the drive or encoder module used. Monitoring can be switched off by setting MD3 6310 $MA_ENC_ZERO_MONITORING[n] to 0 or 100 (n = encoder number: 1, 2). Program Continuation: Clear alarm with the Delete key or NC START.
Source: SINUMERIK 840D sl / 840Di sl / SINAMICS S120 Diagnostics Manual (DAsl, 03/2009)
Last reviewed: 2026-07-01 · Verified against the manufacturer’s official documentation.
About Siemens SINUMERIK 840D CNC Alarms
Siemens controllers include extensive self-diagnostics that flag or halt the machine when a fault is detected. Error code 25021 on the SINUMERIK 840D CNC identifies one specific fault condition — its documented cause and the official corrective steps are listed above, sourced from Siemens's service documentation. Always confirm against the manual for your exact machine configuration and follow LOTO procedures before servicing.
Complete Alarm List: 4014 Siemens SINUMERIK 840D CNC codes.
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