Code 120021: PCU fan monitoring housing fan 1
Issue Description & Safety Warning
The Siemens SINUMERIK 840D CNC has triggered a critical PCU fan monitoring housing fan 1 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
Definitions: Low fan speed of PCU casing fan 1. Interface bit DB10.DB103.4 is being set. Remedy: Switch off the PCU, let it cool off and have the f unction of casing fan 1 of PCU module checked by qualified service personnel (fan problem). Spare part description: Manual Operator Components SINUMERIK 840D/840Di/810D section PCU 50 V3 spare parts, replacement of device fan.
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 120021 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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