Siemens Fault 0a7991 — Best

A maintenance tech named Alex saw alarm 0A7991 appear on a Siemens 840D sl after a power outage. The machine wouldn’t reference an axis. Instead of panicking, Alex checked the drive status via the HMI: the drive was reporting "ready" but the NCK showed the axis as parked. The best fix? Alex went into the startup menu → axis replacement → reparked the axis, then reinitialized the drive parameters from a backup. After a warm reboot, the fault cleared. Lesson learned: 0A7991 often means the NC and drive disagree on axis presence — reparking or re‑acknowledging the drive solves it best.

Check for tight bends or pinches in the communication cables. siemens fault 0a7991 best

If the motor data identification has been activated by mistake or you need to bypass it for urgent testing, it can be deactivated immediately: A maintenance tech named Alex saw alarm 0A7991

Master Guide: Troubleshooting Siemens Alarm A07991 Best Practices The Siemens Sinamics drive alarm A07991 Go to product viewer dialog for this item. The best fix

It is common to see alarm A07991 appear simultaneously with fieldbus alarm . This happens when a master PLC is not actively transmitting data to the SINAMICS control unit, leaving the drive locked in an uncommissioned loop. Observed Symptom Underlying Cause Target Remedy A07991 + Active Bus Fault (BF) LED

: Fieldbus protocol configuration selection ( 0 = No protocol active).

If you recently replaced the drive or motor, the parameters might be incorrect.