A VFD that won't start isn't always a VFD failure

When a drive sits dark — no display, no run, no response to a start command — the instinct is to order a replacement and move on. That instinct is right often enough that it's tempting to skip the diagnostic step entirely. But in the field, a meaningful share of "dead" VFDs are actually fine: the problem is upstream power, a miswired control terminal, a parameter that reset during a firmware update, or a safety interlock that's simply open.

Running through a short fault-isolation sequence before swapping hardware costs you five to ten minutes. Swapping a drive that turns out not to be the problem costs you a return, a re-diagnosis, and the same downtime you were trying to avoid. This checklist walks the nine causes IAC's engineers see most often when a VFD won't start, roughly in the order you should check them.

Before touching anything: lock out and verify zero energy. DC bus capacitors in a VFD can hold a lethal charge for several minutes after the drive is de-energized, even with the display off. Follow your site's LOTO procedure and confirm bus voltage with a meter before opening the enclosure.

Nine causes, roughly in the order to check them

This is the fast-scan version — enough to triage the problem before you dig into any one cause. Details on each item follow in the sections below.

Causes 1 through 6 account for the large majority of "won't start" calls IAC's team walks technicians through. If you've confirmed clean input power, correct control wiring, correct parameters, and a satisfied enable/interlock chain and the drive still won't run, move to causes 7–9 — that's where you're more likely looking at genuine hardware.

Start at the source: is power actually getting to the drive?

The phase-loss trap

Phase loss deserves its own callout because it's easy to misdiagnose. A drive running on two of three input phases will often power up normally, show a healthy display, and then fault on a start attempt because the DC bus can't maintain voltage under load. Techs sometimes read this as "the drive won't run" and swap it — only to have the replacement do the same thing, because the actual fault is a blown fuse or a failed contactor upstream. Always confirm balanced three-phase input before concluding the drive itself is bad.

2-wire vs. 3-wire control

In 2-wire control, the run command is a maintained contact — the drive runs as long as the input stays closed. In 3-wire control, separate momentary start and stop pushbuttons latch the run state internally in the drive. Wiring a 3-wire scheme as if it were 2-wire (or vice versa) is one of the most common reasons a freshly installed or freshly rewired drive "won't start" despite otherwise correct wiring. Check the parameter that defines the control scheme against how the terminals are actually landed.

The drive is powered and wired correctly — so why won't it run?

If power and wiring check out, the next place to look is configuration. Drives that have been recently replaced, reset, or reprogrammed are especially prone to these three issues.

Cause 4: Run command source mismatch

Every VFD has a parameter that determines where it accepts run commands from — the local keypad, the hardwired terminal strip, or a network/fieldbus connection. If that parameter is set to keypad control while the technician is pressing a physical start button on a remote panel, the drive will sit there doing nothing and look faulty. This is especially common after a drive replacement, since default parameter sets don't always match the previous unit's configuration.

Cause 5: Motor and application parameters not programmed

A replacement drive out of the box, or a drive that's had a factory reset, needs the motor nameplate data (voltage, current, frequency, poles) and application-specific parameters (accel/decel time, control mode, min/max frequency) programmed before it will run correctly — some drives won't run at all without at least the motor data entered. If a "new" drive won't start, confirm it's actually been commissioned, not just powered up.

Cause 6: Safety interlock or enable input open

Many VFD installations run the drive's hardware enable input through a safety chain — E-stop circuits, guard door interlocks, or a separate safety relay module. If any link in that chain is open, the drive is functioning exactly as designed: it won't run, and it may not even display a fault, since from the drive's perspective it's simply not been given permission to start. Trace the enable/safety wiring back to its source before assuming the drive has failed.

Ruled out power, wiring, and parameters — now look inward

If you've confirmed causes 1 through 6 and the drive still won't start, you're now in territory that's more likely to point to an actual drive-side or motor-side problem — though not necessarily one that requires replacement.

Cause 7: Encoder or feedback fault

On drives running closed-loop vector or integrated servo control, a missing, damaged, or incorrectly configured encoder feedback signal will typically prevent the drive from starting at all, often with a specific feedback-loss fault code. Check the encoder cable for physical damage, confirm the pulses-per-revolution parameter matches the actual encoder installed, and verify the feedback connector is fully seated — vibration can walk these connectors loose over time.

Cause 8: Thermal trip

A drive that's been running hot will refuse to start on the next attempt even after it cools, if the thermal fault hasn't been cleared. Check for blocked heatsink fins, a failed or unplugged cooling fan, excessive panel ambient temperature, or an undersized enclosure with poor airflow. Thermal trips are also common on drives that have been derated for altitude or ambient temperature but installed in an environment that exceeds those limits.

Cause 9: Latched fault or genuine internal failure

Some faults latch and require an explicit reset sequence — cycling power, holding a reset button, or issuing a reset via the terminal or network — even after the underlying condition clears. If the drive still won't reset after addressing causes 1–8 and confirming a proper reset sequence, you're likely looking at a genuine internal failure: a blown IGBT, a failed DC bus capacitor, or damaged control board. At this point, a resistance check across the power terminals with the unit fully discharged (megohmmeter, following your drive manufacturer's test procedure) will usually confirm it.

Confirmed hardware failure — what's next

Once you've worked through the checklist and landed on a genuine internal fault, the good news is you've also confirmed exactly what needs replacing — no guesswork, no swapping a healthy drive just to be safe. Match the exact part number, voltage class, and HP rating of the failed unit, and if the application uses vector control with encoder feedback, confirm the replacement supports the same feedback type before ordering.

This checklist pairs directly with IAC's guide on VFD overload trips ↗ — if your drive starts but faults during operation rather than refusing to start at all, that's the more relevant reference. Between the two, most VFD "down line" calls are covered start to finish.

If it's confirmed dead, we can have it moving today

IAC stocks VFDs and servo drives across the brands most commonly found on North American plant floors, along with the compatible cross-reference options when an exact OEM part isn't readily available. Every unit is tested before it ships — not just powered on, but run under load where applicable — and backed by a 2-year in-service warranty.

Same-day shipping on confirmed failures

In-stock drives ordered before 4:00 PM Eastern ship same day. If you've worked through this checklist and confirmed the drive needs replacing, call (877) 727-8757 with the nameplate details on hand — model number, voltage class, HP rating, and control type — and IAC's team can typically turn a quote around in under five minutes. You can also submit the part number through the quote form ↗ or email sales@iac.us.com.