The 401 Alarm (VRDY OFF) is one of the most common issues encountered in Fanuc CNC systems. While it indicates that the "Servo Ready" signal is missing, the root cause can vary from internal module failures to external wiring issues.
In this article, we analyze a specific case of a full-axis 401 alarm and provide a step-by-step resolution process.
1. Problem Description
A production facility reported that upon releasing the Emergency Stop (E-Stop) on a specific machine, a 401 Alarm occurred across all axes (X, Y, Z, A, B).
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Symptom: The Power Supply Module (PSM), Spindle Module (SPM), and Servo Modules (SVM) all displayed a flashing dash "-" on their status LEDs.
2. Technical Analysis & Diagnostic Steps
Understanding the 401 Alarm Logic
The 401 alarm (VRDY OFF) is triggered when the Servo Software does not receive the "Ready" confirmation. This involves a handshake between the CNC, the Power Supply Module, and the Magnetic Contactor (MCC).
Figure 1 below illustrates the conditions required for a 401 Alarm (VRDY OFF) to occur.
Figure 1: The causes of the fanuc 401 alarm
Step 1: Diagnostic Data Inspection
We checked DGN358 for all axes. The value was 993, which converts to binary 01111100001.
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Finding: The status indicated a missing CRDY (Converter Ready) signal. This directed our focus to the Power Supply Module (PSM) and the MCC circuit.
Step 2: Hardware Isolation
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Voltage Check: Measurements showed no three-phase input voltage, and the MCC had not pulled in.
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Module Swap: We replaced the PSM unit, but the fault persisted.
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Isolation Test: We disconnected the SPM and SVM modules individually. The PSM still failed to reach the "Ready" state, which ruled out amplifier failure.
Step 3: MCC Circuit Investigation
We shifted the investigation to the external MCC circuit. This specific machine utilizes two MCC paths: the main circuit and the IO/LINK axis circuit.
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Signal Verification: We checked signal F0.6 (SA). It was set to 1, meaning the CNC had issued the "Ready" command and the internal contact at CX3 was closed.
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Continuity Testing: While tracing the wiring for PSM/CX3, we discovered that Line 100 at CX3 was not conducting to Line 100 on the external relay (CRMER).
3. Resolution
After confirming the discontinuity, we temporarily shorted all "100" series points for testing. The machine powered up successfully, and the 401 alarm cleared.
Root Cause: The user had previously swapped I/O units and terminal blocks with a neighboring machine to troubleshoot a different issue. Because the wires were not labeled, they reconnected them based on the neighboring machine's configuration. Although the two machines were the same model and batch, there were slight variations in their electrical wiring, leading to the open circuit on Line 100.
4. Conclusion
The 401 alarm is a systemic error. To solve it efficiently:
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Analyze the Logic: Use DGN signals (like 358) to determine if the issue is internal (software/module) or external (MCC/wiring).
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Verify Hardware: Don't assume a module is broken until the input power and control loops are verified.
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Label Everything: As seen in this case, even "identical" machines can have different wiring nuances.
Reference Source: Beijing FANUC. This article is based on technical documentation provided by Beijing FANUC.
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