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, usually accompanied by Alarm 402 or 404.
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 (JF1/JX4): 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.
Action: 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 is not receiving the 24V signal to close. Check the E-Stop circuit relays.
Open the cabinet and look at the Servo Amplifier Units.
Action: Look for the status LED display on the drive itself.
Result:
Display is blank: No control power. Check input fuses.
Display shows "--" (Not Ready): It is waiting for the VRDY signal.
Display shows a different number (e.g., 8, 9, b): This is the root cause code. Consult the specific servo manual.
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’s Fabrico Agent tracks the frequency of these alarms. If Machine A throws Alarm 401 three times this month, it auto-generates a Work Order: "Inspect Axis Communication Cables for vibration wear," fixing the root cause before it kills a shift.
Stop Googling error codes. Start automating the fix.
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