VFD Fault Codes and Diagnosis Guide
What do VFD fault codes actually tell you? A fault code is the number or mnemonic a drive's control board assigns the instant a monitored value crosses a protection threshold set per IEC 61800-2, covering output current, DC bus voltage, heatsink temperature, or ground leakage current, and logs it with a timestamp in the drive's fault buffer. Read it wrong and a technician swaps a healthy drive for a bad motor cable, or resets an overload fault three times until the IGBTs fail. This guide covers brand fault code formats across ABB, Schneider and Siemens, the five fault families behind most trips, pulling fault history, a first-response checklist, and when a code points upstream to the motor, cable, or supply.
How to Read a VFD Fault Code
Every manufacturer of variable frequency drives uses its own fault code format, and ABB, Schneider and Siemens do not agree on one convention. ABB drives (ACS480, ACS580, ACS880) display a numeric fault code paired with descriptive text directly on the control panel, cross-referenced in the firmware's fault table. Schneider Altivar drives (ATV320, ATV340, ATV6xx) use a short alphabetic mnemonic instead — a handful of capital letters shown on the graphic display or the seven-segment keypad on smaller units. Siemens SINAMICS drives (G120, G120C, V20) use an "F" prefix followed by a numeric code, documented against the specific power module and firmware version installed.
None of these formats is self-explanatory without the manual for that exact model. Firmware revisions renumber or add codes between drive generations, so a code memorized from an older drive will not necessarily match the replacement. Keep the fault list for the exact model on hand — a PDF on a phone works — rather than relying on memory across brands and vintages.
| Criteria | ABB ACS (480/580/880) | Schneider Altivar (ATV3xx/6xx) | Siemens SINAMICS (G120/V20) |
|---|---|---|---|
| Fault code format | Numeric code + descriptive text | 2-4 letter mnemonic | "F" prefix + numeric code |
| Fault history access | Keypad diagnostics menu or Drive Composer PC tool | Keypad fault history log or SoMove PC tool | Keypad fault buffer or Startdrive/TIA Portal |
| Reset method | Keypad reset key or digital input, auto-reset parameter available | Dedicated reset logic input or keypad stop/reset | Digital input or fieldbus acknowledge command |
| Cross-generation caveat | Codes shift between firmware revisions | Mnemonics fairly stable across the ATV3xx/6xx generations | Code meaning depends on power module and firmware combination |
The Five Fault Families Behind Most Trips
Nearly every VFD fault reduces to one of five physical causes, regardless of the code's cosmetic format.
Overcurrent
Output current exceeds the drive's instantaneous current limit — typically 150-200% of rated current for a fraction of a second — usually from a mechanical jam, a shorted motor winding, or acceleration set too fast for the load's inertia. See normal duty vs heavy duty overload ratings for why a drive sized for pumps trips on a conveyor's breakaway torque.
DC Bus Overvoltage
A six-pulse diode rectifier holds the DC bus near a fixed multiple of the incoming line voltage.
Formula: Rectified DC Bus Voltage — Source: three-phase six-pulse bridge rectifier relationship
Vdc = 1.35 × VLL
| Symbol | Description | Unit |
|---|---|---|
| V_dc | Nominal DC bus voltage at the rectifier output | V DC |
| V_LL | Line-to-line AC supply voltage feeding the rectifier | V AC |
On a 400V-class supply the nominal bus sits near 540V DC. A decelerating motor with high inertia pumps energy back into that bus faster than the drive's small internal bleed resistor can dissipate it, and the bus climbs until the overvoltage threshold trips, with margin left below the DC bus capacitor rating. A brake chopper with a resistor, or an active front end, clears the fault by giving the regenerated energy somewhere to go. See dynamic, regenerative and DC injection braking for the sizing method.
Undervoltage
The inverse condition: a line sag, a loose connection, or a dropped phase pulls the DC bus below the level needed to hold the output stage in regulation. Undervoltage trips protect against the drive attempting to regulate output voltage it cannot actually produce, which shows up as a stalling motor rather than a clean stop.
Thermal / Overtemperature
A heatsink or IGBT junction thermistor reports a temperature above the drive's ambient-derated limit, commonly triggered by a blocked cabinet filter, a failed cooling fan, or a carrier frequency set too high for the ambient. See VFD overheating and cooling fan failures for the fan-replacement interval that keeps this from recurring every cooling season.
Ground Fault (Earth Fault)
A current transformer on the output detects current returning to ground instead of through the intended phase conductors: a genuine winding-to-frame short, or, more often on long cable runs, capacitive charging current from PWM edges into the cable shield. Motor cable length limits exist partly because that capacitive leakage scales with cable length and can nuisance-trip a sensitive ground-fault threshold on a perfectly healthy motor.
Communication and Encoder Faults
Fieldbus timeout faults trigger when a PLC or master controller stops responding within the configured watchdog window: a network switch reboot, a loose RJ45, or a PLC scan cycle overrun are the common causes, not the drive itself. Encoder or feedback faults appear only on vector-controlled drives running closed-loop; a broken encoder cable, a wrong pulses-per-revolution parameter, or reversed A/B channels produces a feedback fault code identical to a genuinely failed encoder. What we see in the field: a fault that appears the moment a machine restarts after a plant-wide network change is almost always a communication timeout, not a drive defect. Check the switch and cabling before opening the drive.
