STO Loss 3 (STL3)

STO1–SCM1 and STO2–SCM2 internal loop detection error.

STO1 and SCM1, or STO2 and SCM2 short circuit lines are not connected - Re-connect the short circuit lines.

Hardware failure - After you make sure all the wiring is correct, if STL3 fault still exists after cycling the power.

Poor connection of the IO card

Check if the PIN of IO card is broken.
Check if the IO card connects to the control board correctly, and if the screws are tightened well.