Safe Torque Off (STO) Channel Mismatch

What does FA81 mean?

This fault indicates a discrepancy in the status of the two Safe Torque Off (STO) channels. The drive's diagnostics have detected that the STO channels are not in the same state, or that STO was attempted to be activated in a non-redundant manner (e.g., only one channel activated). This prevents the STO safety function from operating correctly and will cause the drive to trip, leading to an uncontrolled stop or inability to start, posing a safety risk if machine motion is not reliably prevented.

Additional Steps

Steps 1–2 are listed in Priority Fix.

1. 3

   Check parameter 31.22 (STO fault/warning indication delay) in the drive's control program to understand the timing of fault detection and confirm it aligns with system expectations.

2. 4

   If the issue persists after verifying external control and wiring, consult the drive's firmware manual for advanced diagnostics related to STO indications and internal status.

3. 5

   If troubleshooting does not resolve the fault, contact ABB support as this may indicate an internal STO circuit failure within the drive that requires specialized attention.