Key takeaways
See our roundup of condition monitoring software.
FANUC alarm 401 means the CNC control has commanded the servo amplifier to turn on, but the amplifier has not returned its Velocity Ready (VRDY) signal. The control then locks axis movement to prevent a crash, because it cannot confirm the drive is ready to move the axis safely. In short, the brain (CNC) asked the muscle (servo amplifier) to be ready, and the muscle did not answer.
The single most common cause is an interruption in the Emergency Stop (E-Stop) chain, which drops power readiness to the servo drive before any deeper fault. Other frequent causes are a failed Magnetic Contactor (MCC) cutting high-voltage power to the drive, and loose or vibration-worn communication cables between the CNC and the servo amplifier.
Start by inspecting every E-Stop button and the safety circuit, since a broken E-Stop chain is the most common trigger. Then power-cycle and listen for the Magnetic Contactor (MCC) click to confirm control power, read the servo amplifier status LEDs for the underlying fault code, and check communication cables for vibration-related wear.
Working from the safety chain inward to the amplifier isolates the fault quickly instead of swapping parts at random.
The servo amplifier status LEDs reveal the underlying fault code behind the generic 401 VRDY OFF message, which only tells you the Ready signal is missing, not why.
Reading the amplifier LEDs points you to the real root cause, such as a power, overload, or communication fault, so you fix the actual problem rather than the symptom.
The same idea sits behind Fabrico's fault-to-fix loop, where computer vision records exactly when and for how long a machine stopped, and that stop can become a digital work order on the technician's phone instead of a vague stoppage code. You can see how that closed loop works in a live demo .
Yes, tracking the frequency of alarm 401 turns a recurring nuisance stop into a preventable pattern, because a rising count usually points to a degrading component such as a worn contactor or a vibrating cable connector. The article notes alarm-frequency tracking as a preventive measure. A unified OEE plus CMMS platform like Fabrico makes this practical by logging each fault against the machine and so a rising count can trigger a prioritized follow-up task before the intermittent fault becomes a hard line stop, hosted in an AWS EU region for teams that need it.
Error Code: Fanuc Alarm 401 (SERVO ALARM: VRDY OFF)
Meaning: The CNC controller is not seeing the "Velocity Ready" signal from the servo amplifier. Basically, the amplifier isn't turning on.
Severity: High (Machine Down).
Common Culprits: Emergency Stop (E-Stop) chain, MCC (Magnetic Contactor) failure, or loose communication cables.
If you are seeing Fanuc Alarm 401 on your HMI, your machine is in a hard stop. This is one of the most common alarms in CNC machining, sometimes shown together with another servo alarm such as 414. On Series 0 controls you may also see Alarm 403, which is the same VRDY OFF condition on axes 3 and 4.
This guide will walk you through the standard troubleshooting steps to clear the fault and get back to production.
The VRDY OFF signal means the CNC control (the brain) has sent a signal to the Servo Amplifier (the muscle) to turn on, but the amplifier has failed to return a "Ready" signal.
Think of it like turning the key in your car and hearing silence. The brain said "Go," but the engine didn't respond. Because the servos are not powered, the machine locks out all axis movement to prevent a crash.
The E-Stop Chain is Open: This is the #1 cause. If an E-Stop button is pressed, or if the 24V safety circuit is broken (door interlock, over-travel limit switch), the system kills power to the amplifier immediately.
Magnetic Contactor (MCC) Failure: The large contactor that sends high-voltage power to the drive may be stuck open or burned out.
Loose Cable (FSSB / COP10A-COP10B): The communication cable between the CNC and the Spindle/Servo amp may be loose due to vibration.
Warning: Only qualified electrical technicians should perform checks inside the electrical cabinet. High voltage is present.
Before opening the cabinet, check every Emergency Stop button on the machine (operator panel, chip conveyor, magazine).
Action: Twist and pull all E-Stops. Check if the "Over-Travel" release button needs to be held down.
Why: If the safety chain is open, the MCC will never pull in, triggering Alarm 401.
