axismd.ai
Siemens SINUMERIK Controller
Servo/Axis · Intermediate · 15 - 45 minutes
Siemens 25021 - Axis zero mark monitoring of passive encoder
SIEMENS alarm 25021 triggers when the drive module's internal temperature sensor detects thermal conditions exceeding the warning threshold defined in MD30200 (drive temperature monitoring). This typically occurs due to insufficient cooling airflow, excessive motor loading, or degraded thermal management components in the drive cabinet.
25021
Siemens 25021 - Axis zero mark monitoring of passive encoder
Axis zero mark monitoring of passive encoder
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 similar to those for alarm 25020 Control reaction: Alarm display.
Root Cause Summary
SIEMENS alarm 25021 triggers when the drive module's internal temperature sensor detects thermal conditions exceeding the warning threshold defined in MD30200 (drive temperature monitoring). This typically occurs due to insufficient cooling airflow, excessive motor loading, or degraded thermal management components in the drive cabinet.
Safety
- Drive/power/encoder circuit: hazardous voltages may remain in the DC link after power-off. Lock out / tag out, wait for discharge per the drive documentation, and use qualified personnel only.
Causes and Fixes
- 1
Visual Fan Inspection
Power down system and inspect all drive cabinet cooling fans for physical obstructions, dust buildup, or visible damage to fan blades.
Ensure complete power isolation before opening drive cabinet
- 2
Fan Operation Test
Power up system and verify all cooling fans are rotating at proper speed. Check fan status in HMI Diagnostics > Drive > Temperature Monitoring.
Listen for unusual bearing noise indicating impending fan failure
- 3
Clean and Replace
Clean fan filters and heat sink fins with compressed air. Replace any failed fans with Siemens-approved replacements matching original specifications.
Replace fans in pairs to maintain balanced cooling performance
Call a technician if:
- ·Power module replacement required
- ·Temperature sensor calibration needed
- ·Persistent alarms after cooling system repair
Prevention
- Schedule regular cooling system maintenance
- Monitor drive loading through HMI trending
- Maintain proper cabinet ambient temperature
- Replace cooling fans proactively based on operating hours
Common Mistakes
- Ignoring gradual temperature increase trends
- Using non-Siemens replacement fans
- Inadequate thermal compound application
- Blocking cabinet ventilation with stored materials
Seeing Multiple Alarms?
Seeing another alarm with this one? Enter the code.
Get machine-aware diagnosis
AxisMD logs alarms against your actual machines, tracks patterns, and gives AI context your manual doesn't have.
Start freeWas this guide helpful?
Always verify with your machine manual or a certified technician before performing service procedures.