CNC VFD Error Codes Explained: Common Causes and First Checks

A VFD error code usually means the drive detected a condition it could not safely ignore. The code can point toward an overloaded cutting operation, damaged spindle wiring, a connector problem, or an internal VFD fault.

This guide explains several common errors reported on PwnCNC spindle systems, including ERR09, ERR12, ERR31/031, and ERR38.

Error codes are not universal across every VFD manufacturer or model. Always compare the code on your display with the manual for your specific VFD.

Important safety note

If a fault occurs while the spindle is cutting, stop the machine and do not repeatedly reset the VFD to continue the job.

A spindle can stop while the CNC controller continues moving. This can damage the workpiece, tooling, machine, or spindle.

Before inspecting spindle wiring:

1. Stop the machine.

2. Disconnect power from the VFD.

3. Allow the VFD to discharge according to its manual.

4. Do not open the enclosure or touch electrical terminals unless you are qualified to do so.

ERR09: Frequency inverter overloaded

ERR09 generally indicates that the VFD detected a sustained overload condition.

This does not necessarily mean the VFD or spindle has failed. The cutting load may simply be greater than the spindle can maintain at the selected speed and cutting depth.

Common causes include:

• Feed rate that is too high

• Excessive depth of cut

• Full-width slotting

• Heavy chip load

• Dull or damaged tooling

• Chip buildup around the cutter

• A bound collet or spindle

• Cutting at a low RPM where less power is available

• A flexible machine or inadequate workholding

A spindle may be more likely to stall when operating well below its rated speed, especially during a heavy cut. For example, a large tool cutting at full width and full depth can place significant load on the spindle even when the material itself seems easy to machine.

First checks for ERR09

Review the cutting parameters before replacing hardware:

• Tool diameter and flute count

• RPM

• Feed rate

• Depth of cut

• Stepover

• Whether the cut is full-width

• Whether the ramp enters the material too aggressively

• Whether chips are clearing properly

Try a controlled test using a lower feed rate, shallower depth of cut, or more gradual ramp. If the error disappears after reducing the cutting load, the original operation may have exceeded the available spindle torque.

If ERR09 occurs during a light cut or while the spindle is running without cutting, inspect the spindle, collet, cable, and VFD more closely.

ERR12: Output fault or three-phase imbalance

ERR12 is generally associated with an output fault or imbalance between the VFD’s three output phases.

On a CNC spindle system, one of the most common causes is an intermittent problem in the spindle motor wiring.

Possible causes include:

• A loose spindle connector

• A damaged spindle cable

• A cable pinched in a drag chain

• Excessive cable bending

• Strain near the spindle connector

• A poor connection at the VFD output terminals

• A grounding or shielding issue

• An internal spindle or VFD problem

One important clue is whether the error changes when the spindle cable is moved. In one installation, the spindle operated normally during warm-up but generated ERR12 during machine movement. Removing the power cable from a tight drag chain stopped the error. Reinstalling it caused the error to return. A larger drag chain and a more generous bend radius resolved the problem.

First checks for ERR12

With power removed, inspect:

• The entire spindle cable

• Areas entering and leaving the drag chain

• Tight bends and pinch points

• Strain-relief sections

• The spindle-side connector

• The VFD-side connector

• VFD output terminals

• Ground and shield connections

If the error occurs only when the machine moves, suspect cable routing or flex damage before changing VFD parameters.

For machines using both spindle power cable and coolant tubing, verify that the drag chain has enough interior space. A cable that technically fits may still be forced into an unsafe bend radius.

ERR31 or 031: Current detection fault

ERR31, sometimes displayed as 031, indicates a current-detection fault on the VFD.

This is different from a normal overload caused by cutting too aggressively. ERR09 generally points toward the operating load, while ERR31 can indicate a problem with the VFD’s current-sensing circuitry or related electrical conditions.

Possible causes include:

• An internal VFD fault

• A damaged spindle cable

• A grounding problem

• Loose or overheated connections

• Electrical noise

• An issue within the spindle motor

• An electrical event that affected the drive

First checks for ERR31

After safely powering down the system, inspect:

• Spindle cable condition

• Cable connections at both ends

• VFD output terminals

• Spindle and VFD ground connections

• Signs of heat, arcing, or discoloration

• Whether the spindle turns smoothly by hand with power removed

Also note when the error occurs:

• Immediately after starting

• Only after the spindle warms up

• Only while cutting

• Only when the machine moves

• During both unloaded and loaded operation

If ERR31 returns during a light finishing operation, or while the spindle is running without a meaningful cutting load, the VFD may have developed an internal fault.

At that point, continued resetting is not recommended. The practical next step may be testing or replacing the VFD.

You can review the available PwnCNC VFD options.

ERR38: Short between spindle output phases

ERR38 generally indicates that the VFD detected a short or abnormal connection between the spindle’s output phases.

The most common possibilities are:

• Damaged spindle cable

• Shorted or contaminated connector

• Bent or touching connector pins

• Internal spindle motor damage

• Internal VFD damage

• Cable damage near a strain-relief point

• Coolant or moisture exposure

• Damage caused by a machine crash or cable pinch

Power cycling may temporarily allow the spindle to run again, but that does not mean the problem is resolved. Repeatedly resetting ERR38 can damage the VFD or spindle.

