What causes fault code VFD-STO1 (VFD Safe Torque Off Active)?

STO input not energized — safety circuit open. Safety relay output feeding STO is tripped. E-stop circuit active — removing STO enable signal. STO wiring disconnected or broken. STO terminal not jumpered when safety function not used

How do I fix fault code VFD-STO1?

Check STO terminal — should have 24VDC when safe to run. Trace STO wiring back to safety relay or PLC safety output. If STO not used, jumper the STO terminals per drive manual. Reset safety relay if tripped — investigate why it tripped. Check E-stop circuit — all stations released and circuit complete

VFD Safe Torque Off Active

VFD-STO1

Drive Safe Torque Off (STO) function is active. Drive output is disabled by safety circuit — motor cannot run.

medium vfd allen-bradley

vfd sto safe torque off safety drive enable

Probable Causes (ranked)

STO input not energized — safety circuit open
Safety relay output feeding STO is tripped
E-stop circuit active — removing STO enable signal
STO wiring disconnected or broken
STO terminal not jumpered when safety function not used

Fix Steps

1 Check STO terminal — should have 24VDC when safe to run
2 Trace STO wiring back to safety relay or PLC safety output
3 If STO not used, jumper the STO terminals per drive manual
4 Reset safety relay if tripped — investigate why it tripped
5 Check E-stop circuit — all stations released and circuit complete

Field Trick

Put your meter on the STO terminals. 24V = safe to run, should start. 0V = safety circuit open, trace it back.

Suggested Replacement Parts

Parts matched to fault category: Safety Relay

Allen-Bradley Guardmaster 440R-N23126 P/N: 440R-N23126 Dual-channel safety relay, 24VDC, 3 N.O. + 1 N.C., Cat 4 / PLe

AutomationDirect $125.00 Amazon $95.00 Grainger $155.00

Pilz PNOZ X3 Safety Relay P/N: 774310 24VDC/AC, 3 N.O. + 1 N.C., E-stop and safety gate monitoring

AutomationDirect $145.00 Amazon $135.00 McMaster-Carr $165.00

Prices are estimates. Verify part numbers match your equipment before ordering.

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