Fault History: What to Pull Before You Call for Support
Every drive in this class keeps a fault buffer of the last several trips, each with the fault code, a timestamp, and a snapshot of operating conditions at the moment of the trip: output current, DC bus voltage, and drive temperature. Pulling that log before opening a support ticket answers the question that actually matters. Is this fault a one-off event, or a pattern? A single overcurrent trip during commissioning is a parameter to check. The same fault recurring every afternoon at the same ambient temperature points to thermal derating, not a random event.
ABB's Drive Composer, Schneider's SoMove, and Siemens' Startdrive or TIA Portal all read this buffer over USB or the drive's built-in Ethernet port without stopping production, and each exports it as a file a supplier or panel builder can review remotely. Screenshotting the keypad's last-faults screen is the minimum; exporting the full buffer with timestamps is what gets a fault diagnosed on the first call instead of the third.
First-Response Checklist Before Replacing the Drive
Run these checks in order before condemning the drive as failed. Most site visits that end in "the drive was fine" skip straight to step four.
- Read the fault code against the manual for the exact drive model and firmware, not a code memorized from a different generation.
- Pull the fault history buffer and check whether the trip is isolated or recurring.
- Check line voltage at the drive's input terminals under load, not only at no load.
- If the code points to a ground or short-circuit fault, measure motor insulation resistance phase-to-ground with a megger, disconnected from the drive.
- Check ambient temperature and airflow at the drive location, and confirm the cooling fan spins under load.
- Confirm the load's actual current draw against the drive's ND or HD current rating, not just its kW label.
See how to test a VFD for the full multimeter and insulation-resistance procedure behind steps three and four. A drive that passes all six checks and still faults on power-up with no load connected is the case that actually points at internal drive failure rather than an application or wiring issue.
When the Fault Points Upstream: Motor, Cable or Line
Not every VFD fault originates in the drive. A ground fault on a long cable run is frequently the cable's own charging current, not a winding failure — this depends on cable length, shield bonding, and carrier frequency, and a megger test on the disconnected motor is the only way to separate the two. An overvoltage fault during deceleration on a high-inertia fan or centrifuge is the load returning energy the drive has nowhere to put, not a failed bus capacitor. A repeated overcurrent fault on startup can be a mechanically bound load rather than the drive's current limit set too tight.
The overcurrent and overvoltage trip mechanics covered separately go into the physics behind why these two faults dominate nuisance-trip calls; this guide's checklist is the practical companion for narrowing down which one is showing on the keypad. Output filters address the cable-length-related nuisance trips directly, and the VFD engineering guide covers the full power-stage architecture behind every fault family in this article.
Frequently Asked Questions
What does an overcurrent fault mean on a VFD?
It means the drive's output current briefly exceeded its instantaneous current limit, typically 150-200% of rated current. Common causes are a mechanical jam, a shorted motor winding, or an acceleration ramp set faster than the load's inertia allows. Check the fault history for whether it happened on startup, mid-run, or during a specific machine cycle.
Why does my VFD trip on DC bus overvoltage during deceleration?
A high-inertia load pumps energy back into the DC bus faster than the drive's small internal bleed resistor can dissipate it while decelerating, and the bus voltage climbs until the overvoltage threshold trips. A brake chopper with a resistor, a longer deceleration ramp, or an active front end for continuous regen duty all clear it.
What causes a VFD ground fault that a megger test does not find?
Long motor cable runs generate capacitive charging current from PWM output edges into the cable shield, which some ground-fault current transformers read as leakage even though the motor's insulation is healthy. This is a cable-length and carrier-frequency interaction, not a winding fault, and it will not show up on an insulation-resistance test.
How do I clear a VFD fault code and reset the drive?
Most drives clear a fault through a dedicated reset digital input, a keypad reset key, or a fieldbus acknowledge command, but resetting without addressing the underlying cause simply reproduces the same trip. If a fault clears and reappears within a few run cycles, pull the fault history instead of resetting again.
Does a VFD fault code always mean the drive itself failed?
No. Ground faults are frequently cable-related, overvoltage faults are frequently load-related, and communication faults are frequently network-related. The fault code names the protection function that tripped, not automatically the component that caused it.
Where do I find the fault code list for a specific VFD model?
Each brand publishes a fault or parameter manual specific to the drive family and firmware version: ABB's is in the ACS software manual, Schneider's in the Altivar programming manual, Siemens' in the SINAMICS parameter list. Codes are not interchangeable between drive generations even within the same brand.
Conclusion
A fault code is a starting point, not a diagnosis. Reading the correct manual for the exact drive model, pulling the fault history buffer instead of trusting the single code on the display, and running the load-current and insulation checks before opening a support ticket resolves most trips without a drive swap. The five fault families — overcurrent, overvoltage, undervoltage, thermal, and ground fault — cover the overwhelming majority of codes across ABB, Schneider, and Siemens drives, and in most cases the underlying cause sits in the motor, cable, or load rather than in the drive itself.