First checks for ERR38

With the VFD disconnected from power and fully discharged, inspect:

• The full spindle cable for cuts, crushing, abrasion, or heat damage

• Both cable ends

• Strain-relief areas

• Spindle connector pins

• Loose, bent, contaminated, or touching contacts

• Places where the cable enters or exits the drag chain

• Any recent crash, coolant exposure, or wiring modification

If the cable shows visible wear near the spindle connector, replacing the cable may be a reasonable first step. PwnCNC replacement cables are available on the Spindle Cable product page.

However, an ERR38 fault can originate from the VFD, cable, or spindle motor. If replacing the cable does not resolve the issue, the next step may be testing the VFD enclosure or evaluating the spindle motor.

Replacement spindle motors are available through the PwnCNC Spindle Motor product page.

What if the VFD shows no error?

A spindle can stop without the VFD displaying a fault code.

This can happen when:

• The controller drops the spindle run command

• The control signal is interrupted

• The cutting load causes the spindle to stall without triggering a drive fault

• The VFD limits current without entering a fault state

• The controller and VFD lose communication

• The spindle cable has an intermittent connection

When a spindle stops during a job, look at the VFD display before resetting anything.

Ask:

• Is the VFD still showing a run command?

• Is output frequency still displayed?

• Did the displayed frequency drop to zero?

• Did the VFD show an error before the stop?

• Did the controller continue sending motion commands?

A VFD fault relay can notify the CNC controller when the drive enters an actual fault state. However, the relay may not activate if the spindle stalls without the VFD registering a fault.

Advanced monitoring through Modbus, programmable outputs, or controller inputs may be possible on some systems, but the exact behavior depends on the VFD model, parameters, controller, and integration. Consult the PwnCNC VFD manual before changing advanced settings.

What if the VFD display is blank?

A blank display is different from an error code.

The first checks should determine whether the VFD is receiving power or whether the remote keypad connection has failed.

Check:

1. Whether the outlet is working

2. Whether the breaker has tripped

3. Whether a power strip, disconnect, or emergency-stop circuit is interrupting power

4. Whether the VFD power cord is damaged

5. Whether an internal fuse or breaker may have opened

6. Whether the keypad extension cable is firmly connected

7. Whether the keypad cable or connectors are damaged

If the display remains blank after confirming that power is reaching the VFD, send photos of the VFD, keypad, cable connections, model label, and power connection to support.

Do not open the VFD enclosure or touch mains-voltage terminals unless you are qualified to do so.

Communication errors are not always VFD faults

Some problems appear on the controller rather than the VFD.

For example, a Onefinity Redline controller may report a spindle communication error when the controller and VFD are using different settings, the wrong cable, or reversed RS-485 signal wires.

Before assuming the VFD has failed, verify:

• The correct control cable is installed

• The controller is configured for PWM or RS-485 as appropriate

• The VFD is set to automatic control

• The required VFD parameters match the controller guide

• The RS-485 polarity is correct

• The controller and VFD have been power-cycled after programming

The Onefinity Redline controller guide includes setup information for compatible PwnCNC systems.

If the spindle runs manually from the VFD but will not start from G-code, the spindle and VFD may be operating correctly. The problem may instead involve controller settings, the control cable, or the spindle command in the G-code.

When should you stop troubleshooting?

Stop testing and contact support when:

• The same error returns after basic inspection

• The spindle stops during active cutting

• The cable shows heat, crushing, abrasion, or connector damage

• You smell burnt insulation or electronics

• The spindle becomes unusually hot

• The spindle develops vibration or roughness

• ERR31 returns during a light or unloaded operation

• ERR38 returns after power cycling

• The VFD display remains blank despite confirmed power

• The controller continues moving after the spindle stops

Do not keep running a spindle simply because power cycling temporarily clears the code.

Information to collect before contacting support

The fastest troubleshooting usually begins with complete information.

Include:

• VFD error code exactly as displayed

• CNC machine brand and model

• CNC controller brand and model

• Spindle power, voltage, and cooling type

• Approximate spindle age

• Whether the fault occurs unloaded or while cutting

• RPM, feed rate, depth of cut, and tool details

• Whether the error occurs when the machine moves

• Photos of both spindle cable connectors

• Photos of the VFD enclosure and connections

• Photos of any damaged cable or connector

• A short video showing the error, if safe to reproduce

If the system was purchased several years ago, include the original order number when possible. VFD enclosures, connectors, cable options, and spindle configurations can change over time.

Quick reference

Displayed code or symptom	Common meaning	First area to inspect
ERR09	Sustained overload	Cutting load, tooling, chip load, binding
ERR12	Output fault or phase imbalance	Spindle cable, connectors, drag-chain routing
ERR31 / 031	Current-detection fault	VFD, grounding, cable, electrical connections
ERR38	Short between spindle output phases	Cable, connectors, spindle motor, VFD
Communication error	Controller/VFD communication problem	Control cable, settings, RS-485 polarity
Blank display	Power or keypad problem	Outlet, breaker, keypad cable, fuse

A VFD error code is a starting point for diagnosis, not a complete diagnosis by itself. The most reliable troubleshooting combines the displayed code with the timing of the fault, machine movement, cutting conditions, cable condition, and whether the spindle operates normally without a load.

If you need help identifying the correct replacement VFD, spindle cable, or motor, provide photos of the system and the original order information before purchasing.