VFD Troubleshooting: Common Fault Codes and What They Mean

A variable frequency drive rarely fails without warning. It throws a fault code first, resets, runs for another week, then trips again—usually at the worst possible moment. That code on the keypad is the drive telling you exactly where it thinks the problem is, and learning to read it is the difference between a targeted fix and a parts-swapping guessing game.

The catch is that a VFD fault code describes a symptom, not a root cause. An overcurrent trip doesn't mean the drive is bad; it means the drive saw more current than it was willing to pass. The cause could be the motor, the cable, the load, the acceleration ramp, or the drive's own power section. This guide walks through the fault codes you'll actually see on a plant floor, what tends to be behind each one, and how to test a VFD safely when the code alone isn't enough.

What VFD Fault Codes Actually Tell You

Every major manufacturer—ABB, Allen-Bradley, Siemens, Yaskawa, Lenze, Mitsubishi, Emerson—uses its own numbering, so the exact string on the display varies. The underlying protection functions do not. Nearly every fault you'll encounter falls into one of six families: overvoltage, overcurrent, overtemperature, ground fault, undervoltage or input phase loss, and motor overload.

Before you touch anything, do two things. First, write down the exact fault code and the fault history—most drives store the last several faults with a time or run-hour stamp, and a pattern (always on startup, always on decel, always in the afternoon) is often more diagnostic than the code itself. Second, check whether the drive faults with the motor disconnected. A drive that runs clean with no motor attached is pointing you downstream.

VFD Overvoltage Faults

An overvoltage fault means the DC bus climbed above its trip threshold. On a 480 V drive that bus normally sits around 650–680 V DC, and the trip point is typically in the neighborhood of 800 V DC.

• Deceleration that's too aggressive. This is the number one cause. When you command a fast stop, the motor becomes a generator and pushes energy back into the drive. If there's nowhere for that energy to go, the bus voltage rises until the drive trips. Extending the decel time is the first thing to try.
• An overhauling or regenerative load. Downhill conveyors, unwinders, centrifuges coasting down, and any load that drives the motor rather than being driven by it will pump the bus. These applications need a dynamic braking resistor or a regenerative front end, not a longer ramp.
• A failed or disconnected braking resistor. If the application already has a brake resistor, verify it. An open resistor element or a failed braking transistor leaves the drive with no way to dump energy.
• High incoming line voltage. Measure the supply at the drive input. A nominal 480 V system running at 505 V leaves very little headroom, and power factor correction capacitors switching on the same bus can produce transient spikes that trip the drive with no apparent cause.

VFD Overcurrent Faults

Overcurrent is the drive's fastest-acting protection, and it exists to save the output transistors. Because it trips in microseconds, an instantaneous overcurrent almost always points to something abrupt.

• A short or phase-to-phase fault in the motor cable. Damaged insulation, a pinched cable in a drag chain, or water in a junction box will trip the drive instantly on start. Disconnect the motor leads at the drive and megger the cable and motor separately.
• Acceleration that's too fast for the load. High-inertia loads need time. If the drive trips only during startup and the fault clears with a longer accel ramp, you've found it.
• Mechanical binding. A seized bearing, a jammed conveyor, or a brake that isn't releasing will look electrically identical to a short. Always turn the shaft by hand before assuming the fault is electrical.
• A failed IGBT in the output section. When the power module itself is damaged, the drive will typically trip the instant it's commanded to run, sometimes with no motor connected at all. This is a repair-shop diagnosis, not a field one.

Overtemperature and Cooling Faults

Heat is the slow killer of every drive ever built, and overtemperature faults are usually the most preventable trips on this list. The cause is almost always physical rather than electrical.

• A blocked heatsink. Dust, oil mist, and metal fines pack the fins solid over time. Compressed air and a scheduled cleaning interval fix this permanently.
• A failed cooling fan. Drive fans are consumable parts with a finite life. A fan that's slowed or stopped will let the heatsink climb even in a clean environment.
• Enclosure and ambient conditions. A drive rated for 40°C ambient installed in a sealed cabinet in a hot area of the plant will spend its whole life on the edge. Check the panel's own cooling before blaming the drive.
• Carrier frequency set too high. Higher switching frequencies make the motor quieter but generate significantly more heat in the drive. If someone raised the carrier frequency to cure motor noise, that may be the cause.

Ground Faults and Earth Faults

A ground fault trip means the drive detected current leaving the output and returning through ground instead of through the motor windings. In the large majority of cases, the drive is right and the problem is downstream.

Isolate the motor leads at the drive and test insulation resistance from each phase to the motor frame with a megohmmeter. Per IEEE 43, the recommended test voltage for machines rated below 1000 V is 500 V DC, and the recommended minimum one-minute insulation resistance for random-wound stator windings—which covers most general-purpose industrial motors—is 5 MΩ, corrected to 40°C. Readings in the low kilohm range mean the motor has a grounded winding and needs to come out.

Also inspect the cable itself. VFD output cable takes a beating from high-frequency switching, and unshielded cable in a wet or corrosive area degrades faster than most people expect.

