SV0404 V-Ready ON (Ready Signal Stuck) fault in Fanuc drive

SV0404 V-Ready ON (Ready Signal Stuck) fault in Fanuc drive

Description

SV0404 is the opposite of the common SV0401. While SV0401 means "I told the drive to start, but it didn't," SV0404 means "I haven't told the drive to start yet, but it says it's already ready!" This is a safety violation. The CNC checks the status of the "Velocity Ready" (VRDY) signal immediately upon boot-up. If it sees this signal is high (Logic 1) before the Emergency Stop is released, it triggers SV0404. This prevents the machine from starting if a drive is effectively "hot-wired" or stuck in an active state.

Cause

This is almost always a hardware failure in the drive's output or control logic.

• Welded Contactor: The internal relay or MCC that powers the drive is physically welded shut (stuck closed). This keeps the bus energized even when the control is off.
• Short/Bridge on PCB: A metal chip or conductive debris has bridged the pins on the control board responsible for the Ready signal, forcing it permanently ON.
• Wrong Axis Card: In rare cases, using an incompatible axis control card with a specific amplifier firmware can invert the logic of the Ready signal.

Solution

You must determine if the drive is actually live or just saying it is.

1. Isolate the Alarm: Disconnect the command cable (CN1) from the drive. If the alarm persists on the screen, the issue is on the CNC side (axis card). If the alarm changes to a communication error, the drive was indeed sending the false signal.
2. Tap the Relays: Sometimes a relay inside the amplifier is mechanically stuck. A light tap with a screwdriver handle might free it (temporary fix only).
3. Replace Amplifier: In 95% of cases, SV0404 requires replacing the servo amplifier because the internal relay logic or opto-couplers are damaged.