Fanuc O-MC Alarm 405 "Z axis wrong (ZRN)"

[JSaindon]

After a system software upgrade we recently had on two of our Mach 3 Horizontal Machining Centers, the Z axis forgets or loses its home position periodically. The upgrade was to add an option for programming, single direction positioning, and new EPROM chips (5). The home position gets "lost" after we shut down for the night and reboot the next morning. Batteries were just replaced in this machine also. Plus, if batteries were dead all the positions would be lost.
Previous errors were adjusted for but never resolved as to why they occured. I believe before I worked here the previous guy just offset the parameter value of either/or 0510 and 0710. What else would control the machine home position? Is it necessary to adjust grid shift or the 710 ref. point value? The manual is not clear on the grid shift scale, I don't know if a 1 is .1" or .0001". They identify the inch and metric values for the 708-711 params but not the 508-511 ones.
How can the machine forget or lose its Z axis home position by up to a few inches if the parameters aren't touched? No limit switch problems come up either. Anyone else have a suggestion?

[Al_The_Man]

The grid shift values are in 'Units of Detection' IOW detection counts/inch.
The manual shows alarm 405 cause as 'axis card faulty'.
Reference return is abnormal!.
Al.

[JSaindon]

In my manual it says 405 is "A position control error. Reference position return may have failed due to a problem in the NC or the servo system. Restart operations with manual reference position return."

Solution:

The manual was sort of right, although a normal manual reference return wouldn't work. I set bit 2 of parameter 0022 to 0 in order to "forget" the Z axis reference position. Powered down the machine. Powered back up and did a manual reference point return on Z axis and it found its original home position within .002". I plan to adjust the 0510 grid shift parameter to compensate for the .002".
I still suspect a hardware/software issue somewhere since it should never lose its home position to begin with. I would suspect the axis card but both machines started doing this after Fanuc came out here and installed new EPROMs and options just 3 months ago. They will be coming out next week to go over the problem. I will post findings.