❓ Question — from a VMC owner, Faridabad
Our Fanuc 0i-MD machine shows SV0401 (V) READY OFF on all axes immediately after power on. Machine was fine yesterday. Is the servo amplifier dead? It's an αi SVM 3-axis module.
✅ Answer — JJ Automation Technical Team
SV0401 means the servo amplifier's DRDY (drive ready) signal never came on. When it appears on all axes at once, the amplifier itself is usually NOT the first suspect. Check in this order:
- Emergency stop chain — a stuck E-stop button, broken door switch or tripped contactor keeps MCC off. Check diagnostic DGN 358 / X8.4 (*ESP).
- MCC / magnetic contactor — listen for the contactor pulling in ~2s after power-up. No clunk = check the contactor coil circuit and CX3/CX4 connections on the PSM.
- PSM (power supply module) status LED — a number or letter on the 7-segment display? A dark display means no control power (check 200V input and fuses). Post the code in the comments and we'll decode it.
- FSSB optical fibre — a disturbed/dirty fibre between CNC and amplifier gives ready-off on all axes. Reseat COP10A/COP10B connectors.
- 24V DC supply to the amplifiers — measure at CX1A. Low 24V = ready never asserts.
Only after these check out do we suspect the SVM itself. A single-axis SV0401 points more toward that axis's amplifier or its FSSB position.
⚠️ Safety: the DC bus holds 300V+ for several minutes after power-off. Wait for the charge LED to fully extinguish before touching connections.
Got the same alarm with different symptoms? Reply below with your controller model, amplifier model (e.g. A06B-6117-H303) and the PSM LED code.
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