Fanuc Alarm 500 — What It Means
Fanuc Alarm 500 is an overtravel alarm — one or more axes traveled beyond the software overtravel limits stored in the machine parameters. Unlike hardware overtravel alarms (1-4 series), Alarm 500 is a software position check; the control detected that the commanded path would enter or has entered the software travel boundary.
Common Causes
- Machine returned to wrong reference position — After a power cycle or battery change, if the reference return wasn’t completed correctly, the machine’s position understanding is wrong and normal moves trigger 500.
- Part offset or work coordinate set incorrectly — A work coordinate (G54-G59) or tool length offset that positions the program path outside the machine travel envelope triggers 500.
- Program error — A G-code program commanding a position beyond the machine’s soft limits triggers 500 before the move executes.
- Soft limit parameter changed — Incorrect or reduced travel limits in machine parameters cause normal positions to trigger Alarm 500.
Step-by-Step Fix
- Identify which axis triggered 500 — The Fanuc alarm display shows the axis designation. Note it.
- Check current position vs. soft limits — On the Fanuc position display, compare the current machine coordinate to the soft limit values in parameters (typically P1320/P1321 for positive/negative limits per axis).
- Re-execute reference return — If position is uncertain after a power cycle, execute ZRN (reference return) on all axes before any other motion.
- Check work coordinates and offsets — Verify G54 (or active work coordinate) and tool length offsets are correct for the current setup.
- Manual jog out of limit — If the axis is inside the soft limit boundary, use the [OVR cancel] function (hold RESET and press the positive or negative axis jog button on some Fanuc versions) to jog out of the soft limit zone.
Parts Often Needed
| Part | Notes |
|---|---|
| Backup battery | If position was lost due to battery failure |
Technician Tips
CNC alarms should always be diagnosed by reading the full alarm text, not just the number. Modern Fanuc, Haas, and Mazak controls provide detailed alarm descriptions — press the help key while viewing the alarm for diagnostic suggestions. The alarm history screen stores the last 50-100 alarms with timestamps; review this before clearing to identify patterns. A servo alarm (SV series on Fanuc) that appears only during rapid traverse points to a different cause than one that appears during cutting.
Prevention: Back up your CNC parameters monthly. Parameter loss from dead batteries is the #2 cause of unexpected CNC downtime (after mechanical crashes). Replace control batteries annually before they fail.
Need Replacement Parts?
Finding the right replacement parts for your equipment is critical to getting it running again. Check current prices on Amazon — prices and availability change frequently.
When to Call a Pro
If Alarm 500 triggers on moves that were previously fine, a parameter change or position calibration issue needs Fanuc-certified service to diagnose.