Code 402-000B: FK programming: Illegal block reference
Issue Description & Safety Warning
The Heidenhain TNC Series CNC has triggered a critical FK programming: Illegal block reference 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
[Cause of error] From an FK sequence, a block number is used to make reference to the end point or the end tangent of a block that either - does not exist - is too far away (> 64 blocks below or > 32 blocks above) - is not a positioning block - belongs to a category of positioning blocks which cannot be used for references (transitions, CC, FPOL, blocks containing only axis values or machine coordinates) [Error correction] Enter a reference to another block, or do without the reference
Source: HEIDENHAIN NC Error Messages — TNC7/640/620/320/128, CNC PILOT 640, MANUALplus 620 (10/2023)
Last reviewed: 2026-07-01 · Verified against the manufacturer’s official documentation.
About Heidenhain TNC Series CNC Alarms
Heidenhain controllers include extensive self-diagnostics that flag or halt the machine when a fault is detected. Error code 402-000B on the TNC Series CNC identifies one specific fault condition — its documented cause and the official corrective steps are listed above, sourced from Heidenhain's service documentation. Always confirm against the manual for your exact machine configuration and follow LOTO procedures before servicing.
Complete Alarm List: 4497 Heidenhain TNC Series CNC codes.
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