Code 120029: PCU: fatal hard disk error
Issue Description & Safety Warning
The Siemens SINUMERIK 840D CNC has triggered a critical PCU: fatal hard disk error 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: A high number of write/read errors was found on the hard disk. This indicates a hardware failure to come soon (S.M.A.R.T error). Interface bit DB10.DB103.3 is being set. Remedy: Back up the PCU data and have the hard disk replaced by qualified service personnel. Spare part description: Manual Operator Components SINUMERIK 840D/840Di/810D section PCU 50 V3 spare parts. Hard disk replacement: Installation & Start-Up Guide HMI SINUMERIK 840D/840Di/810D section Installation variants/data backup.
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 120029 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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