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Fanuc OM - Alarm 401 (VRDY OFF)


Feb 14, 2020
Got the Alarm 401 (VRDY OFF) running ATC macro
The machine starts up fine, and ref home all axis OK.
When running the ATC Alarm occurs.
It’s a Fanuc OM 1992 machine.

When running G28 X0 Y0 Z0 in “full speed (G00)” - same alarm.
Running G28 X0 Y0 Z0 in Dry Run (lower travel speed) - no alarm
Have found that it is the Y axis that is the issue. (G28 X0 Z0 - works OK)

On my machine the ATC area on the Y axis is in the + direction over the working area.

Could it be a parameter for pos error correction on the Y-axis?



Hot Rolled
Jan 3, 2019
Can you get a Y+ overtravel alarm if you handwheel Y all the way out? Or does that also give you the v-ready loss alarm?
It could be that the mtb chose to pull the P-ready from the drives on tripping the OT limit switches.
if that is true, then why you are hitting the OT on a rapid G28 is the problem to solve.

Is the Y position call for the tool change in a tool change macro? Does it need a Y position call? Easy fix is just eliminate G28 Y0 or substitute maybe G53 Yx.


Feb 5, 2008
I want to use this opportunity to have people check out the official Fanuc alarms app, available on google play (probably apple too? i don't have an iphone)

I'm not affiliated with Fanuc in any way, and will attempt to write the information about your alarm that the app gives. But the app is relatively thorough on information about alarms. And it goes all the way to even "ancient" controls, such as 2, 3, 6, 10, 11, 15, 0, and more

CNC Alarms Tool - Apps on Google Play

Anyway, when you look up 401 in the app for a 0b control:

"The 1st or 2nd axis servo amp READY signal (DRDY) went off. Check 100VAC on analog/digital drives. Check diagnostics x21#4, G121, G128, F148#6, F149#1, 712"

At the very least, check that, and if a fix or cause isn't apparent, you'll be able to post more about what you find here to solve it.