When your Variable Frequency Drive displays a fault code, every minute of downtime costs money. Understanding what these codes mean — and knowing when you can reset safely versus when you need professional repair — can save your operation thousands of dollars in lost production.
At Flexa Systems, we repair hundreds of VFDs every year. Here are the most common error codes we see across the top three drive manufacturers, and what they actually mean for your equipment.
Allen-Bradley PowerFlex Error Codes
Allen-Bradley PowerFlex drives (400, 520, 525, 700, 753, 755) are among the most widely used VFDs in North American manufacturing. Here are the faults we see most frequently:
F2 — Auxiliary Input Fault
This fault indicates an external device connected to a digital input has triggered. Check your safety circuits, door interlocks, and emergency stop wiring. This is usually not a drive issue — it's telling you something external has tripped.
F4 — Undervoltage Fault
The DC bus voltage has dropped below the minimum threshold. Common causes include incoming power sag, loose power connections, or a failing power supply board inside the drive. If this fault occurs repeatedly, the drive's internal capacitors may be degrading and need replacement.
F5 — Overvoltage Fault
The DC bus voltage has exceeded the maximum limit. This typically happens during rapid deceleration when the motor regenerates energy back into the drive. Solutions include increasing deceleration time, adding dynamic braking resistors, or checking if incoming voltage is too high.
F7 — Motor Overload
The drive's electronic overload function has tripped. Check that motor nameplate data is programmed correctly, verify the motor isn't mechanically bound, and ensure adequate cooling. If settings are correct and the motor runs freely, the drive's current sensing may need calibration or repair.
F12 — HW Overcurrent (Hardware)
This is one of the most serious faults. It means the drive detected an instantaneous overcurrent condition at the hardware level. Common causes include output short circuits, ground faults in motor cables, and failed IGBT modules inside the drive. Do not repeatedly reset this fault — it often indicates internal component failure that requires professional repair.
F33 — Auto Restart Tries Exceeded
The drive attempted to restart after a fault the maximum number of times and failed each time. Address the underlying fault before resetting.
Siemens Sinamics Error Codes
Siemens Sinamics drives (G110, G120, S110, S120) use a different fault numbering system from Allen-Bradley. Faults in the F78xx range come from the drive's motor-side monitoring — overcurrent, blocked shaft, speed-controller limits. Faults in the F30xxx range come from the power unit itself: DC-link voltage, output current, and heatsink temperature. Knowing which range a code falls in tells you where to start looking before you reach for the fault list.
For the individual codes — including F7801, F7900 and the F30xxx power unit faults — see our Siemens G120 fault codes guide, which covers the G120, G120C, G120X and G120P in depth with the causes and corrective actions for each.
The cross-brand table below shows how the most common Siemens codes line up against their Allen-Bradley and ABB equivalents.
ABB ACS Error Codes
ABB ACS drives (ACS310, ACS355, ACS550, ACS580, ACS880) have their own fault reporting system:
Fault 1 — Overcurrent
Output current exceeded the trip limit. Check for short circuits, ground faults, and motor insulation. Worn motor bearings can also cause elevated current draw.
Fault 2 — Overvoltage
DC bus voltage too high, usually during deceleration. Extend deceleration ramp or add braking chopper and resistor.
Fault 3 — Earth Fault
Current leakage detected between output phases and ground. This is a serious fault — check motor cable insulation, motor winding insulation, and connections for moisture or damage. Running a motor with an earth fault can cause catastrophic damage.
Fault 8 — Short Circuit
A direct short was detected on the output. Disconnect the motor cables from the drive and measure insulation resistance between each phase and between phases and ground. If the cables and motor test good, the drive's output power stage likely needs repair.
Fault 9 — Undertemperature
The drive is operating below its minimum temperature rating. This is common in outdoor or unheated environments during winter.
VFD Error Code Quick Reference Table
| Fault Type | Typical Codes (per brand) | Common Cause | DIY or Repair? |
|---|---|---|---|
| Auxiliary / external input fault | Allen‑Bradley F2 | External device on a digital input tripped — safety circuits, door interlocks, e‑stop wiring | DIY — check external circuits and reset |
| Undervoltage / DC‑link undervoltage | Allen‑Bradley F4 · Siemens F30003 | Incoming power sag or interruption, loose connections, blown fuses; repeated trips → degrading internal capacitors | DIY after verifying stable supply; repair if it repeats |
| Overvoltage / DC‑link overvoltage | Allen‑Bradley F5 · Siemens F30002 · ABB Fault 2 | Regenerated energy during rapid deceleration; incoming voltage too high | DIY — extend decel ramp or add braking resistor |
| Motor overload | Allen‑Bradley F7 | Wrong motor nameplate parameters, mechanical binding, inadequate cooling | DIY after checking parameters and load; repair if current sensing needs calibration |
| Hardware overcurrent | Allen‑Bradley F12 · Siemens F30001 & F7801 · ABB Fault 1 | Output short circuit, ground fault in motor cables, failed IGBT modules | Repair — do not repeatedly reset |
| Auto restart tries exceeded | Allen‑Bradley F33 | Drive failed the maximum number of restart attempts after a fault | Address the underlying fault before resetting |
| Motor stall / blocked | Siemens F7900 | Mechanical binding, jammed load, seized coupling | DIY if load is bound; repair if motor turns freely (current measurement circuit) |
| Overtemperature | Siemens F30004 | Dirty cooling fans, clogged air filters, high ambient temperature, poor ventilation | DIY — clean fans and improve ventilation, then reset |
| Earth / ground fault | ABB Fault 3 | Current leakage from output phases to ground — cable or winding insulation, moisture | Repair if motor and cables test good — internal drive issue |
| Short circuit | ABB Fault 8 | Direct short on the drive output; failed output power stage | Repair if cables and motor test good |
| Undertemperature | ABB Fault 9 | Drive operating below its minimum temperature rating (outdoor / unheated locations) | DIY — address the environment |
When to Reset vs. When to Call for Repair
Not all fault codes require professional repair. Here's a general guide:
Safe to investigate and reset:
- Overtemperature faults (after cleaning fans and improving ventilation)
- Undervoltage faults (after verifying stable power supply)
- External input faults (after checking safety circuits)
- Motor overload (after verifying parameters and mechanical load)
Call a professional when:
- Hardware overcurrent faults that repeat after reset — likely IGBT failure
- Earth/ground faults when motor and cables test good — internal drive issue
- Short circuit faults with no external short found — power stage failure
- Any fault with visible damage — burn marks, swollen capacitors, burnt smell
- Intermittent faults that appear randomly — developing component failure
Save Up to 70% vs. Replacement
Many facilities assume a faulted VFD needs replacement. In reality, over 85% of VFD faults are repairable at a fraction of the cost of a new unit. At Flexa Systems, we provide:
- Free evaluation and diagnosis
- Component-level repair (not just board swaps)
- Full load testing before return
- 2-year warranty on all repairs
- Rush service available (24-72 hours)
Dealing with a VFD fault code you can't resolve? Request a free evaluation or call us at (254) 254-0005. We'll diagnose the issue and provide a no-obligation repair quote within 24 hours.
