Siemens G120 F01205 - Causes & Fix

Siemens G120 F01205 — What It Means

F01205 on a Siemens SINAMICS G120 means “CU: Time slice overflow.” This fault indicates the Control Unit (CU) did not finish its required processing tasks within the allotted time slice, so the drive trips on an internal execution-time fault. The drive reacts with an OFF2 shutdown and requires a full POWER ON reset to acknowledge the fault. This is an internal firmware or CPU load issue, not a motor or power-stage problem. The plain-language cause is insufficient computation time inside the control unit.

The fault value is stored in the drive diagnostics memory for internal Siemens analysis but is not useful for field measurement or troubleshooting. In most cases, the fault either clears after a power cycle or indicates a control-unit hardware or software failure that requires CU replacement.

Jump to Fix

Common Causes

• Internal firmware or CPU overload The control unit’s processor exceeded its time budget due to software task overrun or internal execution problems.
• Excessive configuration complexity High processing load from complex control logic, communication tasks, or integrated application functions can push the CU beyond its computation capacity.
• Control-unit corruption or internal failure A corrupted firmware state or failing CU hardware prevents the processor from completing its tasks on time.
• Recent configuration or firmware change A new parameter set, firmware update, or added function block may have increased the control unit’s processing demand beyond safe limits.

Step-by-Step Fix

1. Record the fault buffer by accessing the drive diagnostics menu and noting all stored fault messages before you clear or reset anything.
2. Power-cycle the drive completely by removing all power and waiting at least 30 seconds, then restore power for a full POWER ON reset, which is the required acknowledgment method for F01205.
3. Check if the fault returns immediately after the reboot, because a persistent fault indicates a control-unit hardware or software problem rather than a transient overload.
4. Review the control-unit configuration and firmware level using the BOP or STARTER software, especially if the fault appeared after a parameter change, firmware update, or added communication task.
5. Revert recent changes or reload a known-good configuration if the fault coincided with a configuration modification, and test the drive under normal load to see if the fault clears.
6. Replace the Control Unit if the fault persists after a clean power cycle and configuration review, because internal CU faults that survive a POWER ON reset typically indicate hardware failure.
7. Reparameterize the new CU by uploading your saved parameter set or manually reconfiguring the drive, then test under normal operating conditions to verify the fault is cleared.

Parts Often Needed

When to Call a Pro

Call a qualified technician or Siemens-authorized service partner if the fault persists after a power cycle and you do not have experience with VFD parameterization or control-unit replacement. Replacing the CU requires transferring or recreating all drive parameters, and incorrect configuration can damage the motor or machine. Also call a professional if the drive is integrated into a PLC or network control system, because troubleshooting may require analysis of communication load or application software. If your facility does not have Siemens STARTER software or a backup of the drive parameters, professional support is strongly recommended to avoid extended downtime.