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Review the NC program logic around the COUPLE/UNCOUPLE commands. Identify all spindle variables referenced in consecutive COUPLE blocks. Check for any spindle that appears as a following spindle (second parameter) after previously being a leading spindle (first parameter).

Use block search (Ctrl+G) in the HMI program editor to locate all COUPLE and UNCOUPLE calls.

Verify the coupling state using Diagnostics > Spindle Couplings menu (path varies by HMI version: SIEMENS Operate or Sinumerik HMI). Document which spindles are currently coupled and their leader/follower roles.

Restructure the NC program to use a linear coupling sequence: uncouple the existing pair before attempting to couple a new pair involving any previously active spindle. Example: UNCOUPLE(S1,S3) before COUPLE(S3,S2).

Verify machine configuration parameter MD30200 (number of spindles) and MD30210 (spindle assignment) to ensure all referenced spindles are properly configured and active in the channel.

Access Parameters > Machine Data (MD) menu. Navigate to spindle coupling section: MD30200-MD30230 (spindle count, IDs, and coupling configuration). Print or screenshot current settings for all referenced spindles.

Write-protect status: Ensure you have MD write permission; some installations require service mode password.

Cross-reference each spindle's MD30220 (spindle coupling enable) value. Confirm that no spindle has conflicting enable/disable states or contradictory coupling role assignments. Check if any spindle appears in multiple coupling definitions simultaneously.

Verify setpoint data (SD) parameters specific to spindle coupling behavior, particularly SD20200-series (spindle control data). Reset any corrupted or overridden SD values using factory defaults if coupling behavior remains ambiguous.

Perform a full system reset (Diagnostics > System > Initialization or warm restart) to refresh NCK coupling state tables after MD corrections.

Open the machine PLC program (usually accessible via Automation > Program Manager or Service menu). Locate the FB or OB block responsible for spindle coupling control. Search for COUPLE/UNCOUPLE function block calls and inspect the logic flow.

This step requires PLC programming access; some installations restrict this to authorized service personnel.

Trace the PLC coupling state variables (typically BOOL or INT data types in a DB). Verify that UNCOUPLE commands fully reset the state before a new COUPLE is permitted. Check for any unconditional or redundant COUPLE calls in loops or time-triggered OBs.