Undervoltage and Input Phase Loss

Undervoltage faults are common and frequently misread as a drive problem. Check the incoming supply first: a blown input fuse on one phase, a loose lug, a failing contactor pole, or a sagging plant bus during a large motor start will all trip a drive on undervoltage or phase loss.

If the supply measures clean and balanced but the drive still reports low bus voltage, suspect the DC bus capacitors. Electrolytic capacitors dry out with age and heat, and a drive that's been sitting idle for a year or more may need its capacitors reformed before it's put back into service.

Motor Overload Faults

An overload trip is a thermal calculation, not an instantaneous one—the drive has decided the motor is drawing more current for longer than it can safely tolerate. Verify three things in order: that the drive's motor overload parameter actually matches the nameplate full-load amps, that the mechanical load hasn't increased, and that the motor itself is healthy. A motor with a developing bearing failure or a partially shorted winding will pull more current for the same work.

How to Test a VFD Safely

Before any hands-on VFD troubleshooting, understand that the DC bus capacitors hold a lethal charge after power is removed. Follow lockout/tagout, wait the manufacturer's specified discharge time, and then verify with a meter that the bus is at zero volts before opening the drive. Arc flash PPE is not optional on this equipment.

With the drive safely de-energized and the motor leads disconnected, a digital multimeter in diode mode will give you a quick read on the power section:

• Input rectifier check. Measure from each incoming line terminal (L1, L2, L3) to the positive and negative DC bus terminals. You should see a consistent diode drop in one direction and an open in the other, and the readings across all three phases should closely match each other.
• Output IGBT check. Repeat the same measurements from each output terminal (U/T1, V/T2, W/T3) to the positive and negative bus. Again, look for consistency—one leg that reads shorted or wildly different from the other two identifies a failed module.
• Visual and physical inspection. Bulged or vented capacitors, discolored PCB traces, a burnt smell, corrosion on the board, or dust bridging across terminals all tell you something before you ever pick up a meter.

If the diode checks come back inconsistent, the drive has a hardware failure in the power section. That's the point where field troubleshooting ends and component-level repair begins.

Why VFDs Fail in the First Place

Drives are solid-state and have very few moving parts, so when they fail, the cause is usually environmental or electrical rather than mechanical wear:

• Heat. Every 10°C of additional operating temperature meaningfully shortens electrolytic capacitor life. Cooling is the single highest-leverage thing you can control.
• Contamination. Conductive dust, coolant mist, and corrosive atmospheres attack boards and connectors directly.
• Capacitor aging. DC bus capacitors are wear items with a finite service life, independent of how gently the drive is used.
• Power quality. Voltage sags, transients, harmonics, and switching surges from other equipment on the same bus stress the input section over time.
• Vibration. Drives mounted on or near machinery accumulate loose connections and cracked solder joints.

Repairing vs. Replacing an Aging VFD

When a drive is out of warranty, the reflex is often to buy new. That isn't always the right call. Replacing a drive means new mounting, new wiring, re-parameterization, and frequently a change to the communication setup and the PLC program that talks to it—engineering time that dwarfs the cost of the drive itself.

Repair usually makes more sense when the drive is part of a matched system, when it's an obsolete model whose replacement would force changes elsewhere in the machine, when the new-unit lead time is longer than production can tolerate, or when the failure is confined to a serviceable section such as the power supply, cooling, or a single power module. Replacement makes more sense when the drive has failed repeatedly, when the platform is genuinely unsupportable, or when a modernization was already planned.

A Preventive Maintenance Checklist for Drives

• Clean heatsinks and enclosure filters on a set schedule, matched to how dirty the environment actually is.
• Verify cooling fan operation at every inspection and replace fans proactively rather than on failure.
• Check and retorque power terminations—thermal cycling and vibration back connections out.
• Log the DC bus voltage and drive temperature during normal operation so you have a healthy baseline to compare against.
• Record and trend fault history instead of just clearing it. Repeat faults on the same code are a diagnosis waiting to happen.
• Keep parameter backups. A drive replacement without a saved parameter file turns a one-hour job into a full day.
• Energize and reform spare drives periodically so a stored spare is actually ready when you need it.

When to Bring in a Repair Partner

Field troubleshooting will resolve most VFD faults—wiring, cooling, ramp settings, and load problems account for a large share of trips. But once the diode checks point to a failed power section, or the drive faults immediately with nothing connected, or the model is old enough that replacement parts aren't on a shelf anywhere, component-level repair is the fastest way back into production.

IVS Incorporated has been repairing industrial automation equipment for over 30 years, including AC and DC motor drives, servo drives, spindle drives, inverters, amplifiers, and power supplies from ABB, Allen-Bradley, Siemens, Fanuc, Yaskawa, Indramat, Lenze, Reliance, Mitsubishi, and other major manufacturers. Every repair is tested before it ships.

If a drive is holding up production, learn more about our drive repair services, or explore our motor repair services and PLC repair services for the rest of the system. Contact us to discuss your equipment and turnaround requirements.