Have a partner power cycle the control while you stand near the electrical cabinet.
Turn downtime into a number your team can actually act on.
Get a demoAction: Listen for a loud mechanical "Click" or "Clunk." This is the Magnetic Contactor pulling in.
Result:
If you hear the click but it drops out immediately: You likely have a short circuit or a bad servo drive (Check for Alarm 414).
If you hear silence: The MCC coil is not being energized. The power supply module only closes its MCC contact (CX3) when the emergency stop input (CX4) is healthy, so check the E-Stop circuit and its relays first.
Open the cabinet and look at the Servo Amplifier Units. Look, do not touch: with power on and the door open, live parts are exposed, so wear the shock and arc flash PPE your site requires and keep hands and tools out of the cabinet.
Action: Look for the status LED display on the drive itself.
Result:
Before you touch anything in the cabinet: switch the machine off, lock and tag out the main disconnect, wait until the red DC link charge LED on the power supply module is fully off, and confirm zero volts with a meter. Only then check fuses, terminals or connectors.
Display is blank: No control power. With the machine locked out and the charge LED off, check the input fuses.
Display shows "--" (Not Ready): The amplifier has control power but has not been switched on, so it is not sending its Ready signal back to the CNC. Look upstream: the E-Stop chain and the MCC.
Display shows a different number (e.g., 8, 9, b): This is the root cause code. Consult the specific servo manual.
On alpha i drives, the emergency stop contact is wired to connector CX4 on the power supply module (PSM), and the MCC coil is switched by a contact inside the PSM that is wired out through connector CX3. The coil itself runs on its own supply, which must match the contactor's rated coil voltage. If the MCC never pulls in, prove the E-Stop input at CX4 before you suspect the contactor or the PSM.
On older Series 0 controls, the alarm number tells you which axes lost the amplifier Ready signal. Alarm 401 covers axes 1 and 2, and alarm 403 covers axes 3 and 4. If both appear together, look first at something every axis shares, such as the E-Stop chain or the MCC.
If the amplifier shows a code instead of its not ready display, write it down before you reset anything. On alpha i servo amplifier modules, 8 is a DC link current alarm, 8., 9. or A. shown with a dot is an IPM alarm, and b, c or d is abnormal motor current. For these current alarms, Fanuc's first test is to disconnect the motor power leads and release the E-Stop: if the alarm returns with the leads off, replace the amplifier; if it does not, check the insulation of the motor and its power cable to ground.
Safety: releasing the E-Stop with the motor leads disconnected is live work for a qualified electrician. Insulate the loose leads and keep clear of the terminals.
Fanuc Alarm 401 is often caused by loose connections vibrating over time or limit switches sticking due to coolant buildup. These are preventable failures.
You wait for the red light, panic, Google "Alarm 401," and spend 2 hours reading PDF manuals on your phone while the spindle is cold.
Imagine if your machine told you exactly what was wrong, and how to fix it, instantly.
Fabrico Assistant: Instead of digging through 500-page Fanuc PDFs, you simply type "Alarm 401" into the Fabrico app. Our AI, trained on your specific machine manuals, gives you the exact answer: "Check Relay KA-5 and the E-Stop string."
Maintenance Triggers: Fabrico tracks how often each alarm occurs. If Machine A throws Alarm 401 three times this month, that pattern can trigger a follow-up task such as "Inspect Axis Communication Cables for vibration wear," so your team can open a work order and fix the cause before it kills a shift.
Alarm 401 usually stops the machine mid cut, which makes it one of the more expensive stops on a CNC. Those minutes belong in your equipment downtime picture, and when the ready chain fails on the same axis again and again, downtime tracking is what turns three separate call-outs into one visible pattern.
Stop Googling error codes. Start fixing the cause.
See how Fabrico unifies OEE and maintenance in one platform.
Book a demo
Programați o întâlnire individuală cu experții noștri sau înscrieți-vă direct în planul nostru gratuit.
Nu este nevoie de card de credit!
Login
Blog