Fanuc Alarm 2 — Overtravel Plus: What It Means
Fanuc Alarm 2 is an Overtravel (Plus direction) hardware limit — an axis has tripped the positive hardware overtravel limit switch. Unlike the software overtravel limits (set in parameters), the hardware OT switch is a physical limit switch wired directly to the CNC as a safety interlock. When the machine axis travels beyond the positive hardware limit, the drive cuts out and Alarm 2 is stored. The machine will not move the faulted axis in any direction except back off the limit switch using the override procedure.
Common Causes
- Manual jog or programmed move past the soft limit — A program with incorrect coordinate values or a manual jog that inadvertently moved the axis beyond the positive mechanical boundary.
- Soft limit not configured or set too wide — If software overtravel limits (Parameter 1320/1321) are not set correctly, the axis can reach the hardware limit before the software limit trips.
- Homing (reference) position lost — After a power loss or battery failure, the machine may home incorrectly and lose its reference point, causing all subsequent moves to have wrong absolute position.
- Defective or misadjusted OT switch — The hardware limit switch itself has shifted position or failed in a way that triggers prematurely.
Step-by-Step Fix
- Activate OT release function — On most Fanuc systems, press and hold the OT Release (or Hardware OT Release) function key on the MDI panel, or hold the appropriate button combination per your machine’s operation manual. This temporarily overrides the hardware OT signal.
- Jog axis off the limit — While holding OT release, use the handwheel or manual jog to move the axis in the minus direction (away from the positive hardware limit). Move just far enough to clear the limit switch — typically 5–10mm is sufficient.
- Release the OT release button — Once the axis is clear, release the OT release button. Alarm 2 should clear.
- Verify axis position — Check the current machine coordinate position. If the position looks incorrect relative to the known physical position, perform a machine reference (home) cycle.
- Check soft limit parameters — In MDI, access Parameter mode and verify that the positive software limit (Parameter 1320 for each axis) is set inside the physical hardware limit by at least 5mm.
- Inspect the OT switch — If the alarm tripped without apparent cause, check the hardware limit switch for physical shift, contamination, or cable damage.
Parts Often Needed
| Part | Notes |
|---|---|
| Overtravel limit switch | Replace if the switch actuates at the wrong position or shows intermittent behavior |
| Limit switch cable | If wiring to the OT switch is chafed or broken |
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 homes incorrectly after clearing Alarm 2, the absolute encoder battery may be dead or the reference mark may be lost. Incorrect homing on multi-axis machines can cause crashes — have a Fanuc-trained service engineer verify the reference point before resuming production.