Fanuc Alarm 4 — What It Means
Fanuc alarm 4 indicates a stored stroke limit overtravel in the negative direction (Alarm 4: Stored Stroke Limit 1 negative side). The CNC detected that the axis position reached or exceeded the software-defined travel boundary set in the stored stroke limit parameters. This is a software limit — not a physical limit switch — and stops axis motion before the machine reaches a mechanical hard stop.
Common Causes
- Workpiece or fixture placed outside the work envelope — A setup change moved a fixture into a position that requires the tool to travel beyond the configured software limit.
- Incorrect stored stroke limit parameters — The stroke limit parameters (P1320/P1321 or equivalent) are set too conservatively and do not reflect the machine’s actual safe travel range.
- Reference position (zero return) not performed — If the machine lost its reference position after a power cycle or E-stop, the stored limits may be referenced to an incorrect position and trip prematurely.
- Axis overtravel during MDI or program execution — A manually entered MDI command or NC program commanded the axis to a position beyond the stored limit.
Step-by-Step Fix
- Do not force axis motion — When alarm 4 is active, do not attempt to jog the axis further in the negative direction. The machine is at or past its software boundary.
- Switch to MPG or jog mode — On the operator panel, switch the mode selector to Handle (MPG) or Jog mode. Select the faulted axis.
- Jog the axis in the positive direction — Move the axis away from the overtravel position (toward the positive side) until the alarm clears. On most Fanuc controls, the alarm will clear automatically once the axis moves back within the limit boundary.
- Perform a reference return — After recovering the axis, perform a zero return (G28 or the REF return button on the operator panel) to confirm the machine is properly referenced.
- Review stored stroke limit parameters — Navigate to the Parameter screen (typically MDI → System → Param). Review parameters 1320 (positive limit) and 1321 (negative limit) for the affected axis. Adjust if the limit is set incorrectly.
- Review the NC program or MDI command — If the overtravel was caused by a program move, correct the command to stay within the work envelope.
- Reset and resume — After the axis is within limits and properly referenced, reset the alarm via the Reset key and resume operation.
Parts Often Needed
| Part | Notes |
|---|---|
| No parts typically required | Alarm 4 is a software limit; recovery is usually parameter or positioning correction |
| Fanuc servo amplifier | Only if servo fault accompanies the overtravel alarm |
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 the machine repeatedly trips alarm 4 in normal operation after parameter and program correction, the axis position feedback may be drifting due to a faulty encoder or servo drive. Contact a Fanuc-certified service technician for servo system diagnostics.